GB2307610A

GB2307610A - Differential amplifiers and compound transistors using diodes or resistors to balance numbers of base-emitter or collector-base type junctions etc.

Info

Publication number: GB2307610A
Application number: GB9623158A
Authority: GB
Inventors: Theodoros Loizos Pallaris
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-11-21
Filing date: 1996-11-06
Publication date: 1997-05-28
Also published as: GB9623158D0

Abstract

An amplifier has a complementary differential input stage (S1 to S3), two stages (S4, S5) of voltage amplification, and two stages (S6, S7) of current amplification. Compared with a conventional amplifier, additional diodes are included so that any path (e.g. S1 to S7) between the supply rails (V+, V-) can be divided in four quarters (P1 to P4) each containing the same number of diodes, base-emitter junctions or the like, and each containing the same number of resistors, collector-emitter junctions or the like. The long-tail nodes (LT1, LT2) of the input stage are tied together by a pair of diodes (D5a, D5b) or the like. Additional connections (C3 to C10) are made between the input stage and the voltage amplification stages. To counteract the imbalance caused by these connections, compensating diodes (D37a to D38b) are included between the loads and complementary long-tails in the input stage. The transistors in the voltage and current amplification stages are arranged as modified forms of Darlington pair (e.g. T9, T11; T13, T15) and as modified forms of Sziklai pair (e.g. T11, T13). In Figure 57a feedback circuit (R33, R34) is arranged to place as great a load on the amplifier as the output load (R35, R36). Other modifications may be made to the loads and the long-tails of the input stage, and to the input biassing.

Description

TITLE Amplifiers and Compound Transistors DESCRIPTION This invention relates to amplifiers and compound transistors. The various aspects of the invention to be described below may be incorporated into an amplifier which provides excellent linearity, thermal stability, electrical stability (both signal and DC), common mode rejection ratio ("CMRR") and frequency response, with exceptionally low harmonic distortion, offset and noise, and using components which do not need to be of the highest quality and closely matched.

(Compound Transistor Circuits) First to fourth aspects of the present invention relate to compound transistor circuits, and in particular to such circuits which include first and second transistors arranged somewhat like a Darlington or Sziklai configuration, and to amplifier circuits including such compound transistor circuits.

A Darlington pair configuration of transistors is very well known. As will be described in detail below, such a compound transistor has a very high input impedance. It has a voltage gain which is close to unity. Compared with a single-stage emitter-follower, it has a higher current gain and a lower output resistance. It behaves like a single transistor with a current gain equal to the product of the current gains of the two transistors, and this can be very useful where high output currents are required.

A drawback of the Darlington arrangement is that the leakage current of its first transistor is amplified by the second transistor, and hence the overall leakage current may be very high. The overall base emitter drop is twice normal, and the saturation voltage is at least one diode drop, because the emitter of the first transistor must be a diode drop above the emitter of the second transistor. A significant drawback is that the combination tends to act like a rather slow transistor because the first transistor cannot quickly turn off the second transistor. In a known way of attempting to deal with this problem, a resistor is connected from the base to the emitter of the second transistor. This resistor also reduces the amount of leakage current through the first transistor which tends to bias the second transistor into conduction.The choice of the value of the resistor is a compromise between reducing the leakage current, improving the response time, and not overly reducing the gain of the circuit. It is known also to include a further resistor in parallel with the base-emitter junction of the first transistor, for example in the RCA 2N6383, 2N6384 and 2N6385 power transistors.

A Sziklai, or complementary Darlington, arrangement is also very well known. As will be described in detail below, such a compound transistor has a high input impedance. It has only a single base-emitter drop and cannot saturate to less than a diode drop. Like the Darlington pair, the Sziklai arrangement acts like a rather slow transistor, and it is known to connect a resistor from the base to the emitter of its second transistor to improve the speed.

In accordance with the first aspect of the invention, there is provided a compound transistor circuit having a compound base, collector and emitter, the compound transistor comprising: a first transistor having a first base, collector and emitter; and a second transistor having a second base, collector and emitter; wherein: the first base is connected to the compound base; the second base is connected to the first emitter; the second emitter is connected to the compound emitter directly or via a number of forward-conducting diodes; the first emitter is connected to the compound emitter via a number of forward-conducting diodes which is one greater than the number of diodes (if any) connecting the second emitter to the compound emitter; the first collector is connected to the compound collector directly or via a number of forward-conducting diodes; and the second collector is connected to the compound collector via a number of forward-conducting diodes which is one greater than the number of diodes (if any) connecting the first collector to the compound collector.

In accordance with the second aspect of the present invention, there is provided a compound transistor circuit having a compound base, collector and emitter, the compound transistor comprising: a first transistor having a first base, collector and emitter; and a second transistor having a second base, collector and emitter; wherein: the first base is connected to the compound base; the second base is connected to the first collector; the second emitter is connected to the compound collector directly or via a number of forward-conducting diodes; the first collector is connected to the compound collector via a number of forward-conducting diodes which is one greater than the number of diodes (if any) connecting the second emitter to the compound collector; the first emitter is connected to the compound emitter directly or via a number of forward-conducting diodes; and the second collector is connected to the compound emitter via a number of forward-conducting diodes which is one greater than the number of diodes (if any) connecting the first emitter to the compound emitter.

In either case, the circuit may be in the form of an integrated circuit, and it may have an accessible connection to the second base and/or to the second collector.

In accordance with the third aspect of the present invention, there is provided an amplifier circuit having a compound transistor circuit having a compound base, collector and emitter, the compound transistor circuit comprising: a first transistor having a first base, collector and emitter; and a second transistor having a second base, collector and emitter; wherein: the first base is connected to the compound base; the second base is connected to the first emitter; the second emitter is connected to the compound emitter directly or via an impedance; the first emitter is connected to the compound emitter via an impedance across which the quiescent voltage drop is one diode-drop greater than the quiescent voltage drop (if any) between the second emitter and the compound emitter; the first collector is connected to the compound collector directly or via an impedance; and the second collector is connected to the compound collector via an impedance across which the quiescent voltage drop is one diode-drop greater than the quiescent voltage drop (if any) between the first collector and the compound collector.

In accordance with the fourth aspect of the present invention, there is provided an amplifier circuit having a compound transistor circuit having a compound base, collector and emitter, the compound transistor circuit comprising: a first transistor having a first base, collector and emitter; and a second transistor having a second base, collector and emitter; wherein: the first base is connected to the compound base; the second base is connected to the first collector; the second emitter is connected to the compound collector directly or via an impedance; the first collector is connected to the compound collector via an impedance across which the quiescent voltage drop is one diode-drop greater than the quiescent voltage drop (if any) between the second emitter and the compound collector; the first emitter is connected to the compound emitter directly or via an impedance; and the second collector is connected to the compound emitter via an impedance across which the quiescent voltage drop is one diode-drop greater than the quiescent voltage drop (if any) between the first emitter and the compound emitter.

In either case, the, or at least one of the, impedance(s) is provided by a forward-conducting diode, a resistor, or a circuit producing a similar diode-like drop.

(Symmetry) Fifth to seventh aspects of the present invention relate to amplifiers.

Problems which arise in known amplifiers include non-linearity, and thermal drift. These problems are due, to a significant extent, to non-linearity and thermal drift of the active devices in the circuit. The present invention is concerned with reducing to some extent at least some of these problems.

In accordance with the fifth aspect of the present invention, there is provided an amplifier comprising: first and second supply rails; and a plurality of circuit elements including a plurality of active elements and a plurality of diodes arranged in a plurality of paths carrying signals between the supply rails; wherein: each of the paths can be divided into a plurality of sections extending between the supply rails; and each of the sections of each of the paths contains the same number of elements producing a diode-type voltage drop at the quiescent operating point of the amplifier.

Thus a degree of "symmetry" is provided in the amplifier. It is believed that, in view of such symmetry, thermal drift and/or non-linearity of an element which may tend to pull a signal voltage in the amplifier towards one of the supply rails will tend to be counteracted by thermal drift and/or nonlinearity of another element which may tend to pull the signal voltage in the amplifier towards the other supply rail, thereby improving the DC stability, other electrical stability, thermal stability and/or linearity of the amplifier.

In this specification, the term "diode-type voltage drop" is intended to include not only the voltage drop occurring across a forward conducting diode and the voltage drop occurring across a forward conducting base-emitter junction of a bipolar transistor, but also the voltage drop occurring across other devices having a similar characteristic. As is well known, when a silicon p-n junction is forward conducting, a voltage drop of about 0.6V to 0.75V occurs and does not vary substantially in dependence upon the current. The term also covers an impedance in an amplifier circuit across which the voltage drop at the quiescent operating point of the amplifier (i.e. with no input signal) is generally equal to a diode drop.

In accordance with the sixth aspect of the invention, there is provided an amplifier comprising: first and second supply rails; and a plurality of circuit elements including a plurality of active elements and a plurality of diodes arranged in a plurality of paths carrying signals between the supply rails; wherein: each of the paths can be divided into a plurality of sections extending between the supply rails; and each of the sections of each of the paths contains the same number of elements producing a large-swing voltage drop.

Thus a different degree of symmetry is provided in the amplifier.

In this specification the term large-swing voltage drop is intended to include the sort of voltage drop which occurs across a resistor or between the collector and base of a bipolar transistor.

The features of the fifth and sixth aspects of the invention may be combined in a single amplifier. In this case, the number of elements in each section producing a diode-type voltage drop is not necessarily the same as the number of elements in each section producing a large-swing voltage drop, but preferably these numbers do not differ by more than two.

At least one of the elements producing such a large-swing voltage drop may be a resistor, or the collector-base junction of a transistor.

At least one of the elements producing such a diode-type voltage drop may be a forwardconducting diode, or a forward-conducting base-emitter junction of a transistor, or a resistor.

The number of sections into which each of the paths can be divided may be two. However, especially when the amplifier has a large number of stages and a complementary configuration, the number of sections into which each of the paths can be divided is preferably four.

The amplifier may include a long-tailed differential amplifier stage forming at least one of said paths and having a long-tail extending between a long-tail node and one of the supply rails. In this case, the long-tail preferably includes two series resistances preferably of equal value and preferably of total value equal to a value of load resistance between each of the active elements of the long-tailed stage and the other supply rail. The amplifier may include a further such long-tailed differential amplifier stage which is complementary to the first-mentioned long-tailed differential amplifier stage.

In this case, an impedance may be provided, interconnecting the long-tail nodes of the differential amplifier stages, the impedance forming part of one of said paths. The amplifier may also include a voltage amplifying stage forming at least one of said paths. In this case, the voltage amplifying stage may have two cascaded stages of voltage amplification, each forming one of said paths. The amplifier may also include a current amplifying stage forming at least one of said paths. In this case, the current amplifying stage may have two cascaded stages of current amplification, each forming one of said paths.

In accordance with the seventh aspect of the present invention, there is provided a plural stage amplifier having a plurality of signal-carrying paths between a pair of supply rails, each path having symmetrical halves, as regards the number of diode drops and/or large swing voltage drops therein, each half having symmetrical quarters, as regards the number of diode drops and/or large swing voltage drops therein, the amplifier having an input and an output, both at the half-way level.

(Complemenrary Differential Amplifier with Linked Long-tails) Eighth and ninth aspects of the present invention are concerned with complementary differential amplifiers and integrated circuits including differential amplifiers. In particular, these aspects of the invention are concerned with enabling the behaviour of such an amplifier to be affected beneficially.

In accordance with the eighth aspect of the present invention, there is provided a complementary differential amplifier comprising a first long-tailed pair circuit and a second complementary long-tailed pair circuit, the long-tail nodes of the two circuits being connected by an impedance.

By suitable choice of the impedance, it is believed that it may be possible to affect the behaviour of the amplifier beneficially, for example, the common mode rejection ratio ("CMRR"), possible output voltage swing, linearity and/or harmonic distortion of the amplifier.

In accordance with the ninth aspect of the present invention, there is provided an integrated circuit including a differential amplifier comprising a first long-tailed pair circuit and a second complementary long-tailed pair circuit, the long-tail nodes of the two circuits being externally accessible. Such an integrated circuit may be provided in combination with an impedance connecting the long-tail nodes.

The impedance may include at least one resistor, or two series-connected forward-conducting diodes.

Preferably, the impedance is such that, during quiescent operation of the amplifier, its value is about equal to the effective impedance of the active elements of the amplifier between the long-tail nodes.

(Coupling of Input Amplifier to Subsequent Stage(s)) The tenth aspect of the present invention is concerned with improving the electrical and thermal stability, frequency response, offset and noise of amplifiers.

In accordance with the tenth aspect of the present invention, there is provided an amplifier circuit comprising: first and second supply rails, and a plurality of signal-carrying circuit paths extending between the supply rails, wherein: each path comprises first to fourth series-connected sections in that order from the first supply rail to the second supply rail, with each section including the same number of diode-drop devices; the amplifier circuit includes a first stage having at least first and second such paths between the supply rails, and a second stage having such a path between the supply rails independent of the first and second paths of the first stage; a first connection connects the first path of the first stage to the second stage; a second connection connects the second path of the first stage to the second stage.

Preferably, the quiescent potential difference between the first and second connections is one or two diode-drops.

The circuit may be arranged so that: the second connection is connected:- from a point between the first and second sections of the second path of the first stage, to a point between the first and second sections of the path of the second stage. Also, the first connection may be connected:- from a point which is one diode-drop away from a point between the first and second sections of the first path of the first stage, to a point which is one diode-drop away in the same direction from a point between the first and second sections of the path of the second stage.

Alternatively, the circuit may be arranged such that: the second connection is connected:- from a point which is one diode-drop away from a point between the first and second sections of the second path of the first stage, to a point which is one diode-drop away in the same direction from a point between the first and second sections of the path of the second stage; and the first connection is connected:- from a point which is one diode-drop away in the opposite direction from a point between the first and second sections of the first path of the first stage, to a point which is one diode-drop away in that opposite direction from a point between the first and second sections of the path of the second stage.

In this latter case, the circuit may be arranged such that: the first stage includes a third such path between the supply rails; and a third connection is connected:-from a point between the first and second sections of the third path of the first stage, to a point between the first and second sections of the path of the second stage.

The circuit may include a third amplification stage driven by the second stage by a connection at the level between the first and second sections of the paths.

In the case where the circuit includes a third amplification stage, it is preferably driven by the second stage by connections at the same levels as the second and third connections. In this case, it may be arranged that the second and third stages together form a circuit according to the third or fourth aspects of the invention.

The circuit may include a fourth amplification stage driven by the third stage. In this case, it may be arranged that the third and fourth stages together form a circuit according to the third or fourth aspect of the invention.

The diode-drop in the second stage related to the first connection may be provided by an active element of the second stage. Also, the diodedrnp in the second stage related to the second connection may provided by a forward-conducting diode in the second stage, and/or by a forward-conducting emitter-base or base-emitter junction of a transistor in the second stage.

The first stage may be in the form of a complementary pair of long-tailed amplifiers, the first path of the first stage being provided by one of the long-tailed amplifiers, and the second path of the first stage being provided in part by the long-tail of the other long-tailed amplifier.

(Compensalion for Unbalanced Loading) As mentioned above, problems which arise in known amplifiers include non-linearity, and thermal drift, DC instability and general electrical instability. These problems are due, to a significant extent, to non-linearity and thermal drift of the active devices in the circuit. The eleventh aspect of the present invention is concerned with reducing to some extent at least some of these problems.

In accordance with the eleventh aspect of the invention, there is provided an amplifier circuit comprising: a first stage having first and second signal paths between a pair of supply rails; and a second stage having a third signal path between the supply rails; wherein: the second stage is connected to the first stage by a first connection between the first signal path and the third signal path and by a second connection between the second signal path and the third signal path; the second stage places an unbalanced loading on the first stage through the first and second connections; the first stage includes a compensation element which places a corresponding loading on a complementary portion of the first stage to compensate for the loading.provided by the second stage.

Thus, account is taken of an asymmetrical load placed by one stage on a previous stage.

The circuit may be arranged so that: the first stage has a fourth signal path between the supply rails; the second stage is connected to the first stage by a third connection between the fourth signal path and the third signal path; the second stage places a second unbalanced loading on the first stage through the second and fourth connections; the first stage includes a second compensation element which places a corresponding loading on a complementary portion of the first stage to compensate for the second loading provided by the second stage.

In one example, the or each unbalanced loading and compensatory loading are each one or two diodedrops.

In the case where the first stage is in the form of a complementary pair of long-tailed amplifiers, the first and second paths may include first and second loads of one of the long-tailed amplifiers. In this case, the fourth path preferably includes the long-tail of the other long-tailed amplifier. Alternatively, the first and second paths may include a load of one of the long-tailed amplifiers and the long-tail of the other long-tailed amplifier.

In the case of an amplifier according to the seventh aspect of the invention, the input stage may be in the form of a long-tailed differential amplifier, an output point of the input stage being connected to an input point of the second stage, a quartile point of a path including the long-tail being connected to a quartile point of the second stage, the second stage placing a diode drop loading on the input stage between the input point and the quartile point of the second stage, and the input stage having an additional diode drop connected to said quartile point thereof. In this case, preferably said diode drop loading is in one quarter, and the additional diode drop is in the other quarter in the same half.

In accordance with the twelfth aspect of the present invention, there is provided an amplifier circuit comprising a complementary pair of long-tailed differential amplifiers and a subsequent amplification stage, wherein: each differential amplifier comprising a pair of active elements, each connected between a respective load for that active element and a respective long-tail for that differential amplifier; each load includes a first pair of series-connected diode-drop devices; the circuit further comprises for each load a second pair of series-connected diode-drop devices in parallel with the first pair of diode-drop devices of that load; and one diode-drop device of each second pair is provided by the second amplification stage.

In this case, each second pair of diode-drop devices may have a mid-point connected to a point part-way along the long-tail of the other differential amplifier.

(Long-Tails of Differential Amplifier) In accordance with the thirteenth aspect of the present invention, there is provided an amplifier circuit comprising a complementary pair of long-tailed amplifiers, each having a pair of active elements connected between a load for that active element and a long-tail for that amplifier, wherein the long-tail of each amplifier includes a further active element which receives its bias from a diode-drop in one or both of the loads of the other amplifier. The diode-drops may be provided by respective forwardconducting diodes, or by respective resistors across which there is a diode-drop at the quiescent operating point of the amplifier.

Especially when combined with other aspects of the invention, this aspect of the invention has the effect of enabling the number of resistors in the circuit to be reduced, and it is believed that it may further improve the thermal stability of the circuit.

(Loads of Differential Amplifier) In accordance with the fourteenth aspect of the present invention, there is provided an amplifier circuit comprising a complementary pair of long-tailed amplifiers, each having a pair of active elements connected between a load for that active element and a long-tail for that amplifier, wherein the load of each amplifier includes a further active element which receives its bias from a diode-drop in the long-tail of the other amplifier. The diode-drops may be provided by respective forward-conducting diodes, or by respective resistors across which there is a diode-drop at the quiescent operating point of the amplifier.

(Input Biasing) In accordance with the fifteenth aspect of the present invention, there is provided an amplifier according to the sixth aspect of the invention and including a complementary long-tailed configuration, wherein a further pair of said paths each has a mid-point connected to a respective input of the complementary differential amplifier.

In this case, one of said further paths may include first and second diode-drop devices between one of the amplifier inputs and first and second active devices of the amplifier, and the other of said further paths includes third and fourth diode-drop devices between the other amplifier input and third and fourth active devices of the amplifier. In one example, the active devices have a common connection.

(Output Loading) The sixteenth and seventeenth aspects of the invention are concerned with reducing to some extent at least some of problems which arise in known amplifiers of non-linearity, offset and distortion.

In accordance with the sixteenth aspect of the present invention, there is provided an amplifier for driving an output load having a nominal load resistance, the amplifier comprising: an input stage having a primary input and a feedback input; an output stage having an output node; and a feedback network which feeds back to the feedback input a part of the signal at the output node; wherein the feedback network places on the output node a load which is generally equal to the load placed thereon by the output load.

This aspect of the invention evolves from a realisation that the feedback signal will provide an improved representation of the output signal if the feedback network places on the output stage a load which is generally equal to the load placed thereon by the output load.

In accordance with the seventeenth aspect of the present invention, there is provided an amplifier for driving an output load, the amplifier comprising: a pair of supply rails; an input stage having a primary input and a feedback input; an output stage having at least one primary active device connected in a path between one of the supply rails and a preliminary output node, the or each active device producing a diode-type drop which has a particular effective resistance during quiescent operation of the amplifier; a first resistance connected for series connection with the output load, the first resistance having a value greater than or generally equal to said effective resistance; and a feedback network which feeds back to the feedback input a part of the signal at the preliminary output node, the feedback network including a second resistance connected between the feedback input of the input stage and a reference potential such as ground, the second resistance having a value greater than or generally equal to said effective resistance.

Preferably, the values of the first resistance and/or the second resistance are generally twice said effective resistance.

The feedback network may include a resistance between the preliminary output node and the feedback input which is generally equal to twice the internal resistance of the output stage between the preliminary output node and the supply rails during quiescent operation of the amplifier.

Preferably, the feedback network includes a resistance leading to the feedback input which has a value generally equal to said effective resistance, and a resistance is included leading to the primary input which has a value generally equal to twice said effective resistance.

Preferably, the amplifier is for driving an output load having a nominal load resistance which is generally equal to twice the internal resistance of the output stage between the preliminary output node and the supply rails during quiescent operation of the amplifier.

The output stage preferably has a pair of such primary active devices, each connected in a path between a respective one of the supply rails and the preliminary output node.

(Quiescent Operating Point) In accordance with the eighteenth aspect of the present invention, there is provided an amplifier, comprising an active device, or a plurality of series-connected active devices, in a primary path between a pair of supply rails, the amplifier being such that, during quiescent operation of the amplifier, the voltage drop across the active device, or the sum of the voltage drops across the active devices, is generally equal to two-thirds of the supply voltage between the supply rails.

(Further Compound Transistor Circuits) In accordance with the nineteenth aspect of the present invention, there is provided a compound transistor circuit having: first and second parallel paths between first and second nodes; and a transistor arranged so that its collector-emitter forms part of the first path and its base is connected to a third node part-way along the second path; wherein: the number of diode drops between the first and third nodes via the first path is equal to the number of diode drops between the first and third nodes via the second path; the number of large swing voltage drops between the first and third nodes via the first path is equal to the number of large swing voltage drops between the first and third nodes via the second path; the number of diode drops between the second and third nodes via the first path is equal to the number of diode drops between the second and third nodes via the second path; and the number of large swing voltage drops between the second and third nodes via the first path is equal to the number of large swing voltage drops between the second and third nodes via the second path.

The circuit may further include at least one further transistor, the or each further transistor being arranged so that its collector-emitter forms part of one of the paths and its base is connected to a, or a respective, further node part-way along the other path; wherein: the number of diode drops between the first node and the further node, or any of the further nodes, via the first path is equal to the number of diode drops between the first node and that further node via the second path; and the number of large swing voltage drops between the first node and the further node, or any of the further nodes, via the first path is equal to the number of large swing voltage drops between the first node and that further node via the second path.

The circuit may further include at least one other transistor, the or each other transistor being arranged so that its collector-emitter forms part of one of the paths and its base forms a, or a respective, other node.

In one set of examples of such circuits, there are two diode drops and one large swing voltage drop in each of the first and second paths. In another set of examples of such circuits, there are one diode drop and two large swing voltage drops in each of the first and second paths.

In a further set of examples of such circuits, there may be at least two diode drops (for example two, four or six diode drops) and at least two large swing voltage drops in each of the first and second paths. In this case, the first and second paths may each have two halves, each half having the same number of diode drops, and each half having the same number of large swing voltage drops.

In the case where the circuit is combined with a preceding amplifier stage, at least one node in the first or second path may be connected so as to control or correct the preceding stage. Also, in the case where the circuit is combined with a succeeding amplifier stage, at least one node in the first or second path may be connected so as to control, bias or receive a correction from the succeeding stage.

Each diode drop may be provided by the base-emitter junction of a, or one of the, transistors, by a diode, or by a resistor across which there is a diode drop at the quiescent operating point of the circuit, and/or each large swing voltage drop may be provided the collector-base junction of a, or one of the, transistors, or by a resistor.

(General Comments) It should be noted that the teachings of the various aspects of the invention described above may beneficially be combined in a single amplifier or amplifier circuit.

It will also be noted that in many of the circuits which are described in this specification, extensive use is made of diodes (or impedances across which there is a diode drop at the quiescent operating point of the circuit) in places in the circuit where such components would not normally be expected to be found. Whilst these devices have the disadvantage of causing a loss of voltage, this is not a significant problem when high supply voltages are used. However, these devices provide the advantages of enabling correction to be applied in a forward or backward direction in the circuit and of enabling the active devices in the circuit to be properly biased.

It should also be noted that, whereas conventional integrated differential amplifiers cannot normally be used with supply voltages in excess of i22V and thus have a limit on the output voltage swing of typically f15V, the circuits described in this specification can operate with supply voltages of, for example, i60V and produce output voltage swings of, for example, :::: i36V.

(Brief Description of the Drawings) Specific embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: (Compound Transistor Circuits) Fig. 1 is a circuit diagram of a basic Darlington pair configuration; Figs. 2 & 3 are circuit diagrams of known modified Darlington pair configurations; Fig. 4 is a circuit diagram of a basic Sziklai, or complementary Darlington pair, configuration; Fig. 5 is a circuit diagram of a known modified Sziklai configuration; Fig. 6 is a circuit diagram of a specific example of a circuit somewhat like a Darlington pair according to an embodiment of the first and third aspects of the invention; Fig. 7 is a diagram of a more generalised form of the circuit of Fig. 6;; Fig. 8 is a circuit diagram of a specific example of a circuit somewhat like a Sziklai pair according to an embodiment of the second and fourth aspects of the invention; Fig. 9 is a diagram of a more generalised form of the circuit of Fig. 8; (Complementary Differential Amplifier with Linked Long-tails) Fig. 10A is a circuit diagram of a conventional complementary differential amplifier having a pair of complementary long-tailed pair circuits; Figs. 10B & 10C are equivalent circuits of parts of the circuit of Figure 10; Fig. 11 shows how the circuit diagram of Fig. 10A may be modified in accordance with the eighth and ninth aspects of the invention; Figs. 12A & 12B show one example of detail of part of the circuit of Fig. 11, and an equivalent circuit thereof;; Fig. 13 shows another example of detail of part of the circuit of Fig. 11; Figs. 14A & 14B show a further example of detail of part of the circuit of Fig. 11, and an equivalent circuit thereof; Fig. 15 shows how the circuit diagram of Fig. 11 may be further modified in accordance with the eighth and ninth aspects of the invention; Figs. 16 & 17 show detail of part of the circuit of Fig. 15; (Symmetry) Fig. 18 is a circuit diagram of a conventional complementary long-tailed differential amplifier, as in Fig. 10A, with an added single stage voltage amplifier; Fig. 19 is similar to Fig. 18, but with a further stage of voltage amplification added; Fig. 20 is similar to Fig. 19, but with a single stage current amplifier added; Fig. 21 is similar to Fig. 20, but with a further stage of current amplification added;; Figs. 22 to 26 correspond to Figs. 10A and 18 to 21, respectively, but showing examples of how the circuits may be modified in accordance with the fifth to seventh aspects of the invention; (Coupling of Differential Amplifier to Voltage Amplification First Stage) Fig. 27 shows part of the circuit of Fig. 26; Figs. 28 to 30 show a first, second and third modifications to the circuit of Fig. 27 according to the tenth aspect of the invention; (Compensation for Unbalanced Loading) Figs. 31 to 35 show modifications to the circuits of Figs. 28 to 30 according to the eleventh aspect of the invention;; (Coupling of Differential Amplifier to Voltage Amplification First and Second Stages) Fig. 36 shows an amplifier similar to the amplifier of Fig. 26, but with additional couplings between the differential stage and the first and second stages of voltage amplification, in accordance with the tenth aspect of the invention; (Application of Coupling and Compensation Techniques to other Circuits) Fig. 37 shows a complementary differential amplifier with a single complementary voltage amplification stage according to the eleventh aspect of the invention; Fig. 38A shows a specific example of a non-complementary differential amplifier with a single non-complementary voltage amplification stage according to the eleventh aspect of the invention; Fig. 38B is a more generalised form of the circuit of Fig. 38A;; Fig. 39A shows a specific example of a complementary non-differential amplifier with a single complementary voltage amplification stage according to the eleventh aspect of the invention; Fig. 39B is a more generalised form of the circuit of Fig. 39A; Fig. 40A shows a specific example of a non-complementary non-differential.amplifier with a single non-complementary voltage amplification stage according to the eleventh aspect of the invention; Fig. 40B is a more generalised form of the circuit of Fig. 40A; (Loads in Differential Amplifier) Fig. 41 shows a modified form of the circuit of Fig. 32, with modified loads in the differential amplifier according to the fourteenth aspect of the invention; Figs. 42A to 43C show detail of parts of the circuit of Fig. 41;; (Long-tails in Differential Amplifier) Fig. 44 shows another modified form of the circuit of Fig. 32, with modified long-tails in the differential amplifier according to the thirteenth aspect of the invention; Figs. 45A to 46C show detail of parts of the circuit of Fig. 44; (Input Biasing of Differential Amplifier) Figs. 47 & 48 each show a differential amplifier with one stage of voltage amplification, and including input biasing circuits for the differential amplifier according to the fifteenth aspect of the invention; Fig. 49 shows a development to the amplifier of Fig. 48; Fig. 50 shows a development to the amplifier of Fig. 49; (Developments to Coupling and Compensation Techniques) Fig. 51 shows a development to the compensation technique shown in Figs. 31 to 35;; Fig. 52 shows a development to the technique shown in Figs. 28 to 30 of coupling the first stage voltage amplifier to the differential amplifier; Fig. 53 shows a differential amplifier with a first voltage amplification stage employing the developments of both Fig. 51 and Fig. 52; (Current Amplificarion Stages) Fig. 54 is similar to Fig. 36, but shows modifications to the current amplification stages; (Alternative Coupling of All Stages) Fig. 55 shows a differential amplifier having two stages of voltage amplification and two stages of current amplification, coupled in a different manner to that shown in the preceding drawings;; (Output Loading and Feedback) Figs. 56 & 57 shows differential amplifiers of the types described above having two stages of voltage amplification, to which feedback and output loading circuits have been added according to the sixteenth and seventeenth aspects of the invention; (Alternative Input Stage) Fig. 58 shows an alternative form of a non-differential complementary input stage with three further stages of amplification; Fig. 59 shows a modification to the circuit of Figure 58 to include a differential part to the input stage, and with only one stage of further amplification; Figs. 60 to 62 show modifications to the circuit of Figure 59; Fig. 63 shows a modification to Figure 59 to include three further stages of amplification; (Further Circuits) Figs. 64 to 72D show examples of various compound transistor circuits in accordance with the nineteenth aspect of the invention; ; Fig. 73 shows a modification to the circuit of Figure 32; Fig. 74 shows a modification to the circuit of Figure 50; Figs. 75A to 75D show further examples of various compound transistor circuits in accordance with the nineteenth aspect of the invention; and Fig. 76 shows an amplifier circuit having a complementary differential input stage and a parallel-split stage of voltage amplification, embodying the fifth to twelfth and eighteenth aspects of the invention.

Compound Transistor Circuits A Darlington pair configuration of transistors is very well known and an NPN example is shown in Figure 1. The base and collector of an NPN first transistor T1 are connected to base and collector nodes B, C, respectively, of the compound transistor; the collector and emitter of an NPN second transistor T2 are connected to the collector node C and to an emitter node E, respectively, of the compound transistor; and the emitter of the first transistor is connected to the base of the second transistor T2. The compound transistor has a very high input impedance. It has a voltage gain which is close to unity. Compared with a single-stage emitter-follower, it has a higher current gain and a lower output resistance.It behaves like a single transistor with a current gain equal to the product of the current gains of the two transistors, and this can be very useful where high output currents are required.

A drawback of the Darlington arrangement is that the leakage current of the first transistor Tl is amplified by the second transistor, and hence the overall leakage current may be very high. The overall base emitter drop is twice normal, and the saturation voltage is at least one diode drop, because the emitter of the first transistor Tl must be a diode drop above the emitter of the second transistor T2. A significant drawback is that the combination tends to act like a rather slow transistor because the first transistor T1 cannot quickly turn off the second transistor T2. A known way of attempting to deal with this problem is illustrated in Figure 2, in which a resistor R1 is connected from the base to the emitter of the second transistor T2.This resistor R1 also reduces the amount of leakage current through the first transistor T1 which tends to bias the second transistor T2 into conduction. The choice of the value of R1 is a compromise between reducing the leakage current, improving the response time, and not overly reducing the gain of the circuit. It is known also to include a further resistor R2 in parallel with the base-emitter junction of the first transistor Tl, as shown in Figure 3, for example in the RCA 2N6383, 2N6384 and 2N6385 power transistors.

A Sziklai, or complementary Darlington, arrangement is also very well known, and an NPN example is shown in Figure 4. The base and emitter of an NPN first transistor T3 are connected to base and emitter nodes B, E, respectively, of the compound transistor; the collector and emitter of a PNP second transistor T4 are connected to the emitter node E and to a collector node C, respectively, of the compound transistor; and the collector of the first transistor is connected to the base of the second transistor. This compound transistor acts like an NPN transistor, again with a large current gain. The compound transistor has a high input impedance. It has only a single base-emitter drop and cannot saturate to less than a diode drop.Like the Darlington pair, the Sziklai arrangement acts like a rather slow transistor, and it is known to connect a resistor R3 from the base to the emitter of the second transistor T4, as shown in Figure 5, to improve the speed.

Referring to Figure 6, an NPN compound transistor circuit, somewhat like a Darlington pair, in accordance with the invention has base, emitter and collector nodes B, E, C, respectively. An NPN first transistor T1 has its base and collector connected to the base and collector nodes, respectively, and its emitter connected to the base of an NPN second transistor T2. The second transistor T2 has its emitter connected to the emitter node E, and its collector connected via a series p-n diode D2 to the collector node. The diode D2 is oriented so as to conduct collector current to the transistor T2.

Another p-n diode Dl is connected from the base to the emitter of the second transistor T2. The diode D1 is oriented so that it can bypass part of the base-emitter current of the second transistor T2. The voltage gain of the circuit of Figure 6 is close to unity, as in the conventional Darlington circuit. The saturation voltage is at least one diode drop, since the emitter of the first transistor T1 must be a diode drop above the emitter of the second transistor, as in the conventional Darlington circuit. The baseemitter drop is twice normal, as in the conventional Darlington circuit of Figure 1. However, the response speed of the circuit of Figure 6 is higher than that of the conventional circuits because the diodes are providing proper biasing to the two transistors T1, T2.Only part of the leakage current of the first transistor T1 is amplified by the second transistor T2, and therefore the overall leakage current is not as high as the conventional Darlington circuit of Figure 1. The circuit of Figure 6 has a higher current gain, lower output resistance and higher input resistance than a single stage emitter follower, and may, or may not, have as high an input impedance as a conventional Darlington circuit.

In a more generalised form of the circuit of Figure 6, as shown in Figure 7, each diode D1, D2 is provided by a respective impedance Z1, Z2. The types or values of the impedances are chosen such that, during quiescent operation of the circuit, the impedances conduct and each has a voltage drop across it which is generally equal to one diode drop. For example, the impedances Z1, Z2 in the circuit of Figure 7 may be provided by respective resistors having values such that, during quiescent operation of the circuit, each resistor has a voltage drop across it which is generally equal to one diode drop. In this case, some improvement in stability of the circuit can be achieved, as compared with the circuit of Figure 6.

Referring to Figure 8, an NPN compound transistor circuit, somewhat like a Sziklai, in accordance with the invention has base, emitter and collector nodes B, E, C, respectively. An NPN first transistor T3 has its base and emitter connected to the base and emitter nodes B, E, respectively, and its collector connected to the base of a PNP second transistor T4. The second transistor T4 has its emitter connected to the collector node C, and its collector connected via a series p-n diode D4 to the emitter node. The diode D4 is oriented so as to conduct collector current from the second transistor T4. Another p-n diode D3 is connected from the base to the emitter of the second transistor T4. The diode D3 is oriented so that it can bypass part of the emitter-base current of the second transistor T4.

The response speed of the circuit of Figure 8 is higher than that of the conventional circuits because the diodes are providing proper biasing to the two transistors T3, T4.

In a more generalised form of the circuit of Figure 8, as shown in Figure 9, each diode D3, D4 is provided by a respective impedance Z3, Z4. The types or values of the impedances are chosen such that, during quiescent operation of the circuit, the impedances conduct and each has a voltage drop across it which is generally equal to one diode drop. For example, the impedances Z3, Z4 in the circuit of Figure 9 may be provided by respective resistors having values such that, during quiescent operation of the circuit, each resistor has a voltage drop across it which is generally equal to one diode drop. In this case, some improvement in stability of the circuit can be achieved, as compared with the circuit of Figure 8.

It will be appreciated that complementary PNP versions of the circuits described above may be provided by using transistors of opposite polarity to the transistors T1 to T4 and, in the cases of Figures 6 and 8, diodes which are oppositely oriented to the diodes D1 to D4.

The circuits described above may be constructed as integrated circuits.

In addition to the collector, emitter and base nodes of the circuits described above with reference to Figures 6 to 9, the base of the second transistor and other electrode of the first transistor may be made externally accessible as node N1 or N3 so as to receive bias from a preceding amplifier stage or to supply a control or correction signal to the preceding stage. Also, the collector of the second transistor may be made externally accessible as node N2 or N4 so as to supply bias to a succeeding amplifier stage or receive a control or correction signal from the succeeding stage.

Although a cascade of two transistors has been described in each of the arrangements of Figures 6 to 9, it will be appreciated that more transistors, diodes, resistors and nodes may be added to the cascade.

Complementary Differential Amplifier with Linked Long-tails Referring to Figure 10A, a known differential amplifier has inverting and non-inverting inputs VinA, VinB, respectively, and a pair of outputs VoutA, VoutB, respectively, for example for driving a complementary pair of transistors in a subsequent amplification stage. The inverting input VinA is connected to the bases of NPN and PNP transistors T5, T6, respectively, the collectors of which are connected via load resistors R5, R6, respectively, to positive and negative supply rails V+, V-, respectively. Also, the collectors of transistors T5, T6 provide the outputs VoutA, VoutB. The noninverting input VinB is connected to the bases of NPN and PNP transistors T7, T8, respectively, the collectors of which are connected via load resistors R7, R8, respectively, to the positive and negative supply rails V+, V-, respectively.The emitters of the NPN transistors T5, T7 are connected together at a long-tail node LTl, and a long-tail resistor R9 connects node LT1 to the negative supply rail V-.

Similarly, the emitters of the PNP transistors T6, T8 are connected together at long-tail node LT2, and a long-tail resistor R10 connects node LT2 to the positive supply rail V+. Input resistors R11, R12 may be provided, as required, connecting the inputs VinA, VinB to ground E.

The circuit components are chosen such that, in its quiescent state, (i.e. with the voltages at the inputs VinA, VinB both at the 0V potential of the ground E), all of the transistors T5 to T8 are conducting. Typically, values are chosen such that R5=R6=R7=R8 and R9=R10.

It will appreciated that, in operation, if the voltage at the inverting input VinA goes positive, transistor T5 will be turned more on, thus making the voltage at output VoutA less positive. Also, transistor T6 will be turned less on, thus making the voltage at output VoutB more negative. The opposite occurs if the voltage at the inverting input VinA goes negative.

If the voltage at the non-inverting input VinB goes positive, transistor T8 will be turned less on, thus making the voltage at long-tail node LT2 more positive. In turn, this will turn transistor T6 more on, and thus make the voltage at output VoutB less negative. Also, transistor T7 will be turned more on, thus making the voltage at long-tail node LT1 less negative. This has the effect of turning transistor T5 less on, which in turn makes the voltage at output VoutA more positive. The opposite occurs if the voltage at the non-inverting input VinB goes negative.

It may therefore be appreciated that the voltages at the long-tail nodes LT1, LT2 will swing up and down with swings of the voltages at the non-inverting and inverting inputs VinA, VinB.

However, the difference between the voltages at the long-tail node nodes LT1, LT2 will remain approximately constant, equal to the sum of the base emitter voltages of the transistors TS, T6, equal to the sum of the base emitter voltages of the transistors T7, T8, and accordingly equal to approximately two diode drops.

Figure 10B is an equivalent circuit of part of the circuit of Figure 10A, in which the baseemitter junctions of the transistors T5 to T8 are represented by equivalent diodes DebS to Deb8, respectively. Also, Figure 10C is an equivalent circuit of part of the circuit of Figure 10A, in which the base-emitter junctions of the transistors T5 to T8 are represented by equivalent resistors RebS to Reb8, respectively, each having a value equal to the diode drop voltage across the respective junction divided by the respective emitter current at the quiescent operating point of the circuit.

In accordance with the invention, the relationship between the voltages at the long-tail nodes LTl, LT2 in a circuit like that shown in Figure 10A may be modified. For example, Figure 11 shows a portion of the circuit of Figure 10A, but with the addition of an impedance Z5 connecting the longtail nodes LTl, LT2.

As shown in Figure 12A, in one embodiment, the impedance Z5 is provided by a pair of series-connected diodes D5a, D5b, and the point LTm between the two diodes D5a, D5b may be connectable to another part of the circuit. Figure 12B is similar to Figure 10B, but showing the addition of the diodes D5a, D5b. The diodes D5a, D5b are oriented so as to conduct current from the long-tail node LT2 to the long-tail node LT1. It may be appreciated that the additional diodes D5a, D5b will place a stricter limit on the amount of the voltage difference between the long-tail nodes LT 1, LT2, and thus may increase the linearity and reduce the harmonic distortion of the amplifier.It may also be appreciated that the current flowing through the diodes D5a, D5b will reduce the currents which are required to flow through the emitters of the transistors T5 to T8 and therefore reduce the power dissipation in the transistors T5 to T8 and in the load resistors R5 to R8 and increase the available voltage swings of the outputs VoutA, VoutB.

As shown in Figure 13, in another embodiment, the impedance Z5 is provided by a resistor R13. Referring to Figure 10C, the effective resistance Reff between the long-tail nodes LT1, LT2 of the network of base-emitter junctions of the transistors T5 to T8 at the quiescent operating point of the amplifier is: Reff = ((Reb5 +Reb6) . (Reb7 + Reb8))/(RebS + Reb6 + Reb7 + Reb8), and if all of the base-emitter junctions have the same resistance, then Reff = RebS = Reb6 = Reb7 = Reb8. The value of the resistor R13 preferably has the same order of magnitude as, and more preferably is about equal to, the effective resistance Reff. It will be appreciated that, at the quiescent operating point of the amplifier, the voltage drop across the resistor R13 is about equal to two diode drops.

As shown in Figure 14A, the resistor R13 of Figure 13 may be replaced by a pair of seriesconnected resistors R13a, R13b, each having a resistance of one-half of that of the resistor R13, and in this case the point LTm between the two resistors R13a, R13b may be connectable to another part of the circuit. Figure 14B is similar to Figure 10C, but showing the addition of the diodes R13a, R13b, with values such that (R13a+R13b) = RebS = Reb6 = Reb7 = Reb8, since R5 = R6 = R7 = R8.

Figure 15 shows a modification to the circuit of Figures 11 to 14, in which a further impedance Z6 is connected between ground E and the mid-point LTm of the impedance Z5. This has the effect of tying the voltage at the midpoint LTm, and accordingly the voltages at the long-tail nodes LT1, LT2, more closely to ground potential. As shown in Figure 16, the impedance Z6 may be provided by a first pair of series-connected diodes D6a, D6b in parallel with a second pair of seriesconnected diodes D6c, D6d. Alternatively, as shown in Figure 17, the impedance Z6 may be provided by a resistor R14, and this may provide increased stability and a more accurate response to step changes in the input signal.

The amplifier of Figure 10A may be provided in a single integrated circuit package, with the long-tail nodes LT1, LT2 externally accessible. Alternatively, the parts of the amplifier shown in Figures 11 and 15, with the exception of the impedances Z5 and Z6, may be provided in a single integrated circuit package Q1 with the long-tail nodes LT1, LT2 externally accessible, and the impedances Z5, Z6 may be provided by further integrated circuit packages Q2, Q3, respectively, or a single further integrated circuit package.

Many modifications and developments may be made to the embodiments of the invention described above. For example, instead of or in addition to connecting an impedance Z5 between the long-tail nodes LT1, LT2, a signal may be applied to one or both nodes, or between the nodes. Also, the invention is applicable to long-tailed configurations in which the long-tails include active elements so as to make them act like, or more like, constant current sources. Furthermore, the invention is applicable to amplifiers which employ FETs, thermionic valves, and other active amplifying devices.

Symmetry As described above, Figure 10A shows a conventional complementary differential amplifier having a pair of complementary long-tailed pair circuits. Figure 18 shows the addition, in known fashion, of a one-stage voltage amplifier S4 to the outputs VoutA, VoutB of the differential amplifier S1 to S3 to produce an output Vout. The voltage amplifier S4 comprises PNP and NPN transistors T9, T10 having their bases connected at C1, C2, respectively, to the outputs VoutA, VoutB, respectively, their collectors both connected to the output Vout, and their emitters connected to the supply lines V +, V-, respectively, via resistors R1S, R16, respectively.Figure 19 shows the addition, in known fashion, of another stage S5 of voltage amplification to the circuit of Figure 18, by adding PNP and NPN transistors T11, T12 to the transistors T9, T10 of Figure 18 so as to form a complementary pair of darlington pairs T9, T11 and T10, T12. Figure 20 shows the addition, in known fashion, of a stage S6 of current amplification to the circuit of Figure 19, by adding NPN and PNP transistors T13, T14 having their bases connected to the collectors of the transistors T11, T12, respectively, their emitters both connected to the output Vout, and their collectors connected to the supply lines V+, V-, respectively, via resistors R17, R18, respectively.In order to provide biasing for the transistors T13, T14, the collectors of the transistors T11, T12 are not connected directly to the output Vout, but instead are connected thereto via diodes D7, D8, respectively, or other biasing elements. Figure 21 shows the addition, in known fashion, of another stage S7 of current amplification to the circuit of Figure 20, by adding NPN and PNP transistors T15, T16 to the transistors T13, T14 of Figure 20 so as to form a complementary pair of darlington pairs T13, T15 and T14, T16. Also, the diodes D7, D8 of Figure 20 are replaced by pairs of series-connected diodes D7a, D7b and D8a, D8b.

There now follows a description of modifications to the circuits of Figures 10 and 18 to 21 in accordance with the invention.

The circuit of Figure 1 0A may be modified in accordance with the invention as shown in Figure 22. Pairs of forward conducting diodes D9, D10; D11, D12; D13, D14; and D15, D16 have been added in series with the load resistors R5 to R8. Also, the resistors R9, R10 in the long-tails have been replaced by pairs of resistors R9a, R9b; RlOa, RlOb and diodes D17, D18. The values of the resistors are preferably chosen such that R5 = R6 = R7 = R8; R9a = RlOa; and R9b = RlOb. More preferably, R5 = 2.R9a = 2.R9b. As will be appreciated from Figure 22, any signal path between the supply rails V+, V- can be divided into four sections P1 to P4 each containing one diode-drop element and one large-swing voltage-drop element.For example, the path through R5, D9, D10, T5, R9b, D17 and R9a contains: Section Diode-drop element Laree-swing element P1 D9 R5 P2 D10 collector-base of T5 P3 base-emitter of T5 R9b P4 D17 R9a Figure 23 shows a development of the circuit of Figure 22 to include a first stage S4 of voltage amplification, which is a modified form of the stage S4 in Figure 18. By comparison with Figure 18, the stage S4 includes a pair of forward-conducting diodes D19, D20 between the resistors Rips, R16, respectively, and the emitters of the transistors T9, T10, respectively. As will be appreciated from Figure 23, any signal path between the supply rails V+, V- can be divided into four sections P1 to P4, each containing one diode-drop element and one large-swing element. For example, the path through R1S, D19, T9, T5, R9b, D17, R9a contains: Section Diode-dros element Large-swing element P1 D19 RiS P2 emitter-base of T9 collector-base of T5 P3 base-emitter of T5 R9b P4 D17 R9a As another example, the path through RlOa, D18, RlOb, T6, D11, D12, R6 contains:: Section Diode-drop element Large-swing element P1 D18 RlOa P2 emitter-base of T6 RlOb P3 D11 base-collector of T6 P4 D12 R6 Figure 24 shows a development of the circuit of Figure 23 to include a further stage S5 of voltage amplification, which is a modified form of the stage S5 in Figure 19. By comparison with Figure 19, the stage S5 includes forward-conducting diodes D21, D22 between the collectors of transistor Tri 1, T12, respectively, and the output Vout.By comparison with Figure 23, the fourth stage S4 retains a resistor R19 and forward-conducting diode D23 between the supply rail V+ and the emitter of transistor T9, and a resistor R20 and forward-conducting diode D24 between the supply rail V- and the emitter of transistor T10. With these extra elements, any signal path between the supply rails V+, V- can still be divided into four sections P1 to P4 each containing one diode-drop element and one large-swing element.For example, the path through R15, T11, T9, D22, T12, R16 contains: Section Diode-dron element Large-swing element P1 emitter-base of T11 R15 P2 emitter-base of T9 base-collector of T9 P3 D22 collector-base of T12 P4 base-emitter of T12 R16 Figure 25 shows a development of the circuit of Figure 24 to include a stage S6 of current amplification, which is a modified form of the stage S6 in Figure 20. By comparison with Figure 20, the stage S6 includes additional forward-conducting diodes D25, D26 between the resistors R17, R18, respectively, and the collectors of the transistors T13, T14, respectively. Once again, with these extra elements, any signal path between the supply rails V+, V- can still be divided into four sections P1 to P4 each containing one diode-drop element and one large-swing element.For example, the path through R17, D25, T13, T14, D26, R18 contains: Section Diode-drop element Large-swinz element P1 D25 R17 P2 base-emitter of T13 collector-base of T13 P3 emitter-base of T14 base-collector of T14 P4 D26 R18 Figure 26 shows a development of the circuit of Figure 25 to include a second stage S7 of current amplification, which is a modified form of the stage S7 in Figure 21. Also, all of the other stages S1 to S6 as shown in Figure 25 are modified by the inclusion of additional diodes to provide two diode drops in each section P1 to P4 of each signal path. Accordingly, diodes D9 to D22 in the stages S1 to S5 are replaced with pairs of diodes D9a to D22a, D9b to D22b.Furthermore, additional forward-conducting diodes D17c, Dl8c are included in the long-tails of the differential amplifier. Also, in stage S4, additional forward-conducting diodes D27, D28 are included between the collectors of transistors T9, T10, respectively, and the output Vout. In stage SS, additional forward-conducting diodes D29, D30 are included between the resistors R15, R16, respectively, and the supply rails V+, V-, respectively. In stage S6, the collector of transistor T13 is connected to the supply rail V+ by a series-connected pair of forward conducting diodes D31a, D31b and a resistor R21; and the collector of transistor T14 is connected to the supply rail V- by a series-connected pair of forward conducting diodes D32a, D32b and a resistor R22.Also, the emitters of the transistors T13, T14 are connected to the output Vout by forward-connected diodes D33, D34, respectively. In the fmal stage S7, additional diodes D25a, D25b, D35 are included between the collector of transistor T1S and the supply rail V+; and additional diodes D26a, D26b, D36 are included between the collector of transistor T16 and the supply rail V-.

With these extra elements, any signal path between the supply rails V+, V- can still be divided into four sections, but this time each containing two diode-drop elements and one large-swing element. For example, the path through D23a, R19, D23b, T9, DlOb, T5, R9b, D17c, D17a, R9a, D17b contains: Section Two Diode-droD element Larze-swing element P1 D23a, D23b R19 P2 e-b of T9, DlOb c-bofT5 P3 b-e of T5, Dl7c R9b P4 D17a, D17b R9a As another example, the path through D25a, R17, D25b, D35, T1S, T16, T14, T12, R16, D30 contains:: Section Two Diode-drop element Larze-swino element P1 D25a, D25b R17 P2 D35, b-e of T15 c-b of T1S P3 e-b of T16, e-b of T14 c-b of T12 P4 b-eofTl2,D30 R16 In any of the circuits described above with reference to Figures 22 to 26, an impedance Z5 (such as a pair of series-connected diodes D5a, D5b, a resistor R13 or a pair of resistors R13a, R13b) may connected between the long-tail nodes LT1, LT2 as described above in connection with Figures 11 to 17. It should be noted that when the impedance Z5 is provided by a pair of diodes D5a, D5b, this complies with the scheme of symmetry described above with respect to Figures 22 to 26.When the impedance Z5 is provided by a resistor R13, or pair of resistors R13a, R13b, this also complies with the scheme of symmetry described above with respect to Figures 22 to 26, because the voltage drop across the resistor R13 is generally equal to two diode drops, and the voltage drop across each of the resistors R13a, R13b is generally equal to one diode drop during quiescent operation of the circuit.

Coupling of Differential Amplifier to Voltage Amplification First Stage Figure 27 shows part of the circuit of Figure 26, namely the differential input stage S1 to S3 and the first voltage amplification stage S4. It should be noted that the base of the transistor T9 of the stage S4 is connected by connection C1 to the output VoutA between the diodes Diva, DlOb of the first stage S1 to S3, and that the base of the transistor T10 of the stage S4 is connected by connection C2 to the output VoutB between the diodes Dila, Dilb of the first stage S1 to S3.

There now follows a description of a number of techniques by which such a circuit can be modified to provide improved coupling of the voltage amplification stage S4 to the differential input stage Sl to S3. It should be noted that, in the circuit of Figure 27, the component values are preferably chosen so that, during quiescent operation of the amplifier, the points lying on the dotted line between sections P1 and P2 are generally at the same potential above ground potential, and the points lying on the dotted line between sections P3 and P4 are generally at the same potential below ground potential.

First, as shown in Figure 28, a connection CS is made between, on the one hand, the junction between the diode D23b and the transistor T9 in the voltage amplification stage S4 and, on the other hand, the junction between the diodes D18b, DiSc in the input stage S1 to S3. Similarly, a complementary connection C6 is made between, on the one hand, the junction between the diode D24a and the transistor T10 in the voltage amplification stage S4 and, on the other hand, the junction between the diodes D17c, D17a in the input stage S1 to S3.It should be therefore be noted that the connections CS, C6 are connected at the levels between the first and second sections P1, P2 and the third and fourth sections P3, P4, respectively, of the input stage S1 to S3 and the voltage amplification stage S4. It should also be noted that the connections CS, C6 are connected to the long-tail part S2 of the differential stage S1 to S3.

In an alternative arrangement, as shown in Figure 29, a connection C7 is made between, on the one hand, the junction between the diode D23b and the resistor R19 in the voltage amplification stage S4 and, on the other hand, the junction between the diode D13b and the resistor R7 in the input stage S1 to S3. Similarly, a complementary connection CS is made between, on the one hand, the junction between the diode D24a and the resistor R20 in the voltage amplification stage S4 and, on the other hand, the junction between the diode D16a and the resistor RS in the input stage S1 to S3.It should be therefore be noted that the connections C7, C8 are connected at levels which are one diode drop nearer the supply rails V +, V-, respectively, than the levels between the first and second sections P1, P2 and the third and fourth sections P3, P4, respectively, of the input stage Sl to S3 and the voltage amplification stage S4. It should also be noted that the connections C7, C8 are connected to the side S3 of the differential stage S1 to S3 which complements the side S1 from which the outputs VoutA, VoutB are taken.

In a further arrangement, as shown in Figure 30, the connections CS, C6 of Figure 28 and the connections C7, C8 of Figure 29 are all included.

This aspect of the invention is also applicable to circuits, such as those described with reference to Figures 23 to 25, in which there is only one diode drop and one large voltage swing drop in each section P1 to P4 of the circuit. Similarly to the arrangements described with reference to Figures 28 to 30, those other circuits may include a connection CS from, on the one hand, the junction between the diode D19 and the transistor T9 and, on the other hand, the junction between the diode D18 and the resistor RlOb, with a complementary connection C6 in the complementary portion of the circuit.Alternatively or additionally, they may include a connection C7 from, on the one hand, the junction between the diode D19 and the resistor RiS and, on the other hand, the junction between the diode D13 and the resistor R7, with a complementary connection C8 in the complementary portion of the circuit.

Compensation for Unbalanced Loading There now follows a description of modifications to the circuits of Figures 28 to 30 to compensate for the loading which the first voltage amplification stage places on the differential input stage.

In Figure 30, it will be seen that the base-emitter junction of transistor T9 of the voltage amplification stage S4 places a diode-drop tie between the anode of diode Di8c and the cathode of diode DlOa. Also, the diode D23b places a diode-drop tie between the anode of diode D13b and the cathode of diode D18b. Both of these diode drop ties cause imbalance between the sides S1 and S3 of the differential amplifier. As shown in Figure 31, in order to compensate for this effect, an impedance Z7a is connected between the cathode of diode D14a and the anode of diode D18c, and a further impedance Z7b is connected between the cathode of diode D18b and the anode of diode D9b. Similar impedances Z8a, Z8b are added to the complementary part of the input amplifier.The values of the impedances Z7a, Z7b, Z8a, Z8b are preferably chosen so that, during quiescent operation of the amplifier, a diode drop occurs across each of them. More particularly, the impedances Z7a, Z7b, Z8a, Z8b are preferably provided by forward-conducting diodes D37a, D37b, D38a, D38b, respectively, as shown in Figure 32, or other devices which produce a similar effect.

The circuit of Figure 29, which does not include the connections CS, C6, may be modified in accordance with this aspect of the invention in a similar manner. Alternatively, in a further modification, the point between the impedances Z7a, Z7b (or diodes D37a, D37b) is not connected to the point between the diodes D18b, D18c, and the point between the impedances Z8a, Z8b (or diodes D38a, D38b) is not connected to the point between the diodes D17a, D17c, as shown in Figure 33. This latter modification may be employed, even if the connections CS, C6 are included, as shown in Figure 34.Alternatively, instead of the connections CS, C6 being connected to the point between the diodes D18b, Dl8c and the point between the diodes D17a, D17c, respectively, they may be connected to the point between the impedances Z7a, Z7b (or diodes D37a, D37b) and the point between the impedances Z8a, Z8b (or diodes D38a, D38b), respectively, as shown in Figure 35.

Coupling of Differential Amplifier to Volrage Amplification First and Second Stages As shown in Figure 24, the base of transistor T11 of stage S5 is connected by connection C3 to the point between the emitter of transistor T9 and the cathode of diode D23 of stage S4, and the base of transistor T12 of stage SS is connected by connection C4 to the point between the emitter of transistor T10 and the anode of diode D24 of stage S4. In a modification of this circuit, the point between the emitter of transistor T1 1 and the resistor R1S of stage SS is connected by connection C9 to the point between the resistor R19 and the anode of diode D23 of stage S4, and the point between the emitter of transistor T12 and the resistor R16 of stage SS is connected by connection C10 to the point between the resistor R20 and the cathode of diode D24 of stage S4, as shown by dotted lines in Figure 24. It will then be appreciated that the transistors T9, T11 and diodes D21, D23 form a compound transistor, and the transistors T10, T12 and diodes D22, D24 form another compound transistor, as described with reference to Figure 6.This modification may be provided in addition to the modifications described with reference to Figures 11 to 17, in addition to the modifications described with reference to Figures 28 to 30, and in addition to the modifications described with reference to Figures 31 to 35. For example, Figure 36 shows an amplifier based on the amplifier of Figure 26, but with the additional long-tail diodes DSa, DSb, with the additional connections CS to C10 between the differential stage S1 to S3 and the first and second stages S4, S5 of voltage amplification, and with the additional compensation diodes D37a, D37b, D38a, D38b.

The effect of these additional connections and diodes is to make significant improvements in the thermal and electrical stability, frequency response, noise and offset of the amplifier.

As will be seen, the circuit of Figure 36 has a plurality of signal-carrying circuit paths extending between the supply rails V+, V-, each path comprising first to fourth series-connected sections P1 to P4 in that order from the supply rail V+ to the supply rail V-. Each section of each path includes two diode-drop devices. The amplifier circuit includes a first complementary long-tailed differential amplifier stage S1 to S3, which has four primary paths between the supply rails, i.e. (1) from D9a via R9b to D17b, (2) from D13a via R9b to D17b, (3) from D18a via RlOb to D12b, and (4) from D18a via RlOb to D16b.The first stage also has a further path from Di8a to D17b due to the inclusion of the diodes DSa, DSb interconnecting the long-tail nodes LT2, LT1 of the complementary amplifiers.

The amplifier also includes a complementary first voltage amplification stage S4 having a path between the supply rails independent of the four primary paths of the differential amplifier S1 to S3.

The first and third paths of the differential amplifier S1 to S3 have a complementary pair of connections C1, C2 to the stage 54, and the stage S4 has a first complementary pair of connections CS, C6 to the long-tails of the differentail amplifier S1 to S3. Importantly, these connections are at the one-quarter and three-quarter levels from the supply rail V+ to the supply rail V-. The emitter-base junctions of the transistors T9, T10 of the stage S4 provide diode-drops between the connections C1, C2 and the connections CS, C6, and the differential amplifier has a pair of diodes D37b, D38b which counteract the those diode-drops of the stage S4.

The stage S4 also has a second pair of connections C7, C8 to the second and fourth paths of the differential amplifier S1 to S3, and the stage S4 has diodes D23b, D24a which cause diode-drops between the connections CS, C7 and between the connections C6, C8, respectively. The differential amplifier S1 to S3 has a further pair of diodes D37a, D38a which counteract those diode-drops of the stage S4.

The circuit further comprises a complementary second voltage amplification stage S5 extending between the supply rails V+, V-. There is connection C3 from between the first and second sections of the path of the stage S4 to between the first and second sections of the path of the stage S5, i.e. at the one-quarter level from the supply rail V+ to the supply rail V-. Also there is a complementary connection C4 from between the third and fourth sections of the path of the stage S4 to between the third and fourth sections of the path of the stage S5, i.e. at the three-quarters level from the supply rail V+ to the supply rail V-.

The emitter-base junctions of the transistors T11, T12 of the stage S5 produce respective diode-drops in the first and fourth sections, respectively, of the path of the stage S5. Further connections C9, C10 are provided such that those diode-drops of the stage S5 are connected in parallel with diodes D23b, D24a of the stage S4.

The amplifier of Figure 36 also has first and second current amplification stages S6, S7, as shown, each having the feature that the path through each stage comprises first to fourth seriesconnected sections in that order from the first supply rail to the second supply rail, each section of each path including the same number of diode-drop devices.

It will be noted that the transistors T9, T11, diodes D23b, D27, D21a, D21b, the connections C3, C9 and the output line Vout together form a Darlington-type arrangement as described above with reference to Figures 6 and 7. The anodes of the diodes D27, D21b may be connected, as shown by a dotted line in the drawing. The connections C1, C5 ensures proper biasing of the transistor T9, and the connections CS, C7 apply correction to the differential input stage S1 to S3. The connection between the collector of transistor T11 and the base of transistor T13 provide biasing to the subsequent stage S6. Similar comments apply to the complementary part of the circuit.

It will also be seen from Figure 36 that the diodes D29, D31a and the resistors R15, R21 of the stages S5, S6 are in parallel. The emitter of transistor Tri 1 may be directly connected to the anode of diode D31b, as shown by connection C12. It should then be noted that the transistors Till, T13, the diodes D21a, D3 1b and the connections therebetween together form a Sziklai-type arrangement as described with reference to Figures 8 and 9. Also, the anodes of diodes D21b, D33 may be directly connected. The connections C3, C9 ensures proper biasing of the transistor Tri 1, and the connection C9 applies correction to the preceding stage S4. Similar comments apply to the complementary part of the circuit.

It will also be seen from Figure 36 that the diodes D31a, D25a, the resistors R21, R17, and the diodes D31b, D25b of the stages S6, S7 are in parallel. The collector of transistor T13 may be directly connected to the anode of diode D35, as shown by connection C 14. It should then be noted that the transistors T13, T15, the diodes D33, D35 and the connections therebetween together form a Darlington-type arrangement as described above with reference to Figures 6 and 7. Similar comments apply to the complementary part of the circuit.

In the circuit of Figure 36, the values of the resistors are preferably chosen such that: RS = R6 = R7 = R8; R9a = RlOa; R9b = RlOb; R19 = R20; RiS = R16; R21 = R22; and R17 = R18.

The circuit of Figure 36 may be modified so as to incorporate any of the features of the other circuits described above.

It will be appreciated from the above that the amplifiers which have been described possess the feature that each circuit path between the supply rails can be divided into four quarters, each including the same number of diode-drops and each including the same number of resistive-type drops, and therefore there will be a tendency for thermal drifts in the amplifiers to cancel out at a half-way level between the supply rails, which is the level at which the output is taken. Furthermore, where the input stage is loaded by a diode drop in a subsequent stage, one or more further diodes are added in an attempt to balance that loading so as to compensate for variations therein.

In the circuit shown in Figure 36, the diodes D27, D28 and/or the diodes D21a, D22b and/or the diodes D21b, D22a and/or the diodes D33, D34 may be replaced by more complex biasing arrangements than the simple diodes which are shown, and the facility for trimming may be included so as to enable the class of operation to be adjusted for example between pure Class A and Class B.

For example, these diodes may be replaced by diodes in parallel with, or in series with, resistors, by transistors having biasing resistors, or by other biasing circuits. However, these alternative arrangements preferably act predominantly like forward biased diodes so that, when forward conducting, the voltage drop does not vary greatly in dependence upon the current. In the case of a diode which carries a current which does not vary greatly throughout the operating range of the amplifier, the diode may be replaced by, for example, a simple resistor (or a more complex biasing circuit) which provides the required diode drop voltage. Similar comments apply to the diodes shown in the other drawings in this application.

Application of Coupling and Compensation Techniques to other Circuits Figure 37 shows a complementary differential amplifier S1 to S3 with a single complementary voltage amplification stage S4 to which the techniques described above have been applied.

As shown in Figures 38A and 38B, the circuit of Figure 37 may be modified by omission of the components of one of the complementary parts of the circuit to provide a non-complementary differential amplifier S1 to S3 with a single non-complementary voltage amplification stage S4. As can be seen in Figure 38A, the additional diodes DSa, DSb, the additional connections CS, C7 and the compensation diodes D37a, D37b are included. More generally, as shown in Figure 38B, an impedance ZS may be used in place of the diodes DSa, DSb so as to have two diode-drops thereacross at the quiescent operating point of the amplifier, and compensation impedances Z7a, Z7b may be used in place of the compensation diodes D37a, D37b each having a diode-drop thereacross at the quiescent operating point of the amplifier.By comparison with Figure 37, in Figures 38A and 38B the transistor T10 is replaced by a series-connected resistor RT10 and diode DT10 having an equivalent impedance at the quiescent operating point of the amplifier. The additional connection C6 may also be included, if required.

As shown in Figures 39A and 39B, the circuit of Figure 37 may also be modified by omission of the components of one of the differential parts of the circuit to provide a complementary nondifferential amplifier S1 to S2 with a single complementary voltage amplification stage S4. As can be seen in Figure 39A, the additional diodes D5a, DSb, the additional connections CS, C6 and the compensation diodes D37b, D38b are included. More generally, as shown in Figure 39B, an impedance ZS may be used in place of the diodes DSa, DSb so as to have two diode-drops thereacross at the quiescent operating point of the amplifier, and compensation impedances Z7b, Z8b may be used in place of the compensation diodes D37b, D38b each having a diode-drop thereacross at the quiescent operating point of the amplifier.The impedances Z7b, Z8b, or diodes D37b, D38b, compensate for the diode drop loadings produced by the emitter-base junction of transistor T9 and the base-emitter junction of transistor T10, respectively. In the circuits of Figures 39A and 39B, the values of the resistors are preferably chosen such that: PS = R6; R9a = RlOa; R9b = RlOb; and RIS = R16.

As shown in Figures 40A and 40B, the circuit of Figure 3? may furthermore be modified by omission of the components of one of the differential parts of the circuit and one of the complementary parts of the circuit to provide a non-complementary non-differential amplifier S1 to S2 with a single non-complementary voltage amplification stage S4. As can be seen in Figure 40A, the additional diodes D5a, D5b, the additional connection CS and the compensation diode D37b are included.More generally, as shown in Figure 40B, an impedance ZS may be used in place of the diodes DSa, DSb so as to have two diode-drops thereacross at the quiescent operating point of the amplifier, and a compensation impedance Z7b may be used in place of the compensation diode D37b having a diodedrop thereacross at the quiescent operating point of the amplifier. By comparison with Figure 37, in Figures 40A and 40B the transistor T10 is replaced by a series-connected resistor RT1O and diode DOT10 having an equivalent impedance at the quiescent operating point of the amplifier. The additional connection C6 may also be included, if required.

It should be noted that, in Figures 40A and 40B, there are a plurality of signal-carrying circuit paths extending between the supply rails V +, V-, each path comprising first to fourth series-connected sections Pl to P4 in that order from the supply rail V+ to the supply rail V-. Each section of each path includes one diodedrop device. For example, the path through R5, D9, D10, T5, R9b, D17, R9a includes the diode D9 in section P1, the diode D10 in section P2, the base-emitter junction of T5 in section P3, and the diode D17 in section P4.The amplifier circuit includes a first stage (R5, D9, D10, TS, R9b, D17, R9a, RlOa, D18, RlOb, Z5) having a first such path (R5, D9, D10, TS, R9b, D17, R9a) and a second such path (RlOa, D18, RlOb, ZS (or DSa, DSb), R9b, D17, R9a) between the supply rails V+, V-. The amplifier circuit also includes a second stage S4 having such a path (RlS, D19, T9, RT10, DT10, D20, R16) between the supply rails V+, V- independent of the first and second paths of the first stage. The first stage has a connection C1 from the first path to the second stage.The second stage also has a connection C5 to the second path of the first stage. Importantly, this connection CS is one quarter of the way from the supply rail V+ to the supply rail V-. The emitterbase junction of transistor T9 of the second stage S4 provides a diode-drop between the connection C1 and the connection CS. The first stage has the impedance Z7b (Figure 40B), such as the diode D37b (Figure 40A), between the first and second paths which counteracts the diode-drop of the emitter-base junction of transistor T9 of the second stage. The circuit may also have a connection C6 between the anodes of the diodes D17, D20, i.e. at the three-quarters level from the supply rail V+ to the supply rail V-.

It will be noted that the diode-drop of the emitter-base junction of transistor T9 of the second stage S4 is in the second section of its path, and the impedance Z7b (Figure 40B) or diode D37b (Figure 40A) of the first stage is in the first section of its path.

It should also be noted that the impedance ZS (Figure 40B) or diodes DSa, D5b (Figure 40A) may be as described above with reference to Figures 11 to 17, the mid-point LTm being at the halfway level between the supply rail V+ and the supply rail V-, and optionally being connected via a feedback path to the output Vout.

In the circuits of Figures 40A and 40B, the values of the resistors are preferably chosen such that: RlOa = RlOb; R9a = R9b; and RT10 = R16.

It should also be noted that, in the circuit of Figures 38A and 38B, the first stage S1 to S3 has a third such path (R7, D13, D14, T7, R9b, D17, R9a) between the supply rails V+, V-, and the second stage S4 has a further connection C7 to the third path of the first stage. A diode D19 of the second stage S4 provides a second diode-drop between the connections CS, C7. Preferably, the first stage additionally has the impedance Z7a (Figure 38B), such as the diode D37a (Figure 3 8A), providing a diode-drop between the second path (RlOa, D18, RlOb, Z5 (or D5a, DSb), R9b, D17, R9a) and the third path (R7, D13, D14, T7, R9b, D17, R9a) which counteracts the second diode-drop of the diode D19 of the second stage.The second diode-drop D19 of the second stage S4 is in the first section P1 of its path, and the diode-drop of the impedance Z7a (Figure 38B) or diode D37a (Figure 38A) of the first stage S1 to S3 is in the second section P2 of its path.

In the circuits of Figures 38A and 38B, the values of the resistors are preferably chosen such that: R5 = R7; RlOa = RlOb; R9a = R9b; and RT10 = R16.

It will also be noted that, like Figures 40A and 40B, the second path (RlOa, D18, RlOb, ZS (or DSa, DSb), R9b, D17, R9a) of the first stage S1 to S3 includes the impedance ZS (Figure 38A) or diodes D5a, DSb (Figure 38A), which may be as described above with reference to Figures 11 to 17, the mid-point LTm being at the half-way level between the supply rail V+ and the supply rail V-.

In the circuits of Figures 39A, 39B, 40A and 40B, the node LTm may be connected via a feedback path to the output Vout. In the circuits of Figures 38A and 38B, the node LTm may be connected by an impedance Z6 to ground E, as described above with reference to Figures 15 to 17, and the input VinB may be connected to the output Vout by a feedback circuit. In the circuits of Figures 38A to 40B, additional diodes may be included in each section P1 to P4 of each path between the supply rails so that each section contains two or more diode drops and one large swing voltage drop, as described for example with reference to Figure 26.

Figure 76 shows a further amplifier circuit having a complementary differential input stage S1 to S3, which is similar in many respects to the differential input stages described above, having inputs VinA, VinB, transistors T5 to T8, resistors R5 to R8, R9a, R9b, RlOa, RlOb and diodes D9 to D18. The circuit also includes a voltage amplification stage S4, which comprises, in order from the supply rail V+ to the supply rail V-, resistor R15 and diode D19, and then in parallel, on the one hand, the emitter to collector of transistor T9 and the collector to emitter of transistor T10 and, on the other hand, the emitter to collector of transistor T9A and the collector to emitter of transistor T1OA, and then diode D20 and resistor R16. The point between the cathode of diode D19 and the emitters of transistors T9, T9A is connected by connection CS to the point between the cathode of diode D18 and the resistor RlOb, and similarly the point between the anode of diode D20 and the emitters of transistors T10, T1OA is connected by connection C6 to the point between the anode of diode D17 and the resistor R9b. The base of transistor T9 is connected by connection Cl to the point between the cathode of diode D10 and the collector of transistor TS, and similarly the base of transistor T10 is connected by connection C2 to the point between the anode of diode D11 and the collector of transistor T6.The base of transistor T9A is connected by connection CiA to the point between the cathode of diode D14 and the collector of transistor T7, and similarly the base of transistor TlOA is connected by connection C2A to the point between the anode of diode D1S and the collector of transistor T8. The output Vout is taken from the point between the collectors of the transistors T9, T10. The point Cli between the collectors of the transistors T9A, T1OA provides a node which may be connected possibly via a resistor to earth and possibly via a network to the feedback input VinB.

It should be noted therefore that the connections C1, CiA between the input stage S1 to S3 and the stage S4 are at the same physical level of the amplifier and at the same voltage level at the quiescent operating point, and similarly that the connections C2, C2A between the input stage S1 to S3 and the stage S4 are at the same physical level of the amplifier and at the same voltage level at the quiescent operating point.

Furthermore, the connection C5 is one diode drop above the level of the connections C1, C1A, and similarly the connection C6 is one diode drop below the level of the connections C2, C2A. In order to compensate for the loading placed by the base-emitter junction of transistor T9 on the input stage S1 to S3, a compensation diode D37b is connected between the connection C5 and the junction of the anode of the diode D9 with the resistor R5.Similarly, in order to compensate for the loading placed by the base-emitter junctions of transistors (a) T9A, (b) T10, (c) T1OA on the input stage S1 to S3, compensation diodes (a) D37S, (b) D38b, (c) D38S, respectively, are connected between the connections (a) C5, (b) C6, (c) C6, respectively, and the junctions of (a) the anode of the diode D13 with the resistor R7, (b) the cathode of the diode D12 with the resistor R6, (c) the cathode of the diode D16 with the resistor R8, respectively.

The circuit of Figure 76 may be modified so that the connections C1, Cl A are at a different level, but the same level as each other, and similarly so that the connections C2, C2A are at a different level, but the same level as each other.

Loads in Differential Amplifier There now follows a description of how the loads in the differential amplifier may be modified. Figure 41 shows a first modified form of the circuit of Figure 32, in which each of the diodes D9a, D12b, D13a, D16b in the loads and the resistors R5 to R8 connected thereto are replaced by a respective transistor T17 to T20 having its emitter connected to the respective supply line V+, V-, its collector connected to the remaining diodes D9b, D12a, D13b, D16a, and its base connected to the respective supply line V+, V- via the diode D18a, D17b in the long-tail of the complementary part of the amplifier. Each of these transistors T17 to T20 is therefore biased on by the diode drop across the respective diode D18a, D17b.This modification has the effect of reducing the number of resistors and diodes in the circuit by four, at the expense of four additional transistors. However, it is believed that thermal stability of the circuit is improved in this way. Even with this modification, any signal path between the supply rails V+, V- of the amplifier can still be divided into four sections each containing two diode-drop elements and one large-swing element.

Figures 42A and 43A show the parts of the circuit of Figure 41 containing the transistors T17 to T20 and the diodes D18a, D17b. These parts of the circuit may be modified as shown in Figures 42C and 43C to include impedances Z9a, Z9b in place of the diodes 18a, 17b, respectively. The values of the impedances Z9a, Z9b are preferably each chosen so that, during quiescent operation of the circuit, about one diode drop is produced across each impedance. The impedances Z9a, Z9b may, in particular, be provided by respective resistors RD18a, RD17b, as shown in Figures 42D and 43D.As shown in Figures 42B and 43B, the diodes D9a, D13a, D12b, D16b of Figure 36 may be expressed or analysed as respective resistors RD9a, RD13a, RD12b, RD16b, across each of which there is a diode drop at the quiescent operating point, the values of resistors RD9a, RD13a, RD12b, RD16b therefore each preferably being twice the value of each of the resistors RD18a, RD17b.

Long-tails in Differential Amplifier Figure 44 shows another modified form of the circuit of Figure 32, in which the diode D18a and the resistor RlOa in the upper long-tail of the circuit of Figure 27 are replaced by a transistor T2l having its emitter connected to the supply rail V+, its collector connected to the remainder of the longtail and its base connected to the two diodes D9a, D13a connected to the supply rail V+ in the loads of the complementary part of the circuit. The transistor T2 1 is therefore biased on by the diode drop across the diodes D9a, D13a. A similar modification is made to the complementary part of the circuit with the inclusion of transistor T22.These modifications have the effect of reducing the number of resistors in the circuit by two, and reducing the number of diodes by two, at the expense of two additional transistors. However, it is believed that thermal stability of the circuit is also improved in this way. Even with this modification, any signal path between the supply rails V+, V- of the amplifier can still be divided into four sections each containing two diode-drop elements and one largeswing element.

Figures 45A and 46A show the parts of the circuit of Figure 44 containing the transistors T21, T22 and the diodes D9a, D13a, D12b, D16b. These parts of the circuit may be modified as shown in Figures 42C and 43C to include impedances ZlOa to ZlOd in place of the diodes D9a, D13a, D12b, D16b, respectively. The values of the impedances ZlOa to ZlOd are preferably each chosen so that, during quiescent operation of the circuit, about one diode drop is produced across each impedance. The impedances ZlOa to ZlOd may, in particular, be provided by respective resistors RD9a, RD13a, RD12b, RD16b, as shown in Figures 45D and 46D.Also, as shown in Figures 42B and 43B, the diodes D18a, D17b of Figure 36 may be expressed or analysed as respective resistors RD18a, RD17b, across each of which there is a diode drop at the quiescent operating point, the values of resistors RD18a, RD17b therefore each being half the value of each of the resistors RD9a, RD13a, RD12b, RD16b.

It will be appreciated that these modifications are not exclusively applicable to the circuit of Figure 36, and may be applied to the other circuits described herein and also to other amplifier circuits.

Input Biasing of Differential Amplifier Figures lOA, 11 and 15 show resistors R11 and R12 connecting the inputs VinA, VinB of the differential amplifier to ground E, and for simplicity input biasing elements have not been shown in the drawings of the other differential amplifiers described above. Other biasing arrangements will now be described with reference to Figures 47 to SO.

In Figure 47, a series-connected chain of resistor R25, diode D39, resistor R26, diodes D40, D41, resistor R27, diode D42 and resistor R28 extends in that order from the supply rail V+ to the supply rail V-, with the diodes D39 to D42 arranged to be forward conducting, to form a potential divider. The mid-point of the chain, between the cathode of diode D40 and the anode of diode D41, provides the input VinA and is connected to the bases of the transistors T5, T6. A similar arrangement comprising resistor R29, diode D43, resistor R30, diodes D44, D45, resistor R31, diode D46 and resistor R32 is provided, as shown, for the other input VinB of the differential amplifier.

A modification may be made to the arrangement of Figure 47, as shown in Figure 48, to include two diode-drops and one large voltage swing in each section S1 to S4. Thus, in Figure 48, each of the diodes D39 to D46 of Figure 47 is replaced by a pair of series-connected forwardconducting diodes D39a to D46a, D39b to D46b.

In a modification to the circuit of Figure 48, the potential of each of the transistors T5, T7 is raised by one diode-drop, and the potential of each of the transistors T6, T8 is lowered by one diodedrop, as shown in Figure 49. Specifically, by comparison with Figure 48, the bases of the transistors T5, T7 are connected instead to the anodes of diodes D40b, D44b, respectively, and the bases of the transistors T6, T8 are connected instead to the cathodes of diodes D41a, D45a, respectively. Also, the diodes DlOb, Dl ia, D14b, D15a are omitted, but additional forward-conducting diodes D47, D49 are connected between the emitters of transistors T5, T7 and the long-tail node LT1, and additional forward-conducting diodes D48, D50 are connected between the emitters of transistors T6, T8 and the long-tail node LT2.

A modification to the circuit of Figure 49 is shown in Figure 50. In Figure 50, the diodes D47 to D50 are omitted, and, instead, the emitters of transistors TS to T8 are directly connected to the node LTm between the diodes DSa, DSb.

Developments to Coupling and Compensation Techniques Figure 51 shows a development to the compensation technique shown in Figure 32. As shown, the resistor RlOa, diode D18b and compensation diode D37b are replaced by a transistor T23 having its collector connected to the cathode of diode D18a, its base connected to the point between the resistor R5 and the anode of diode D9b, and its emitter connected to the connection CS. A complementary transistor T26 replaces the complementary resistor R9a, diode D17a and compensating diode D38b.Also, the resistor RlOb, diode Dl8c and compensation diode D37a are replaced by a transistor T24 having its collector connected to the long-tail node LT2, its base connected to the point between the cathode of diode D14a and the anode of diode D14b, and its emitter connected to the connection C5. A complementary transistor T25 replaces the complementary resistor R9b, diode Dl7c and compensating diode D38a.

Figure 52 shows a development to the technique shown in Figure 32 of coupling the first stage voltage amplifier to the differential amplifier. As shown in Figure 52, the resistor R19 and diode D23b of Figures 28 to 30 are replaced by a transistor T27 having its collector connected to the cathode of diode D23a, its base connected by connection C7 to the point between resistor R7 and the anode of diode D13b, and its emitter connected the connection CS and the emitter of transistor T9. A complementary transistor T28 replaces the diode D24a and the resistor R20. It will therefore be seen that, not only does the first stage S4 of voltage amplification have symmetrical halves, but also each half has symmetrical quarters.If the stage S4 is the final stage, then the output Vout is taken from the point between the cathode of diode D27 and the anode of diode D28.

Figure 52 also shows a modified second stage S5 of voltage amplification comprising, from the supply rail V+ to the supply rail V-, the emitter to collector of transistor T29, diodes D51, D52, the collector to emitter of transistor T30, the emitter to collector of transistor T31, diodes D53, D54, and the collector to emitter of transistor T32. The base of transistor T29 is connected to the point between the cathode of diode D23a and the collector of transistor T27, and the base of transistor T30 is connected to the point between the collector of transistor T9 and the anode of diode D27. The complementary part of the stage S5 is connected to the stage S4 in a complementary fashion.Also, the point between the emitters of transistors T30, T3 1 is connected by connection Cii to the point between the cathode of diode D27 and the anode of diode D28. If the stage S5 is the final stage, then the connection Cli may provide the output Vout. It will be noted that, like the stage S4, not only does the second stage S5 of voltage amplification have symmetrical halves, but also each half has symmetrical quarters.

Figure 52 furthermore shows a modified stage S6 of current amplification comprising, from the supply rail V+ to the supply rail V-, diode D31a, the collector to emitter of transistor T33, the emitter to collector of transistor T13, diode D33, diode D34, the collector to emitter of transistor T14, the emitter to collector of transistor T34, diode D32b. The bases of transistors T33, T13, T14, T34 are connected to the collectors of transistors T29, T30, T3l, T32, respectively, in the preceding stage S5.The output Vout is taken from the point between the cathode of diode D33 and the anode of diode D34, and this point may, if required, be connected to the connection Cli. It will be noted that, like the stages S4 and S5, not only does this stage S6 of current amplification have symmetrical halves, but also each half has symmetrical quarters.

Figure 53 shows a differential amplifier with a first voltage amplification stage employing the developments described with reference to Figure 51 and Figure 52, that is employing the transistors T23 to T28, and also employing the developments described with reference to Figures 41 to 43, that is employing the transistors T17 to T20. It should be noted that all of the elements of the circuit of Figure 53, other than the interconnections, are provided by semiconductors, that is diodes and transistors, and that there are no resistors, and therefore very low levels of noise can be expected in such a circuit.However, in order to limit the currents flowing in the circuit, it may be required to include some resistive elements, for example by replacing the diodes D18a, D17b by resistors RD18a, RD17b, respectively, as described above with reference to Figures 42D and 43D, and/or by replacing diodes D23a, D24b and/or diodes D27, D28 with resistors of equivalent value at the quiescent operating point.

Current Amp lifi cation Stages Figure 54 shows modifications to the current amplification stages S6, S7 of the amplifier shown in Figure 36. In Figure 54, the elements of the stage S6 are arranged in order from the supply rail V+ to the supply rail V- as: diode D3 1 a, collector to emitter of transistor T13, diode D31b, resistor R21, diodes D33, D34, resistor R22, diode D32a, emitter to collector of transistor T14, and diode D32b. The bases of the transistors T13, T14 are connected to the emitters of the transistors Tri 1, T12 of the preceding stage S5.The elements of the stage S7 are arranged in order from the supply rail V+ to the supply rail V- as: diodes D25a, D2Sb, collector to emitter of transistor T15, resistor R17, diodes D35, D36, resistor R18, emitter to collector of transistor T16, and diodes D26a, D26b. The bases of the transistors T15, T16 are connected to the emitters of the transistors T13, T14 of the preceding stage S6, and the output Vout between the diodes D35, D36 is connected to the point between the diodes D33, D34 of the stage S6.The output Vout may, or may not, as required, also be connected to the connection Cii between the diodes D2 1 b, D22a of the stage S5 and between the diodes D27, D28 of the stage S4.

Alternative Coupling of All Stages In the differential amplifiers described above, the outputs VoutA, VoutB have been taken by the connections C1, C2 lying in the sections P2, P3 of the circuit. There now follows a description with reference to Figure 55 of a modification of the circuit of Figure 36 in which the outputs VoutA, VoutB from the differential amplifier are taken by connections C1*, C2* lying in the sections P1, P4 of the circuit.By comparison with Figure 36, in Figure 55: the elements in sections 1 and 2 of the stages S4 to S7 have been mirror-imaged about the line between sections 1 and 2 and have been denoted with an asterisk (*); the connections C1, C7, C9 in sections 1 and 2 have been mirror-imaged about the line between sections 1 and 2 to form connections C1*, C7*, C9*, respectively; the compensation diodes D37a, D37b in sections 1 and 2 have been mirror-imaged about the line between sections 1 and 2 to form compensations diodes D37a*, D37b*, respectively; the elements in sections 3 and 4 of the stages S4 to S7 have been mirror-imaged about the line between sections 3 and 4 and have been denoted with an asterisk (*); the connections C2, C8, C10 in sections 3 and 4 have been mirror-imaged about the line between sections 3 and 4 to form connections C2*, CS*, C10*, respectively; the compensation diodes D38a, D38b in sections 3 and 4 have been mirror-imaged about the line between sections 3 and 4 to form compensation diodes D38a*, D38b*, respectively; and the polarities of the transistors and diodes which have been mirror-imaged have been reversed.

The output Vout of the circuit is taken from the point between diodes D25a*, D26b*, D31a*, D32b*, and this may or may not, as required, be connected to the connection Cii between the diodes D29*, D30*, D23a*, D24b*.

A similar transformation may be made to the other circuits described above.

Output Loading and Feedback Figure 56 shows an amplifier of the type described above with a differential input stage S1 to S3 followed by two stages S4, S5 of voltage amplification. Figure 56 also shows a feedback circuit comprising series-connected resistors R34, RB1 connected between the output Vout (in this section now referred to as the "preliminary output Vout") of the stages S4, S5 and the input VinB of the differential amplifier Si to S3, and a resistor R33 connected between the junction of the resistors R34, RB1 and ground E. Figure 56 also shows an output loading circuit comprising a resistor R35 connected between the preliminary output Vout and an output Vout* (in this section now referred to as the final output Vout*") of the whole circuit, and a resistor R36 connected between the final output Vout* and ground E. Furthermore, a resistor RA1 is connected between the input VinA and the bases of the transistors T5, T6.

The circuit is biased such that, in quiescent operation with the input VinA of the differential amplifier S1 to S3 grounded, the voltages at the emitters of the transistors Till, T12 of stage S5 are about 2/3 of the respective power supply voltages. For example, if V+ = 60V and V- = -60V, the transistors are biased so that Ve(Ti 1) = 40V and Ve(T12) = 40V.

The values of the resistors R35, R33 are chosen so that each is greater than or about equal to the effective resistance of the base-emitter junctions of the transistors T9, Toll, or T10, T12 during quiescent operation. For example, if the values of resistors R15 and R19 in parallel are chosen to be lkfl (and if the current through resistor R7 is negligible by comparison), then the emitter current, which is approximately equal to the collector current, of each transistor T9, Tri 1 will be lOmA, and if the base-emitter voltage of each of the transistors is 0.667V, then the effective resistance of each base-emitter junction is 66.7n. The base-emitter junctions of the transistors T9, T11 act in parallel, and so their combined resistance is 333.30. Therefore, the values of the resistors R35, R33 are each preferably chosen to be generally equal to or greater than 33.3n, and more preferably twice that amount, that is 66.7Q.

The values of the resistors R34, R36 are each chosen to be twice the internal resistance of the output stage between the preliminary output Vout and the supply rails V +, V-. In the example given, the output resistance due to the upper half of stages S4, S5 is 60V / 20mA = 3kn and similarly due to the upper half of stages S4, S5 is 3kin. These act in parallel between the preliminary output Vout and the supply rails V+, V-, and therefore the resultant internal resistance is 1.5kin. Accordingly, the values of resistors R34, R36 are each preferably chosen to be equal to 3k0.

The value of the resistor RB1 is chosen to be approximately one half of the values of the resistors R33, R35, that is to have a value of about 33.31) in the above example. The value of the resistor RAl is chosen to be approximately equal to the values of the resistors R33, R35, that is to have a value of about 66.7n in the above example.

It should be noted from classical amplifier theory that the voltage gain G of the amplifier up to the preliminary output Vout, assuming that the gain without feedback is high, is given by: G = Vout / VinA = ( R33 + R34) / R33. Therefore, with the example values given above, and if R33 = 33.3n, the gain G = 3033.3n/33.3n = 91. Preferably, more feedback is applied than would be produced by these example values. In particular, the values of the resistors R35, R33 are each preferably twice the value of the effective resistance of the base-emitter junction, and therefore R33 = 66.70. In this case, the gain is reduced by about 50% to 3066.7n/66.7n = 46.

Figure 57 shows a circuit similar to that of Figure 56, but with two diode drops and one large swing voltage drop in each section P1 to P4. The values of the supply voltages, emitter voltages of transistors Til, T12, and resistors R1S, R16, R33 to R36 are the same as for Figure 56.

Quiescent Operating Point The circuits described above are preferably biased such that, in quiescent operation with the input VinA of the differential amplifier S1 to S3 grounded, the voltages at the levels between the sections P1, P2 and between the sections P3, P4 are about 2/3 of the respective power supply voltages.

For example, if V+ = 60V and V- = -60V, the voltage at the level between the sections P1, P2 is preferably about 40V, and the voltage at the level between the sections P3, P4 transistors is preferably about 90V.

Also, in all of the circuits described above, the components of the circuits are preferably arranged so that, under normal quiescent operating conditions withpout an input signal, and more preferably throughout the whole normal operating range, the transistors and diodes are all in their active regions of operation, without moving into their saturation or cut-off regions, and in their safe operating areas. Also, the ohmic values and electrical specifications of the comnponents are deduced under these conditions. Components which are chosen which are not under-rated and not excessively over-rated for the intended purpose.

More specifically, and referring for example to Figure 56, in the case where the supply voltages V+, V- are 40V and 40V, respectively, and R15 = R16 = R19 = R20 = 2k#, the currents I4, I5 along each of the paths S4 and S5 are approximately I4 = I5 = 1/3.40V/R1S = 6.67mA. If, at the quiescent operating point, the emitter-base voltage drops Veb9 of the transistors T9 to T12 are each Veb9 = 0.667V, the effective resistances Reff9 of those emitter-base junctions are each about Reff9 = Veb9/I4 = loon. The resistors R33, R35 and RA1 are each chosen to have approximately that value Reff9, i.e.R33 = R35 = RAl = 100n, and the resistor RBI is chosen to have half of that value, l/2Reff9, i.e. RB1 = son. The internal resistance Ri of the output stages S4, SS between the output Vout and the supply rails V+, V- is about Ri = '4.40V/I4 = l.Skfl, and therefore the resistors R34, R36 are each chosen to have about twice that value, 2.Ri, i.e. R34 = R36 = 3kQ.At the quiescent operating point, the effective resistance Z5 of the diodes D5a, D5b in series is preferably chosen to be about equal to the effective resistance of the resistors R15, R19 in parallel, or R16, R20 in parallel, and therefore Z5 = l/2.R15 = ikfl. As described above, the effective resistance ReffS of the base-emitter junction of each of the transistors T5 to T8 is preferably about equal to the effective resistance Z5 of the diodes D5a, D5b in series, and therefore ReffS = Z5 = ikll approximately.If, at the quiescent operating point, the emitter-base voltage drop VebS of each of the transistors T5 to T8 is VebS = 0.667V, then the collector current I1 in each of the transistors T5 to T8 is I1 = VebS/ReffS = 0.667mA approximately. Given that the voltage between the sections P1, P2 is to be 2/3.40V = 26.67V, the voltage across each of the resistors R5 to RS is 40V - 26.67V = 13.33V approximately, and therefore each of the resistors RS to R8 is chosen to have a value of 13.33V/I1 = 20k# approximately. The values of the resistors R9a, R9b, RlOa, RlOb are each chosen to be approximately equal to half of the value of each of the resistors R5 to R8, and therefore R9a = R9b = RlOa = RlOb = l/2.R5 = lOkQ. Thus, in a preferred example of the circuit of Figure 56, the values of the resistors and the effective resistances of the base-emitter junctions of the transistors at the quiescent operating point are as follows:: R5 to R8 20k# R35 lOOQ R9a to RlOb lOkQ R36 3kQ R15, R16 2kQ RA1 lOOQ R19, R20 2kn RB1 50# R33 loon Reb(TS to T8) ikn R34 3kû Reb(T9 to T12) ioon It may be usefully noted that in the above selection of values, the square root of the product of the value of each resistor RS to R8 and the effective value of the effective resistances (at the quiescent operating point) of the base-emitter junctions of the transistors T9, T11 (or T10, T12) in parallel is equal to the effective value of the resistors R1S, R19 (or R16, R20) in parallel, that is (RS.[Reff9//Reff11]) = [R15//Rl9], or (20k#..100#) = .2k# = 1k#. As mentioned above, the gain G of such a circuit is approximately (R33 + R34) / R33, and therefore for this example is 31.

It may also be noted that the ratio of the value of the resistor RS (or R6, R7 or R8) to the effective resistance (at the quiescent operating point) of the base-emitter junction of the transistor T5 (or T6, T7 or T8) is equal to the ratio of the value of the resistor R19 (or R15, R20 or R16) to the effective resistance (at the quiescent operating point) of the base-emitter junction of the transistor T9 (or T11, T10 or T12), that is RS/Reb(TS) = 20k0/1kQ = R19/Reb(T9) = 2k#/100# = 20. It may furthermore be noted that the effective resistance Z5 of the diodes DSa, D5b in series (at the quiescent operating point) is equal to the value of the resistors R19 and R15 (or R20 and R16) in parallel.

In the case where the power supply voltages are increased to f60V, and the values of R15, R16, R19, R20 remain at 2kn, then applying the above analysis, I4 = I5 = 1/3.60V/R15 = lOrnA, approximately. In this case, if the emitter-base voltage drops Veb9 of the transistors T9 to T12 (at the quiescent operating point) remain at Veb9 = 0.667V, Reff9 = Veb9/14 = 66.7n, approximately.

Therefore, R33 = R35 = RA1 = Reff9= 66.7n, and RB1 = Reff9 = 33.3n, approximately. Ri = 4/4.60V/14 = 1.5k# approximately, and therefore, R34 = R36 = 2.Ri = 3kQ approximately. At the quiescent operating point, ZS = 1/2.RlS = Ikn approximately. ReffS = Z5 = ikn approximately.

If, at the quiescent operating point, VebS = 0.667V, then I1 = VebS/ReffS = 0.667mA approximately. Given that the voltage between the sections P1, P2 is to be 213.60V = 40V, the voltage across each of the resistors R5 to R8 is 60V - 40V = 20V approximately, and therefore R5 = R6 = R7 = RS = 20V/I1 = 30k# approximately. R9a = R9b = RlOa = RlOb = '/2.R5 = 15k#. Thus, in a preferred example of the circuit of Figure 56 with supply voltages of +60V, the values of the resistors and the effective resistances of the base-emitter junctions of the transistors at the quiescent operating point are as follows:: RS to R8 30kQ R35 66.?n R9a to RlOb 15k# R36 3kn R1S, R16 2kn RA1 66.7S2 R19, R20 2k# RB1 33.3n R33 66.?n Reb(T5 to T8) 1k# R34 3kfl Reb(T9 to T12) 66.?n In this case, the gain G of the circuit has increased to approximately (R33 + R34) / R33 = 46, as compared with the gain of 31 with supply voltages of i40V. In other words, with voltage amplifying circuits of this type, the gain G is dependent on the supply voltage V+, V-. Again it may be noted that (RS.[Reff9//Reffli])' = [RlS//Rl9] ikn.

Alternative Input Stage There now follows a description of a number of circuits which have an alternative input stage S1 to S3. The amplifier of Figure 58 has a complementary non-differential input stage S1, S2 having transistors TS, T6, diodes D9, D12, D17, D18, D5a, D5b, D55, D56 and resistors R5a, R5b, R6a, R6b, R9a, R9b, RlOa, RlOb. The input stage has an input Vin, and a node LTm which may be used as a feedback input. The diodes D9, D12, D17, D18 provide the symmetry feature discussed above.

The resistors R9b, RlOb and diodes DSa, DSb link together the long-tail nodes LT1, LT2 in the manner discussed above. The input stage S1, S2 has complementary outputs VoutA, VoutB, which are connected to the next stage S4 by connections C1, C2. Additional connections CS, C6 are also provided in the manner described above. Importantly, they are at the level between the first and second quarters P1, P2 and at the level between the third and fourth quarters P3, P4 of the circuit. The diodes D37b, D38b are included in the first stage S1, S2 to compensate for the diode-drop loading on the outputs of the first stage S1, S2 provided by diodes D23, D24 in the stage S4. Further stages S5, S6 are provided, similar to those described with reference to Figure 25, which together provide a complementary Sziklai-like configuration.

In the circuit of Figure 58, the preferred relationship between the resistance values is: RSa = R6b; R5b = R6a; R9a = RlOa; R9b = RlOb; Rl9 = R20; R15 = R16; and R17 = R18.

Figure 59 is similar to Figure 58, except that: the first stage S1 to S3 is symmetrical as between the primary input VinA and a feedback input VinB; additional diodes have been included in each section P1 to P4 so that there are two diode-type drops and one large voltage swing drop in each section P1 to P4; and further stages after stage S4 have not been shown for simplicity - however further stages such a stages S5 to S7 may be added, as described above. Thus, the circuit of Figure 59 includes additional transistors T7, T8, diodes D13a, D13b, D14, D1S, D16a, D16b, D37a, D38a, DS7, D58, D10, Dull, D17c, DiSc and resistors R7a, R7b, R8a, R8b.Also, the diodes D9, D12, D17, D18 of Figure 58 have been replaced by pairs of diodes suffixed "a" and ib". Furthermore, additional connections C7, C8 are made as shown. The diodes D37a, D38a are also compensation diodes.

In the circuit of Figure 59, the preferred relationship between the resistance values is as for the Figure 58 circuit, and in addition R5a = R6b = R7a= R8b; R5b = R6a = R7b = R8a.

The circuit of Figure 60 is similar to that of Figure 59, except that the resistor Rl9 and diode D23b have been replaced by a transistor T27, and the resistor R20 and diode D24a have been replaced by a transistor T28, in the manner described above with reference to Figure 52.

The circuit of Figure 61 is similar to that of Figure 59, except: the outputs VoutA, VoutB of the differential amplifier S1 to S3 are taken, as connections C1*, C2*, from the other ends of resistors R5b, R6a, respectively; the transistor T9 and diode D27 are interchanged, and the transistor T10 and diode D28 are interchanged; resistors R37a, R37b, R38a, R38b are included in series with the compensation diodes D37a, D37b, D38a, D38b, respectively; the compensation diodes and resistors are connected to the opposite ends of resistors RSa, R6b., R7b, R8a, respectively; and the connections C7, C8 are replaced by connections C7*, CS* at the opposite ends of the resistors R7a, R19; RSb, R20, respectively.

The circuit of Figure 62 is similar to that of Figure 59, except that a mirror image transformation has been made of some of the circuit elements, in the manner described above with reference to Figure 55.

The circuit of Figure 63 is similar to that of Figure 59, except that three further stages S5 to S7 of amplification have added, as described above with reference to Figure 26. It will be appreciated that the circuit of Figure 63 may be further modified with a mirror image transformation, in a similar manner to the transformation of the circuit of Figure 59 to form the circuit of Figure 62.

Further Circuits Referring now to Figures 64 to 72D, examples of a number of compound transistor circuits will be described. In each of these circuits (and in many of the other circuits described in this specification), there is a first node N1 and a second node N2, with a pair of paths between the nodes N1, N2, shown in the drawings to the left and the right and hereinafter referred to as "the left path" and "the right path". All of the circuits include at least one transistor, having its collector-emitter in the left path (see TA, TA1, TA2) or in the right path (see TB, TB1, TB2).In all of the circuits, the left and right paths each contain the same number of diode drops between the first and second nodes N1, N2, and each contain the same number of large swing voltage drops between the first and second nodes N1, N2. In all of the circuits, at least one of the transistors is a bridging transistor having its base connected at a bridging node N3, N3a, N3b in the other of the paths to the one containing its collector-emitter, and this node N3 N3a, N3b may be accessible to other circuitry or for testing purposes. In all of the circuits, the left and right paths each contain the same number of diode drops between the node N1 and the node N3, N3a or N3b, and each contain the same number of large swing voltage drops between the node N1 and the node N3, N3a or N3b.In some of the circuits, at least one of the transistors is a non-bridging transistor with its base providing a node N4, N4a, N4b, N4c which is accessible to other circuitry or for testing purposes. In all of the circuits, the other nodes N5, N5a to N5f of the circuit may, if required, be accessible to other circuitry or for testing purposes. The accessibility of the nodes enables the voltages of the nodes to be measured and the currents flowing in the components of the circuit to be determined by applying Kirchhoff's and Ohm's laws.

Accordingly, tight control may be exercised over the components of the circuit and the operation of the circuit.

Referring specifically to the circuit of Figure 64, from the first to second nodes N1, N2, the left path contains, in order, the collector-emitter of NPN non-bridging transistor TA and a forwardconducting diode DA, and the right path contains, in order, a forward-conducting diode DB and the collector-emitter of NPN bridging transistor TB. The bridging node N3 of the transistor TB connects to the emitter of the transistor TA and the anode of the diode DA. The circuit therefore corresponds to that of Figure 6.

In the circuit of Figure 65, from the first to second nodes N1, N2, the left path contains a forward-conducting diode DA and the collector-emitter of NPN non-bridging transistor TA, and the right path contains the emitter-collector of PNP bridging transistor TB and a forward-conducting diode DB. The bridging node N3 of the transistor TB connects to the collector of the transistor TA and the cathode of the diode DA. The circuit therefore corresponds to that of Figure 8.

In the circuits of Figures 64 and 65, there are two diode drops and one large swing voltage drop in each of the left and right paths between the first and second nodes N1, N2. In the circuit of Figure 64, there are one diode drop and one large swing voltage drop in each of the left and right paths between the first node N1 and the bridging node N3. In the circuit of Figure 65, there are one diode drop and no large swing voltage drops in each of the left and right paths between the first node Ni and the bridging node N3.

In the circuit of Figure 66, from the first to second nodes N1, N2, the left path contains a resistor RA and the collector-emitter of NPN non-bridging transistor TA, and the right path contains the collector-emitter of NPN bridging transistor TB and a resistor RB. The bridging node N3 of the transistor TB connects to the collector of the transistor TA and the lower end of the resistor RA.

In the circuit of Figure 67, from the first to second nodes N1, N2, the left path contains the collector-emitter of NPN non-bridging transistor TA and a resistor RA, and the right path contains a resistor RB and the emitter-collector of PNP bridging transistor TB. The bridging node N3 of the transistor TB connects to the emitter of the transistor TA and upper end of the resistor RA.

In the circuits of Figures 66 and 67, there are one diode drop and two large swing voltage drops in each of the left and right paths between the first and second nodes N1, N2. In the circuit of Figure 66, there are no diode drops and one large swing voltage drop in each of the left and right paths between the first node N1 and the bridging node N3. In the circuit of Figure 67, there are one diode drop and one large swing voltage drop in each of the left and right paths between the first node N1 and the bridging node N3.

In the circuits of Figures 68A to 71E, there are two diode drops and two large swing voltage drops in each of the left and right paths between the first and second nodes N1, N2. Furthermore, each of the left and right paths can be considered as two halves, one above the other, each containing one diode drop and one large swing voltage drop.

Specifically, in the circuit of Figure 68A, from the first to second nodes N1, N2, the left path contains, in order, the emitter-collector of PNP non-bridging transistor TA, forward-conducting diode DA, and a resistor RA. The right path contains, in order, a forward-conducting diode DB, the collector-emitter of NPN bridging transistor TB, and a resistor RB. The bridging node N3 of the transistor TB connects to the collector of the transistor TA and the anode of the diode DA.

In the circuit of Figure 68B, from the first to second nodes N1, N2, the left path contains, in order, forward-conducting diode DA, the collector-emitter of NPN non-bridging transistor TA, and a resistor RA. The right path contains, in order, the emitter-collector of PNP bridging transistor TB, a resistor RB and a forward-conducting diode DB. The bridging node N3 of the transistor TB connects to the collector of the transistor TA and the cathode of the diode DA.

Figure 68C shows a modification of Figure 68B in which the right path contains the diode DB, the resistor RB and the emittercollector of the PNP bridging transistor TB. In this case, the bridging node N3 of the transistor TB connects to the emitter of the transistor TA and the upper end of the resistor RA.

Figure 68D shows a modification of the circuit of Figure 68C in which the transistor TA is a bridging transistor providing a second bridging node N3b connected to the lower end of the resistor RB and to the emitter of transistor TB.

In the circuits of Figures 68B to 68D, the positions of the diode DB and resistor RB may be interchanged.

Figure 68E shows a modification to the circuit of Figure 68D in which the bridging transistors TA, TB of Figure 68D become non-bridging transistors TA, TB2 in Figure 68E, and in which the diode DB and resistor RB of Figure 68D are replaced in Figure 68E by a PNP bridging transistor TB1 having its emitter connected to the node N1 and providing a bridging node N3 connected to the cathode of diode DA and the collector of transistor TA.

In the circuit of Figure 69A, from the first to second nodes N1, N2, the left path contains, in order, the emitter-collector of PNP non-bridging transistor TA, the forward-conducting diode DA and the resistor RA, and the right path contains, in order, the resistor RB, the diode DB and the emittercollector of PNP bridging transistor TB providing a bridging node N3 connected to the cathode of the diode DA and the upper end of the resistor RA.

Figure 69B shows a modification of the circuit of Figure 69A in which the positions of the diode DB and the resistor RB are interchanged, and the PNP non-bridging transistor TA of Figure 69A is replaced by an NPN non-bridging transistor TA in Figure 69B having its collector connected to node N1.

In the circuit of Figure 69C, the left path contains NPN bridging transistor TA, diode DA and resistor RA. The right path contains resistor RB, PNP bridging transistor TB and diode DB.

In Figure 69D, the left path contains NPN non-bridging transistor TA, diode DA and resistor RA. The right path contains diode DB, NPN bridging transistor TB and resistor RB. In a modification of the circuit of Figure 69D, the positions of the diode DA and resistor RA may be interchanged.

In Figure 69E, the left path contains NPN non-bridging transistor TA1 and PNP bridging transistor TA2. The right path contains diode DB, NPN non-bridging transistor TB and resistor RB.

In Figure 69F, the left path is the same as in Figure 69E. However, the right path contains diode DB1, resistor RB1, diode DB2 and resistor RB2. In a modification of the circuit of Figure 69F, the positions of the diode DB1 and resistor RB1 may be interchanged.

Figure 69G shows a further modification to the circuit of Figure 69F, in which the transistor TA2, diode DB2 and resistor RB2 are mirror imaged about the node N3 and the polarities of the diode DB2 and transistor TA2 are reversed. In a modification of the circuit of Figure 69G, the positions of the diode DB1 and resistor RBl may be interchanged.

In the circuit of Figure 70A, from the first to second nodes N1, N2, the left path contains, in order, diode DA, the resistor RA and NPN bridging transistor TA, and the right path contains, in order, PNP bridging transistor TB, resistor RB and diode DB.

In the circuit of Figure 70B, from the first to second nodes N1, N2, the left path contains, in order, diode DA, the resistor RA and NPN non-bridging transistor TA, and the right path contains, in order, resistor RB, PNP bridging transistor TB and diode DB. In a modification of the circuit of Figure 70B, the positions of the diode DA and resistor RA may be interchanged.

Figure 70C shows a modification of the circuit of Figure 70B in which the right path is mirrorimaged about the bridging node N3, and the polarities of the diode DB and transistor TB are reversed.

In a modification of the circuit of Figure 70C, the positions of the diode DA and resistor RA may be interchanged.

Figure 70D shows a further modification to the circuit of Figure 70C in which the diode DA and resistor RA of Figure 70C are replaced by a PNP bridging transistor TAl in Figure 70D.

Figure 70E shows a modification of the circuit of Figure 70D, in which the bridging transistor TB of Figure 70D is replaced by a resistor RB1 and diode DB2, without any direct connection from them to the left path. In a modification of the circuit of Figure 70E, the positions of the diode DB2 and resistor RB2 may be interchanged.

In the circuit of Figure 70F, from the first to second nodes N1, N2, the left path contains, in order, NPN bridging transistor TA1 and PNP non-bridging transistor TA2, and the right path contains, in order, resistor RB, PNP bridging transistor TB and diode DB.

Figure 70G shows a modification of the circuit of Figure 70F, in which the bridging transistor TB of Figure 70F has become a non-bridging transistor TB, and the polarity of the non-bridging transistor TA2 has been reversed, with its emitter now being connected to node N2.

In the circuit of Figure 71A, from the first to second nodes N1, N2, the left path contains, in order, NPN non-bridging transistor TA, resistor RA and diode DA, and the right path contains, in order, diode DB, NPN bridging transistor TB and resistor RB. In a modification of the circuit of Figure 71 A, the positions of the diode DA and resistor RA may be interchanged.

Figure 71 B shows a modification of the circuit of Figure 71A in which the right path contains the resistor RB, diode DB and NPN bridging transistor TB.

In the circuit of Figure 71C, from the first to second nodes N1, N2, the left path contains, in order, NPN bridging transistor TA, resistor RA and diode DA, and the right path contains, in order, resistor RB, PNP bridging transistor TB and diode DB. In a modification of the circuit of Figure 71C, the positions of the diode DA and resistor RA may be interchanged.

In the circuit of Figure 71D, from the first to second nodes N1, N2, the left path contains, in order, NPN non-bridging transistor TA1 and NPN bridging transistor TA2, and the right path contains, in order, resistor RB, PNP non-bridging transistor TB and diode DB.

In the circuit of Figure 71E, from the first to second nodes N1, N2, the left path contains, in order, PNP non-bridging transistor TAl and NPN bridging transistor TA2, and the right path contains, in order, resistor RB1, diode DB1, resistor RB2 and diode DB2. In a modification of the circuit of Figure 71E, the positions of the diode DB1 and resistor RBl may be interchanged.

In the circuits of Figures 64 and 65 there are two diode drops and one large swing voltage drop in each of the left and right paths between the nodes N1, N2, and in the circuits of Figures 66 and 67 there are one diode drop and two large swing voltage drops in each of the left and right paths between the nodes N1, N2. In the circuits of Figures 68A to 71E, there are two diode drops and two large swing voltage drops in each of the left and right paths between the nodes N1, N2, with the same numbers in each of the upper and lower halves of those paths.The techniques described with reference to Figures 64 to 71E may be extended to include, for example: three diode drops and two large swing voltage drops in each of the left and right paths between the nodes N1, N2; two diode drops and three large swing voltage drops in each of the left and right paths between the nodes N1, N2; three diode drops and three large swing voltage drops in each of the left and right paths between the nodes N1, N2; four diode drops and two large swing voltage drops in each of the left and right paths between the nodes N1, N2, with the same numbers in each of the upper and lower halves of those paths; and six diode drops and two large swing voltage drops in each of the left and right paths between the nodes N1, N2, with the same numbers in each of the upper and lower halves of those paths.

Examples of four diode drops and two large swing voltage drops are shown in Figures 72A to 72D. In Figure 72A, the left path contains, in order from node N1 to node N2, PNP bridging transistor TA1, diode DAl; NPN non-bridging transistor TA2 and diode DA2. The right path contains diode DB1, diode DB2; NPN bridging transistor TB, diode DB3 and resistor RB. In Figure 72B, the NPN non-bridging transistor TA2 of Figure 72A is replaced by a PNP non-bridging transistor TA2.

Also, the diode DB3 and resistor RB of Figure 72A are replaced by a PNP non-bridging transistor TB2. In Figure 72C, the left path contains, in order from node N1 to node N2: diode DA1, resistor RA, diode DA2, NPN bridging transistor TA and diode DA3. The right path contains diode DB1, NPN bridging transistor TB, resistor RB and diodes DB2, DB3. In Figure 72D, the left path contains, in order from node N1 to node N2, diode DA1, NPN non-bridging transistor TA1, NPN non-bridging transistor TA2, and diode DA2. The right path contains diode DB1, diode DB2, NPN bridging transistor TB1 and PNP non-bridging transistor TB2.

Examples of six diode drops and two large swing voltage drops are shown in Figures 75A to 75D. In Figure 75A, the left path contains, in order from node N1 to node N2, two diodes DAl, DA2, resistor RA, diode DA3; diode DA4, NPN bridging transistor TA and diode DA5. The right path contains diodes DB1, DB2, NPN bridging transistor TB; diode DB3, resistor RB and diodes DB4, DB5. If required, the order of the diodes DA1, DA2 and the resistor RA may be changed, and the order of the diode DB3 and resistor RB may be changed.In Figure 75B, the left path contains, in order from node Ni to node N2, two diodes DA1, DA2, NPN non-bridging transistor TA1; PNP bridging transistor TA2 and diodes DA3, DA4. The right path contains diode DB1, PNP bridging transistor TB, diode DB2; diode DB3, resistor RB and diodes DB4, DB5. If required, the order of the resistor RB and the diodes DB4, DB5 may be changed. In Figure 75C, the left path contains, in order from node N1 to node N2, two diodes DA1, DA2, NPN non-bridging transistor TA; diode DA3, resistor RA and diodes DA4, DA5. The right path contains diode DB1, PNP bridging transistor TB1, diode DB2; diode DB3, NPN bridging transistor TB2 and diode DB4. If required, the order of the diode DA3 and the resistor RA may be changed.In Figure 75D, the left path contains, in order from node N1 to node N2, diode DA1, NPN non-bridging transistor TA1, diode DA2; diode DA3, PNP bridging transistor TA2 and diode DA4. The right path contains diode DB1, PNP non-bridging transistor TB, diode DB2; diodes DB3, DB4, resistor RB and diode DB5. If required, the order of the resistor RB and the diode DB5 may be changed.

In the circuits described above with reference to Figures 64 to 72D and 75A to 75D, additional nodes N5, N5a to N5j are shown. When used in multi-stage amplifier circuits, signals may be taken from selected nodes on the first path for supply to a preceding stage as feedback or correction or applied from the preceding stage for biasing of the transistor(s). Also, signals may be taken in at selected nodes on the second path from a succeeding stage for feedback or correction, and applied from such nodes to the succeeding stage for biasing. Also, for all of the circuits described above with reference to Figures 64 to 72D and 75A to 75D, complementary circuits may be provided with the polarities of the transistors and diodes reversed.Also, in all of these circuits, each of the diodes may be replaced by some other impedance, such as a resistor, across which there is a diode drop at the quiescent operating point of the circuit.

Figure 73 shows a modification to the differential input stage S1 to S3 of the circuit of Figure 32. In Figure 73, the long-tail nodes LT1, LT2 are provided by the anode of diode D17c and the cathode of diode D18c, respectively. Also, the emitters of the transistors T5 to T8 are not directly connected to their respective long-tail nodes LT1, LT2, but instead are connected to them via respective resistors R9b2, Rob2, R9b3, Rob3, forming large voltage swing drops. As shown, the long-tail nodes Loth, LT1 may be connected, but in this case by a pair of large voltage swing resistors RlObl, R9bl in series with the diodes D5a, D5b.Preferably, the values of the resistors are such that R5 = R6 = R7 = R8; R9b2 = R9b3 = RiOb2 = Rob3; R9a = RlOa; R9bl = RlObl; and R19 = R20.

Figure 74 shows a further amplifier circuit which is a modified form of the upper half of the circuit of Figure 50. By comparison: the diodes D39a, D9a, D13a, D43a, D23a, D18a, D40b, D44b, D27 of Figure 50 are replaced by series-connected pairs of diodes D39a, D39c; D9a, D9c; D13a, D13c; D43a, D43c; D23a, D23c; D18a, D18d; D40b, D40c; D44b, D44c; D27a, D27b, respectively; the cathodes of diodes D40c, D44c, D27b are connected to ground E; the emitters of transistors T5, T7 and the cathode of diode D5a are connected to ground E via a forward-conducting diode D5c; the inputs VinA, VinB are applied to the bases of the transistors T5, T7 (with the use of coupling capacitors, if required); and the output is taken either from the emitter of transistor T9 as Voutl, or from the collector of transistor T9 as Vout2.

It should be noted that, in each path, each of the sections P1, P2 contains one large-swing voltage drop and three diode drops. The diodes may be replaced by resistors which produce a diode drop at the quiescent operating point of the amplifier, or by other devices producing a similar effect.

Other Modifications and Developments It should be noted that the inventions described above are not limited to the use of bipolar transistors and p-n junction diodes. Other types of transistor, such as FETs, MOSFETs and GaAs devices, and other active devices, such as thermionic valves may be employed, together with complementary diode-like devices.

Claims

(For convenience, the following claims are arranged in groups with sub-headings, and the claims include reference numerals. The sub-headings and reference numerals do not form part of the definition of the manerfor which protection is sought.) (Symmetry) 1. An amplifier comprising: first and second supply rails (V+, V-); and a plurality of circuit elements including a plurality of active elements (T) and a plurality of diodes (D) arranged in a plurality of paths carrying signals between the supply rails; wherein: each of the paths can be divided into a plurality of sections (P1 to P4) extending between the supply rails; and each of the sections of each of the paths containing the same number of elements producing a diode-type voltage drop at the quiescent operating point of the amplifier.
2. An amplifier comprising: first and second supply rails (V+, V-); and a plurality of circuit elements including a plurality of active elements (T) and a plurality of diodes (D) arranged in a plurality of paths carrying signals between the supply rails; wherein: each of the paths can be divided into a plurality of sections (P1 to P4) extending between the supply rails; and each of the sections of each of the paths containing the same number of elements producing a large-swing voltage drop.
3. An amplifier as claimed in claim 2, wherein at least one of the elements producing such a large-swing voltage drop is a resistor.
4. An amplifier as claimed in claim 2 or 3, wherein at least one of the elements producing such a large-swing voltage drop is the collector-base junction of a transistor.
5. An amplifier as claimed in any of claims 2 to 4, wherein each of the sections of each of the paths contains the same number of elements producing a diode-type voltage drop at the quiescent operating point of the amplifier.
6. An amplifier as claimed in any preceding claim, wherein at least one of the elements producing such a diode-type voltage drop is a forward-conducting diode.
7. An amplifier as claimed in any of claims 1, 5 and 6, wherein at least one of the elements producing such a diode-type voltage drop is a forward-conducting base-emitter junction of a transistor.
8. An amplifier as claimed in any of claims 1 and 5 to 7, wherein at least one of the elements producing such a diode-type voltage drop is a resistor.
9. An amplifier as claimed in any preceding claim, wherein the number of sections into which each of the paths can be divided is two.
10. An amplifier as claimed in any preceding claim, wherein the number of sections into which each of the paths can be divided is four.
11. An amplifier as claimed in any preceding claim, and including a long-tailed differential amplifier stage (S1 to S3) forming at least one of said paths and having a long-tail extending between a long-tail node (LT1) and one of the supply rails ( V-).
12. An amplifier as claimed in claim 11, wherein the long-tail includes two series resistances (R9a, R9b) of equal value and of total value equal to a value of load resistance (R5, R7) between each of the active elements (T5, T7) of the long-tailed stage and the other supply rail (V+).
13. An amplifier as claimed in claim 11 or 12 when dependent on claim 10, and including a further such long-tailed differential amplifier stage which is complementary to the first-mentioned longtailed differential amplifier stage.
14. An amplifier as claimed in claim 13, and including an impedance (Z5; D5a, D5b; R13; R13a, R13b; D5a, D5b, R9b, RlOb) interconnecting the long-tail nodes (LT1, LT2) of the differential amplifier stages, the impedance forming part of one of said paths.
15. An amplifier as claimed in any preceding claim, and including a voltage amplifying stage (S4, S5) forming at least one of said paths.
16. An amplifier as claimed in claim 15, wherein the voltage amplifying stage has two cascaded stages (S4, S5) of voltage amplification, each forming one of said paths.
17. An amplifier as claimed in any preceding claim, and including a current amplifying stage (S6, S7) forming at least one of said paths.
18. An amplifier as claimed in claim 17, wherein the current amplifying stage has two cascaded stages (S6, S7) of current amplification, each forming one of said paths.
19. A plural stage amplifier having a plurality of signal-carrying paths between a pair of supply rails (V+, V-), each path having symmetrical halves (P1, P2; P3, P4), as regards the number of diode drops and/or large swing voltage drops therein, each half having symmetrical quarters (P1, P2, P3, P4), as regards the number of diode drops and/or large swing voltage drops therein, the amplifier having an input (VinA) and an output (Vout), both at the half-way level.

(Compound Transistor Circuits)
20. A compound transistor circuit having a compound base (B), collector (C) and emitter (E), the compound transistor comprising: a first transistor (T1) having a first base, collector and emitter; and a second transistor (T2) having a second base, collector and emitter; wherein: the first base is connected to the compound base; the second base is connected to the first emitter; the second emitter is connected to the compound emitter directly or via a number of forward-conducting diodes; the first emitter is connected to the compound emitter via a number of forward-conducting diodes (D1) which is one greater than the number of diodes (if any) connecting the second emitter to the compound emitter; the first collector is connected to the compound collector directly or via a number of forwardconducting diodes; and the second collector is connected to the compound collector via a number of forward-conducting diodes (D2) which is one greater than the number of diodes (if any) connecting the first collector to the compound collector.
21. A compound transistor circuit having a compound base (B), collector (C) and emitter (E), the compound transistor comprising: a first transistor (T3) having a first base, collector and emitter; and a second transistor (T4) having a second base, collector and emitter; wherein: the first base is connected to the compound base: the second base is connected to the first collector; the second emitter is connected to the compound collector directly or via a number of forward-conducting diodes; the first collector is connected to the compound collector via a number of forward-conducting diodes (D3) which is one greater than the number of diodes (if any) connecting the second emitter to the compound collector; the first emitter is connected to the compound emitter directly or via a number of forwardconducting diodes; and the second collector is connected to the compound emitter via a number of forward-conducting diodes (D4) which is one greater than the number of diodes (if any) connecting the first emitter to the compound emitter.
22. A circuit as claimed in claim 20 or 21, and in the form of an integrated circuit.
23. An amplifier circuit having a compound transistor circuit having a compound base (B), collector (C) and emitter (E), the compound transistor circuit comprising: a first transistor (T1) having a first base, collector and emitter; and a second transistor (T2) having a second base, collector and emitter; wherein: the first base is connected to the compound base; the second base is connected to the first emitter; the second emitter is connected to the compound emitter directly or via an impedance; the first emitter is connected to the compound emitter via an impedance (Z1; D1) across which the quiescent voltage drop is one diode-drop greater than the quiescent voltage drop (if any) between the second emitter and the compound emitter: the first collector is connected to the compound collector directly or via an impedance; and the second collector is connected to the compound collector via an impedance (Z2; D2) across which the quiescent voltage drop is one diode-drop greater than the quiescent voltage drop (if any) between the first collector and the compound collector.
24. An amplifier circuit having a compound transistor circuit having a compound base (B), collector (C) and emitter (E), the compound transistor circuit comprising: a first transistor (T3) having a first base, collector and emitter; and a second transistor (T4) having a second base, collector and emitter; wherein: the first base is connected to the compound base; the second base is connected to the first collector; the second emitter is connected to the compound collector directly or via an impedance; the first collector is connected to the compound collector via an impedance (Z3;D3) across which the quiescent voltage drop is one diode-drop greater than the quiescent voltage drop (if any) between the second emitter and the compound collector; the first emitter is connected to the compound emitter directly or via an impedance; and the second collector is connected to the compound emitter via an impedance (Z4; D4) across which the quiescent voltage drop is one diode-drop greater than the quiescent voltage drop (if any) between the first emitter and the compound emitter.
25. A circuit as claimed in claim 23 or 24, wherein the, or at least one of the, impedance(s) is provided by a forward-conducting diode (D1 to D4).
26. A circuit as claimed in any of claims 23 to 25, wherein the, or at least one of the, impedance(s) is provided by a resistor.
27. A circuit as claimed in any of claims 20 to 26, and having an accessible connection to the second base.
28. A circuit as claimed in any of claims 20 to 27, and having an accessible connection to the second collector.
29. A compound transistor circuit having: first and second parallel paths between first and second nodes (N1, N2); and a transistor arranged so that its collector-emitter forms part of the first path and its base is connected to a third node (N3, N3a, N3b) part-way along the second path; wherein: the number of diode drops between the first and third nodes via the first path is equal to the number of diode drops between the first and third nodes via the second path; the number of large swing voltage drops between the first and third nodes via the first path is equal to the number of large swing voltage drops between the first and third nodes via the second path; the number of diode drops between the second and third nodes via the first path is equal to the number of diode drops between the second and third nodes via the second path; and the number of large swing voltage drops between the second and third nodes via the first path is equal to the number of large swing voltage drops between the second and third nodes via the second path.
30. A circuit as claimed in claim 29, further including at least one further transistor, the or each further transistor being arranged so that its collector-emitter forms part of one of the paths and its base is connected to a, or a respective, further node (N3, N3a, N3b) part-way along the other path; wherein: the number of diode drops between the first node and the further node, or any of the further nodes, via the first path is equal to the number of diode drops between the first node and that further node via the second path; and the number of large swing voltage drops between the first node and the further node, or any of the further nodes, via the first path is equal to the number of large swing voltage drops between the first node and that further node via the second path.
31. A circuit as claimed in claim 29 or 30, further including at least one other transistor, the or each other transistor being arranged so that its collector-emitter forms part of one of the paths and its base forms a, or a respective, other node (N4, N4a, N4b).
32. A circuit as claimed in any of claims 29 to 31, wherein there are two diode drops and one large swing voltage drop in each of the first and second paths; or wherein there are one diode drop and two large swing voltage drops in each of the first and second paths.
33. A circuit as claimed in any of claims 29 to 31, wherein there are at least two diode drops and at least two large swing voltage drops in each of the first and second paths.
34. A circuit as claimed in claim 33, wherein the first and second paths each have two halves, each half having the same number of diode drops, and each half having the same number of large swing voltage drops.
35. A circuit as claimed in any of claims 29 to 34, in combination with a preceding amplifier stage, wherein at least one node in the first or second path is connected so as to control, correct or bias the preceding stage.
36. A circuit as claimed in any of claims 29 to 35, in combination with a succeeding amplifier stage, wherein at least one node in the first or second path is connected so as to control, bias or receive a correction from the succeeding stage.
37. A circuit as claimed in any of claims 29 to 36, wherein each diode drop is provided by the base-emitter junction of a, or one of the, transistors (TA, TAl, TA2, TB, TB1, TB2), by a diode (DA, DA1 to DA5, DB, DBl to DB5), or by a resistor across which there is a diode drop at the quiescent operating point of the circuit.
38. A circuit as claimed in any of claims 29 to 37, wherein each large swing voltage drop is provided by the collector-base junction of a, or one of the, transistors (TA, TA1, TA2, TB, TB1, TB2), or by a resistor (RA, RB, RB1, RB2).

(Complemenrary Differential Amplifier with Linked Long-tails)
39. A complementary differential amplifier (S1 to S3) comprising a first long-tailed pair circuit (R5, TS, R7, T7, R9) and a second complementary long-tailed pair circuit (R6, T6, R8, T8, R10), the long-tail nodes (LT1, LT2) of the two circuits being connected by an impedance (Z5; D5a, D5b; R13; R13a, R13b; D5a, D5b, R9b, RlOb).
40. An integrated circuit including a differential amplifier comprising a first long-tailed pair circuit (R5, T5, R7, T7, R9) and a second complementary long-tailed pair circuit (R6, T6, R8, T8, R10), the long-tail nodes (LT1, LT2) of the two circuits being externally accessible.
41. An integrated circuit as claimed in claim 40, in combination with an impedance (Z5; D5a, D5b; R13; R13a, R13b; D5a, D5b, R9b, RlOb) connecting the long-tail nodes.
42. An amplifier or circuit as claimed in claim 39 or 41, wherein the impedance includes at least one resistor (R13; R13a, R13b).
43. An amplifier or circuit as claimed in any of claims 39, 41 and 42, wherein the impedance includes two series-connected forward-conducting diodes (DSa, D5b).
44. An amplifier or circuit as claimed in any of claims 39 and 41 to 43, wherein the impedance is such that, during quiescent operation of the amplifier, its value is about equal to the effective impedance of the active elements (T5 to T8) of the amplifier between the long-tail nodes.

(Coupling of Input Amplifier to Subsequent Stage(s))
45. An amplifier circuit comprising: first and second supply rails (V+, V-), and a plurality of signal-carrying circuit paths extending between the supply rails, wherein: each path comprises first to fourth series-connected sections (P1 to P4) in that order from the first supply rail (V+) to the second supply rail (V-), with each section including the same number of diode-drop devices; the amplifier circuit includes a first stage (S1 to S3) having at least first and second such paths between the supply rails, and a second stage (S4) having such a path between the supply rails independent of the first and second paths of the first stage; a first connection (Cl) connects the first path of the first stage to the second stage; a second connection (C5; C7) connects the second path of the first stage to the second stage.
46. A circuit as claimed in claim 45, wherein: the quiescent potential difference between the first and second connections is one or two diode-drops.
47. A circuit as claimed in claim 45 or 46, wherein: the second connection (C5) is connected: from a point between the first and second sections (P1, P2) of the second path of the first stage, to a point between the first and second sections of the path of the second stage.
48. A circuit as claimed in claim 47 when dependent on claim 46, wherein: the first connection (C1) is connected: - from a point which is one diodedrop away from a point between the first and second sections of the first path of the first stage, to a point which is one diode-drop away in the same direction from a point between the first and second sections of the path of the second stage.
49. A circuit as claimed in claim 46, wherein: the second connection (C7) is connected: - from a point which is one diode-drop away from a point between the first and second sections (P1, P2) of the second path of the first stage, to a point which is one diode-drop away in the same direction from a point between the first and second sections of the path of the second stage; and the first connection (C1) is connected: - from a point which is one diode-drop away in the opposite direction from a point between the first and second sections of the first path of the first stage, to a point which is one diodedrop away in that opposite direction from a point between the first and second sections of the path of the second stage.
50. A circuit as claimed in claim 49, wherein: the first stage includes a third such path between the supply rails; and a third connection (C5) is connected: - from a point between the first and second sections (P1, P2) of the third path of the first stage, to a point between the first and second sections of the path of the second stage.
51. A circuit as claimed in any of claims 45 to 50, and including a third amplification stage (S5) driven by the second stage (S4) by a connection (C3) at the level between the first and second sections of the paths.
52. A circuit as claimed in claim 50, and including a third amplification stage (S5) driven by the second stage (S4) by connections (C3, C9) at the same levels as the second and third connections (C7, C9).
53. A circuit as claimed in claim 51 or 52, wherein the second and third stages together form a circuit as claimed in any of claims 23 to 28.
54. A circuit as claimed in any of claims 51 to 53, and including a fourth amplification stage (S6) driven by the third stage (S5).
55. A circuit as claimed in claim 54, wherein the third and fourth stages together form a circuit as claimed in any of claims 23 to 28.
56. A circuit as claimed in any of claims 48 to 55, wherein the diode-drop in the second stage related to the first connection (C1) is provided by an active element (T9) of the second stage.
57. A circuit as claimed in any of claims 49 to 56, wherein the diode-drop in the second stage related to the second connection (C7) is provided by a forward-conducting diode (D23b) in the second stage.
58. A circuit as claimed in any of claims 49 to 57, wherein the diode-drop in the second stage related to the second connection (C7) is provided by a forward-conducting base-emitter or emitter-base junction of a transistor (T27) in the second stage.
59. A circuit as claimed in any of claims 45 to 58, wherein the first stage is in the form of a complementary pair of long-tailed amplifiers, the first path of the first stage being provided by one of the long-tailed amplifiers, and the second path of the first stage being provided in part by the long-tail of the other long-tailed amplifier.

(Compensation for Unbalanced Loading)
60. An amplifier circuit comprising: a first stage (S1 to S3) having first and second signal paths between a pair of supply rails (V+, V-); and a second stage (S4) having a third signal path between the supply rails; wherein: the second stage is connected to the first stage by a first connection (C1) between the first signal path and the third signal path and by a second connection (C5) between the second signal path and the third signal path; the second stage places an unbalanced loading on the first stage through the first and second connections; the first stage includes a compensation element (Z7b; D37b; R37b, D37b) which places a corresponding loading on a complementary portion of the first stage to compensate for the loading provided by the second stage.
61. A circuit as claimed in claim 60, wherein: the first stage has a fourth signal path between the supply rails; the second stage is connected to the first stage by a third connection (C7) between the fourth signal path and the third signal path; the second stage places a second unbalanced loading on the first stage through the second and fourth connections; the first stage includes a second compensation element (Z7a; D37a; R37a, D37a) which places a corresponding loading on a complementary portion of the first stage to compensate for the second loading provided by the second stage.
62. A circuit as claimed in claim 60 or 61, wherein the or each unbalanced loading and compensatory loading are each one or two diode-drops.
63. A circuit as claimed in any one of claims 60 to 62, wherein the first stage is in the form of a complementary pair of long-tailed amplifiers, the first and second paths including first and second loads (R5, R7) of one of the long-tailed amplifiers.
64. A circuit as claimed in claim 63 when dependent on claim 61, wherein the fourth path includes the long-tail (R10) of the other long-tailed amplifier.
65. A circuit as claimed in any one of claims 60 to 62, wherein the first stage is in the form of a complementary pair of long-tailed amplifiers, the first and second paths including a load (R5) of one of the long-tailed amplifiers and the long-tail (R10) of the other long-tailed amplifier.
66. An amplifier as claimed in claim 19, wherein the input stage is in the form of a long-tailed differential amplifier, an output point (VoutA) of the input stage being connected (by Cl) to an input point of the second stage, a quartile point of a path including the long-tail being connected (by CS) to a quartile point of the second stage, the second stage placing a diode drop loading on the input stage between the input point and the quartile point of the second stage, and the input stage having an additional diode drop (D37b) connected to said quartile point thereof.
67. An amplifier as claimed in claim 66, wherein said diode drop loading is in one quarter (P2), and the additional diode drop is in the other quarter (P1) in the same half.
68. An amplifier circuit comprising a complementary pair of long-tailed differential amplifiers (S1 to S3) and a subsequent amplification stage (S4), wherein: each differential amplifier comprising a pair of active elements (T5 to T8), each connected between a respective load (R5 to R8) for that active element and a respective long-tail (R9, R10) for that differential amplifier; each load includes a first pair of series-connected diode-drop devices (D9b, DlOa; D13b, D14a; Dllb, D12a; D15b, D16a); the circuit further comprises for each load a second pair of series-connected diode-drop devices (D37b, T9; D37a, D13b;D38b, T10; D38a, D15b) in parallel with the first pair of diode-drop devices of that load; and one diode-drop device (T9, Di3b, T10, D15b) of each second pair is provided by the second amplification stage.
69. A circuit as claimed in claim 68, wherein each second pair of diode-drop devices has a midpoint connected to a point part-way along the long-tail of the other differential amplifier.

(Long-Tails in Differential Amplifier)
70. An amplifier circuit comprising a complementary pair of long-tailed amplifiers (S1 to S3), each having a pair of active elements (T5 to T8) connected between a load for that active element and a long-tail (R9b, RlOb) for that amplifier, wherein the long-tail of each amplifier includes a further active element (T22, T21) which receives its bias from a diode-drop (ZlOa to ZlOd) in one or both of the loads of the other amplifier.
71. A circuit as claimed in claim 70, wherein the diode-drops are provided by respective forwardconducting diodes (D9a, D13a, D12b, D16b).
72. A circuit as claimed in claim 70, wherein the diode-drops are provided by respective resistors (RD9a, RD13a, RD12b, RD16b) across which there is a diode-drop at the quiescent operating point of the amplifier.

(Loads in Differential Amplifier)
73. An amplifier circuit comprising a complementary pair of long-tailed amplifiers (S1 to S3), each having a pair of active elements (T5 to T8) connected between a load for that active element and a long-tail (R9a, R9b, RlOa, RlOb) for that amplifier, wherein the load of each amplifier includes a further active element (T17 to T20) which receives its bias from a diode-drop (Z9a, Z9b) in the longtail of the other amplifier.
74. A circuit as claimed in claim 73, wherein the diode-drops are provided by respective forwardconducting diodes (D18a, D17b).
75. A circuit as claimed in claim 73, wherein the diode-drops are provided by respective resistors (RD18a, RD17b) across which there is a diode-drop at the quiescent operating point of the amplifier.

(Input Biasing of Differential Amplifier)
76. An amplifier as claimed in claim 13 or 14, wherein a further pair of said paths each has a midpoint connected to a respective input of the complementary differential amplifier.
77. An amplifier as claimed in claim 76, wherein one of said further paths includes first and second diode-drop devices (D40b, D41a) between one of the amplifier inputs (VinA) and first and second active devices (T5, T6) of the amplifier, and the other of said further paths includes third and fourth diode-drop devices (D44b, D45a) between the other amplifier input (VinB) and third and fourth active devices (T7, T8) of the amplifier.
78. An amplifier as claimed in claim 77, wherein the active devices (T5, T6, T7, T8) have a common connection (LTm).

(Output Loading and Feedback)
79. An amplifier for driving an output load having a nominal load resistance (R36), the amplifier comprising: an input stage having a primary input (VinA) and a feedback input (VinB); an output stage having an output node (Vout); and a feedback network (R33, R34) which feeds back to the feedback input a part of the signal at the output node; wherein the feedback network places on the output node a load which is generally equal to the load placed thereon by the output load.
80. An amplifier for driving an output load (R36), the amplifier comprising: a pair of supply rails (V+, V-); an input stage (S1 to S3) having a primary input (VinA) and a feedback input (VinB); an output stage (S4, S5) having at least one primary active device (T9, T11) connected in a path between one of the supply rails (V+) and a preliminary output node (Vout*), the or each active device producing a diode-type drop which has a particular effective resistance during quiescent operation of the amplifier; a first resistance (R35) connected for series connection with the output load, the first resistance having a value greater than or generally equal to said effective resistance; and a feedback network (R33, R34) which feeds back to the feedback input a part of the signal at the preliminary output node, the feedback network including a second resistance (R33) connected between the feedback input of the input stage and a reference potential such as ground, the second resistance having a value greater than or generally equal to said effective resistance.
81. An amplifier as claimed in claim 80, wherein the values of the first resistance and/or the second resistance are generally twice said effective resistance.
82. An amplifier as claimed in claim 80 or 81, wherein the feedback network includes a resistance (R34) between the preliminary output node and the feedback input which is generally equal to twice the internal resistance of the output stage between the preliminary output node and the supply rails during quiescent operation of the amplifier.
83. An amplifier as claimed in any of claims 80 to 82, wherein the feedback network includes a resistance (rib1) leading to the feedback input which has a value generally equal to said effective resistance, and a resistance (RAl) is included leading to the primary input which has a value generally equal to twice said effective resistance.
84. An amplifier as claimed in any of claims 80 to 83, wherein the amplifier is for driving an output load having a nominal load resistance (R36) which is generally equal to twice the internal resistance of the output stage between the preliminary output node and the supply rails during quiescent operation of the amplifier.
85. An amplifier as claimed in any of claims 80 to 84, wherein the output stage has a pair of such primary active devices (T11, T12), each connected in a path between a respective one of the supply rails and the preliminary output node.

(Quiescent Operating Point)
86. An amplifier, comprising an active device, or a plurality of series-connected active devices, in a primary path between a pair of supply rails, the amplifier being such that, during quiescent operation of the amplifier, the voltage drop across the active device, or the sum of the voltage drops across the active devices, is generally equal to two-thirds of the supply voltage between the supply rails.

(Omnibus Claims)
87. A compound transistor circuit substantially as described with reference to the drawings.
88. An amplifier substantially as described with reference to the drawings.