GB1602296A

GB1602296A - Current sources

Info

Publication number: GB1602296A
Application number: GB4747577A
Authority: GB
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1977-11-15
Filing date: 1977-11-15
Publication date: 1981-11-11
Also published as: FR2408948A1; FR2408948B1; NL7811241A; CA1135349A; BE872025A; WO1979000295A1

Abstract

A current source in which the output current from an output amplifier stage (2) is controlled by an input signal, originating in a signal source (11), to an input amplifier stage (1) incorporates, additionally to a negative feedback path from the current input path of the output stage (2) to the input amplifier (1) monitoring means (5) in the signal input path to the output amplifier (2) and control means (4) in an input path to the input amplifier (1). Monitoring means (5) monitors the current flow in the input path to the output amplifier (2) and applies, via control means (4) a signal to the input amplifier (1) such that the dependence of the current flow in the current output path on the gain of the output stage (2) is reduced. A circuit is described, as well as an application to driving a telephone subscriber line and using two current sources working in push-pull mode.

Description

(54) IMPROVEMENTS IN OR RELATING TO CURRENT SOURCES (71) We, THE GENERAL ELECTRIC COMPANY LIMITED, of 1 Stanhope Gate, London W1A IEH, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement:- This invention relates to current sources and in particular to such current sources in which the magnitude of the output current of the source is determined by the magnitude of a control voltage.

According to one aspect of the present invention in a current source arrangement in which the magnitude of current flowing in an output path of a transistor amplifier stage is arranged to be controlled by a control signal applied to an input of the arrangement, said control signal being applied to the transistor amplifier stage by way of a differential amplifier to an input of which is also applied a negative feedback signal whose magnitude is dependent upon that of said current in said output path, there are provided means to provide a further feedback signal to an input of said differential amplifier whose value is dependent upon the input current to said transistor amplifier stage such as to reduce the dependence of the current flowing in said output path on the gain of the transistor amplifier stage.

The control signal and the said further feedback signal are conveniently applied to a non-inverting input of the differential amplifier, and the negative feedback signal to an inverting input thereof. The negative feedback signal may be derived from the voltage drop across a resistive element in the emitter circuit of an output transistor in the transistor amplifier stage. The said further feedback signal is conveniently derived from a first resistive element in the control signal input path of the transistor amplifier and applied to the non-inverting input via a second resistive element, with the control signal being applied to the same input of the differential amplifier via a third resistive element.

In accordance with another aspect of the present invention a current source arrangement comprises a differential amplifier having inverting and non-inverting inputs and an output, and a transistor having an input electrode, an output electrode, and a control electrode, the input electrode being connected by way of a resistive element to one pole of power supply means and the output electrode being connected by way of a load circuit to the other pole of said power supply means, the output of the differential amplifier being electrically coupled to the control electrode of the transistor, means to apply a control signal to one input of the differential amplifier, feedback means to apply to another input of the differential amplifier a signal dependent upon the voltage developed in operations across said resistive element, and further means for deriving a signal dependent on the current flow through the control electrode of the transistor, said further means being electrically connected to said one input of the differential amplifier.

Preferably, said differential amplifier is a high gain amplifier having substantially linear amplification characteristics, of the kind known as operational amplifiers.

Said further means for deriving a signal dependent on the current flow through the control electrode of the transistor conveniently comprises a first resistive element connected between the output of the differential amplifier and the control electrode of the transistor, thereby also electrically coupling said output to said control electrode, a second resistive element by means of which the output of the differential amplifier is connected to said one input thereof, and a third resistive element connecting said one input to a source of said control signal whereby said control signal is applied to said one input.

The control signal applied in operation of the current source to said one input may be a DC signal voltage derived from a resistance divider network.

Said control signal may also include an A.C. component superimposed on the D.C. signal in a known manner.

A current source arrangement in accordance with the present invention is suitable for use with electronic line units for telephone systems.

A current source arrangement in accordance with the present invention will now be described by way of example with reference to the accompanying drawing, of which: Figure 1 shows the current source arrangement schematically; Figure 2 shows the current source arrangement diagrammatically; and Figure 3 shows in diagrammatic form, a circuit incorporating two complimentary current source arrangements in push-pull mode.

Referring first to Figure 1, a current source in accordance with the present invention comprises, a high gain linear amplification stage provided by an operational amplifier 1, and an amplifier stage in the form of a junction transistor 2, whose control electrode, or base, 8 is electrically coupled with the output of the amplifier 1. The input electrode, or emitter 7, of the transistor 2 is connected to the inverting input of the amplifier 1 and also, via a close tolerance resistor 3, to a voltage rail 9. Connected to the non-inverting input of the amplifier 1 is a voltage source 11 supplying a control signal voltage to the amplifier and hence to the base 8 of the transistor 2, the value of the control voltage determining the current flow at the output electrode, or collector 6, of the transistor 2.Additionally control means 4 and 5 are connected to the non-inverting input and the output respectively of the amplifier 1, arranged to derive a signal dependent on base current of the transistor 2 and to apply a signal to the non-inverting input of the amplifier 1 such that the dependence of the collector current on the gain of the transistor is reduced. A load circuit (not shown) to be driven by this current source arrangement is connected in use to the collector 6 of the transistor 2.

A signal indicative of the base current is derived by means 5 and is fed by means of the control means 4 into the non-inverting input of the amplifier, any deviation of this signal from its design value as determined by the control voltage causing a variation in the signal applied to the non-inverting input. A consequent change in current flow through the emitter and hence the resistor 3 results in a change of the voltage applied to the emitter and thus compensates for this signal variation thereby bringing the collector current nearer to the intended value.

The operation of a particular embodiment of this constant current source will become apparent from the description below with reference to Figure 2 of the drawings, in which parts easily identifiable as being equivalent to components of Figure 1 carry the same reference.

Referring now to Figure 2, the output of the operational amplifier 1 is coupled by means of a resistor 16 to the base 8 of the transistor 2, whose emitter 7 is connected to the voltage rail 9 by way of a resistor 3 and to the inverting input of the amplifier 1 through a resistor 14 which serves as a feedback path and can also be arranged to equalize the source impedances to the two inputs of the amplifier. A resistance divider network comprising resistors 18 and 19 connected across the voltage rails 9 and 10, from which power is also supplied to the amplifier 1, provides the biasing D.C. control voltage onto which may be superimposed on A.C. control voltage in the form of an A.C. signal applied to the terminal 20 of the capacitor 21.

The biassing voltage derived from the resistance network 18, 19 and, where applicable, the A.C. signal is applied to the non-inverting input of the amplifier I through a resistor 17, with a resistor 15 connecting this input to the output of the amplifier. The influence of resistors 14, 18, and 19 on the A.C. performance of the current source is negligible. The accuracy of the current source can be made dependent solely on resistor matching, thus obviating the need to adjust each individual source in dependence on the individual performance characteristics of the transistor used, as the following calculation shows, in which: Vjn=the voltage obtained from the resistance divider network, Vrerthe voltage at the non-inverting input of the amplifier, VO-the voltage at the output of the amplifier, Vthe voltage between base and emitter of the transistor, Ib=the base current of the transistor, le=the emitter current of the transistor, Ic=the collector current of the transistor, re=the dynamic emitter-base resistance of the transistor, R3=the resistance value of resistor 3, etc.

Summing currents at the non-inverting input of the operational amplifier, one obtains:

Rearranging this expression gives:

But, since current flow through the resistor R3 will tend to stabilise at a value such that the voltage at the emitter electrode 7 of the transistor 2 is virtually equal to Vref: Vo=Vwf+ Vh+IbRl6 (3) Defining VbDc=Vb-Iere (4a) re =Vb-Vref , (4b) R3 writing re/R3=k and substituting for Vb from equation (4b) in equation (3) yields:: Vo=Vef (l+k)+VbDs+tbR,6 (5) Using this equation (5) in equation (2) gives

However,

and by substitution from equation (6)

In order to make the collector current Ic independent of the base current Ib, and hence of the current amplification A, where it is necessary to ensure that

or given k is small, to a first order that

Thus under D.C. conditions the accuracy of the current source subject to the restrictions (9) and (10) above is indeed dependent only on resistor matching and independent of the current gain A of the transistor.Any deviation of Ic from the predicted value is caused by variations in VbDc and resistive tolerances which result in imperfect cancellation of the base current 1h and hence in an error in the transconductance

of the circuit, i.e. the variation of collector current Ic in dependence on Vin.

The error in the quiescent current under D.C. conditions is the same as that in the A.C. case dealt with below, except for the additional error due to VbDr variations. As this error is substantially independent of the absolute value of the collector current Ic, only the differential of Ic with respect to VbDc needs to be calculated. Thus, from equation (8)

which on account of equation (10) yields

and therefore

i.e. the effect of variation in VbDc, as defined by equation (4a), on the collector current IC is inversely proportional to the resistance value of resistor 16.

Although being sufficient for a calculation of the accuracy of the constant current source under D.C. conditions, the first order approximation expressed in equation (9) above is not precise enough to permit an assessment of the accuracy of the current source under A.C. conditions. A good approximation is, however, possible by using a sensitivity analysis, for a given resistor R3, involving the transconductance

and the current gain p of the transistor.

Rearranging equation (8), using the relationship (8a) between Ic and Ib, and defining a resistor ratio X as gives

The sensitivity factor S for a fractional change Agm gm caused by a fractional change is defined by

providing a measure for the dependence between the two variables.

In the limit of infinitesimal changes dgm and dss expression (13) becomes

Substituting for gm from equation (12) gives

It follows that the dependence of gm of fi is only slight by virtue of X being a very small number approaching 0 in the case of ideal resistor matching. Ideally, the ss dependence of the transconductance is removed completely but for practical values of X there still is a very slight dependence on account of the finite current gain.

Expression similar to (16 can be derived for the sensitivity of the current source to changes of the transconductance gm with respect to R3 and X, viz: and

Equation (17) shows that variations in gm are directly proportional to variations in Ra, underlining the necessity for a very low tolerance resistor 3.

The effect of variations in the resistor ratio on gm is very low as shown by (18), the physical explanation being that the tolerance of resistor ratio X proportionally effects the base current, but the base current itself is only a small proportion of the total collector current.

The performance of the circuit shown in Figure 2 has been evaluated in practical tests, with the resistor ratio X being realized to an accuracy of 0.03%.

Measurements carried out showed no discernible difference over the audio frequency range between the predicted and the actual transconductance.

Using different transistors 1 with current gains between 20 and 100.. worst possible resistor tolerances of +0.1% and a perfect operational amplifier, the error between predicted and measured performance is still less than +i).2'j;; under A.C.

conditions.

Typical variations in VbDc for a power transistor are +100 mV resulting in a change of the collector current of approximately f501A, i.e. +0.2% of the quiescent current.

The near-independence of the output current with respect to the gain of the transistor as achieved in the foregoing current source arrangement allows easy matching of two or more such current source arrangements.

Although a current source in accordance with the invention will give, for the same applied DC control signal voltage, the same current output over a large range of transistor gains, the value at which the source output stabilizes may vary slightly between different sources, for instance on account of resistor tolerances. These variations will, in general, be small enough to be of no consequence when replacing one source with another.However, if two current sources are simply connected in series to work in a push-pull mode, e.g. to drive a telephone subscriber's line, even the slightest mismatch between the current outputs of the two sources may present problems in that, as, each of the current sources attempts to keep the common output current to a value appropriate to its own stable state, one or the other of the sources may saturate and cease to operate in the above described manner. A circuit arrangement designed to overcome this problem is shown in Figure 3.

Thus, each of the two current sources making up the circuit comprises an operational amplifier 1,1', whose output is connected, by way of a resistor 16.161 to the base of the transistor 2,21 respectively, with transistor 2 being an n-p-n transistor, and transistor 21 being a nominally matched, complementary p-n-p transistor. The use of a matched pair of complementary transistors 2,21 provides for the impedance presented by each current source to the respective line of the two wire line 31 to be equal, since thereby the collectors 6 and 6' of the transistors 2 and 2t are connected respectively to the positive line (+) and the negative line (-) of the two wire line 31. The other end of the two wire line 31 is connected to a load circuit (not shown) such as e.g. a subscriber's instrument.

The feedback paths of each of the two current sources, that is the negative feed back path via resistors 14 and 141 and the further feed back path including resistors 15 and 16, and 151 and 16' respectively, are identical in arrangement and function to the corresponding feed back paths of the circuit shown in Figure 2.

The power supply to the arrangement is by means of voltage rail 29, carrying a negative voltage of suitable magnitude, and grounded voltage rail 30.

Also connected between the voltage rails 29 and 30 is a resistor divider network comprising resistors 22,23,24,25, all having the same value, and equal resistors 19 and 19'.

The resistor network determined the average voltage of the two wire line, i.e.

the mean of the voltages on the positive and the negative line of the two wire line 31.

If the two current sources of the arrangement are perfectly matched, the average voltage of the line pair lies half way between the voltage levels at rails 29 and 30. If, however, the two current sources are imperfectly matched, that is to say that they stabilize individually at different current levels of collector current for the same applied control input voltage, the average voltage of the two wire line 31 moves away from this half way point, in a direction so as to decrease the voltage between the collector and the corresponding current rail of that transistor which draws the higher current and increase the corresponding voltage at the other circuit.This shift of the average voltage at the two wire line, also termed common mode shift causes an equal and opposite change, with respect to the nearest current rail, of the input voltage levels Vln and Van1, the change being such that the current flow through that transistor, which initially drew the higher current, is reduced and the current flow through the other transistor is increased. This common mode shift is arrested when both transistors draw the same current, with the average voltage stabilizing at the new value. Resistors 22 to 25 thus form, in conjunction with resistors 19 and 191, a third feed back path which ensures that the collector currents of the two transistors 2 and 21 are equal, i.e. that II,'.

A differential voltage change, on the other hand, which causes the two lines of the two wire line 31 to move individually away from the average voltage in opposite directions, leaves the average voltage unchanged, and thus will produce no change in the levels of Vln and V,n1. If therefore an antiphase AC signal is applied to the terminals 20 and 201, and the output currents I, and lC1 vary, in antiphase, in accordance with that signal, no feed back effect will be produced. Provided also, that the resistors 22 to 24 are approximately equal to twice the appropriate value of resistor 18 of Figure 2 in order to leave the above calculations unchanged, the third feed back path does not, therefore, interfere with the AC operation of the arrangement.

In a similar way, the currents Ic and IC1 are not affected by any differential signal produced within the two wire line, such as may be produced in a subscriber's instrument.

As aforesaid, the collector currents Ic and IC1 are largely unaffected by differential signals generated within the two wire line circuit, but respond only to either a common mode voltage shift, due to e.g. imperfect matching of the two current sources or an asymmetry in the line circuit on account of leakage currents, or to antiphase signals applied to the terminals 20 and 201. Therefore, by providing a further circuit (not shown) which is unaffected by variations in the collector currents, but detects differential signals which are generated within the two wire line circuit, the present arrangement may be incorporated in, and form part of an electronic hybrid circuit. The detection of such line generated signals may be achieved in the following way.When signals are sent to the subscriber's instrument, that is when antiphase signals are applied to the terminals 20 and 201, then the emitter and collector voltages generated by a given transistor are out of phase with each other, and by suitable addition these voltages may be made to cancel each other, and consequently no output signal is provided by said further circuit.

Differential signals which are generated within the line circuit do, however, produce in-phase collector and emitter voltages at each of the transistors, resulting in an output signal at the said further circuit. The arrangement including a said further circuit thus provides for a separation of incoming and outgoing signals as is required for an electronic hybrid circuit such as may be used in the conversion from two wire to four wire transmission and vice versa.

Resistors 26 and 27, and non-linear devices 28 and 29 on the positive and the negative line respectively form part of an overvoltage or lightning protection arrangement. The devices 28 and 29 may e.g. be non-linear resistors, or zener diodes. In the constant current sources described above, known means for providing the DC control voltages, other than resistor divider networks may of course be employed, and any other modifications of the current soruces above, which are obvious to those skilled in the art are included in the scope of the present invention.

WHAT WE CLAIM IS: 1. A current source arrangement in which the magnitude of current flowing in an output path of a transistor amplifier stage is arranged to be controlled by an input-signal applied to an input of the arrangement, said input-signal being applied to the transistor amplifier stage by way of a differential amplifier to an input of which is also applied a negative feedback signal whose magnitude is dependent upon that of said current in said output path, and wherein there are provided means to provide a further feedback signal to an input of said differential amplifier whose value is dependent upon the input current to said transistor amplifier stage such as to reduce the dependence of the current flowing in said output path on the gain of the transistor amplifier stage.

2. A current source arrangement as claimed in claim 1, wherein said input signal being applied to the transistor amplifier stage by way of a differential amplifier is applied to a non-inverting input thereof, to which is also applied said further feedback signal.

3. A current source arrangement as claimed in claim 1 or 2, wherein the negative feedback signal is derived from the voltage drop across a resistive element in an emitter circuit of an output transistor in the transistor amplifier stage.

4. A current source arrangement as claimed in claims 1 to 3, in which the said further feedback signal is derived from at least one first resistive element in the input-signal input path of the transistor amplifier and applied to the non-inverting input of the differential amplifier via at least one second resistive element, with the input-signal being applied to the same input of the differential amplifier via at least one third resistive element.

5. A current source arrangement as claimed in any preceding claim, wherein the said input-signal has a direct current component derived from a resistor divider network.

6. A circuit arrangement for driving a two wire telephone subscriber line incorporating two current source arrangements in accordance with any preceding

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. that the resistors 22 to 24 are approximately equal to twice the appropriate value of resistor 18 of Figure 2 in order to leave the above calculations unchanged, the third feed back path does not, therefore, interfere with the AC operation of the arrangement. In a similar way, the currents Ic and IC1 are not affected by any differential signal produced within the two wire line, such as may be produced in a subscriber's instrument. As aforesaid, the collector currents Ic and IC1 are largely unaffected by differential signals generated within the two wire line circuit, but respond only to either a common mode voltage shift, due to e.g. imperfect matching of the two current sources or an asymmetry in the line circuit on account of leakage currents, or to antiphase signals applied to the terminals 20 and 201. Therefore, by providing a further circuit (not shown) which is unaffected by variations in the collector currents, but detects differential signals which are generated within the two wire line circuit, the present arrangement may be incorporated in, and form part of an electronic hybrid circuit. The detection of such line generated signals may be achieved in the following way.When signals are sent to the subscriber's instrument, that is when antiphase signals are applied to the terminals 20 and 201, then the emitter and collector voltages generated by a given transistor are out of phase with each other, and by suitable addition these voltages may be made to cancel each other, and consequently no output signal is provided by said further circuit. Differential signals which are generated within the line circuit do, however, produce in-phase collector and emitter voltages at each of the transistors, resulting in an output signal at the said further circuit. The arrangement including a said further circuit thus provides for a separation of incoming and outgoing signals as is required for an electronic hybrid circuit such as may be used in the conversion from two wire to four wire transmission and vice versa. Resistors 26 and 27, and non-linear devices 28 and 29 on the positive and the negative line respectively form part of an overvoltage or lightning protection arrangement. The devices 28 and 29 may e.g. be non-linear resistors, or zener diodes. In the constant current sources described above, known means for providing the DC control voltages, other than resistor divider networks may of course be employed, and any other modifications of the current soruces above, which are obvious to those skilled in the art are included in the scope of the present invention. WHAT WE CLAIM IS:

1. A current source arrangement in which the magnitude of current flowing in an output path of a transistor amplifier stage is arranged to be controlled by an input-signal applied to an input of the arrangement, said input-signal being applied to the transistor amplifier stage by way of a differential amplifier to an input of which is also applied a negative feedback signal whose magnitude is dependent upon that of said current in said output path, and wherein there are provided means to provide a further feedback signal to an input of said differential amplifier whose value is dependent upon the input current to said transistor amplifier stage such as to reduce the dependence of the current flowing in said output path on the gain of the transistor amplifier stage.

claim operating in push-pull mode and having outputs of the respective transistor amplifier stages connected to respective lines of the two wire line, with the phase of the output current of one current source arrangement being inverted with respect to that of the other, wherein there is provided a third feedback loop arranged to ensure that the output currents of the said two current source arrangement are of substantially equal magnitude.

7. A circuit arrangement as claimed in claim 6, wherein said third feedback path forms part of a resistor divider network arranged to hold the mean voltage of both lines of said two wire line at a level intermediate the voltage provided by power supply means to the circuit arrangement.

8. An electronic hybrid circuit for conversion between two wire and four wire telephone transmission, incorporating a circuit arrangement as claimed in claim 6 or 7.

9. A current source arrangement comprising a differential amplifier having inverting and non-inverting inputs and an output, and a transistor having an input electrode, an outputfelectrode, and a control electrode, the input electrode being connected by way ofa resistive element to one pole of power supply means and the output electrode being connected by way-of a load circuit to the other pole of said power supply means, the output of the differential amplifier being electrically coupled to the control electrode of the transistor, means to apply a control signal to one input of the differential amplifier, feedback means to apply to another input of the differential amplifier a signal dependent upon the voltage developed in operations across said resistive element, and further means for deriving a signal dependent on the current flow through the control electrode of the transistor, said further means being electrically connected via another feedback path to said one input of the differential amplifier.

10. A current source arrangement as claimed in claim 9, wherein said means for deriving a signal dependent on the current flow through the control electrode of the transistor comprises a first resistive element connected between the output of the differential amplifier and the control electrode of the transistor, thereby also electrically coupling said output to said control electrode, a second resistive element by means of which the output of the differential amplifier is connected to said one input thereof, and a third resistive element connecting said one input to a source af said control signal whereby said control signal is applied to said one input.

11. A current source as claimed in claim 9 or 10 wherein the control signal applied in operation of the current source to said one input has a DC signal voltage component derived from a resistor divider network.

12. A current source arrangement substantially as herein described with reference to, and as shown in, Figures 1 and 2 of the accompanying drawings.

13. A circuit arrangement substantially as herein described with reference to, and as shown in Figure 3 of the accompanying drawings.