DE3511591A1 - BROADBAND SIGNAL AMPLIFIER - Google Patents

Abstract

A multiple-stage, direct current-coupled signal amplifier with transistors of various types of conductivity, in particular for amplification of the analogue entry signal in an oscilloscope, whereby each of the amplification stages has paired symmetrical transistors of different conductivity type between the supply potentials of different polarity, which receive and process the complete wanted signal in the same phase condition. The respective sequential transistors, which are connected with a supply potential of the same polarity, are also of different conductivity types. The entry phase is designed as an emitter sequence. The successive stages have a cascade circuit, or in other words, they form successive stages in the emitter and base circuits.

Description

Description

The invention relates to a broadband signal amplifier of the type specified in the preamble of claim 1.

In the case of such broadband amplifiers, high demands are usually placed on the transmitting frequency band, which must range from 0 Hz to the megahertz range. In addition, the highest possible signal level should be free of distortion be transmitted. In addition, there is the stress on the signal from noise sources within the amplifier must be kept as small as possible. Especially with broadband amplifiers, the negative feedback is over several Stages usually associated with considerable difficulties. For example, it is known in the case of a differential amplifier counter-couple each individual stage in order to obtain a dissimated gain (Tietze / Schenk, "semiconductor circuit technology", 2nd edition, p. 357).

The disadvantage here is that the negative feedback via individual stages reduces the overall gain behavior of the amplifier is difficult to control and otherwise complex input voltage components must be provided whose dividing behavior is influenced by the repercussions of the input conditions of the amplifier.

The previous attempts to design operational amplifiers so broadband and single phase that they meet the requirements may suffice for use in a broadband oscilloscope, fail because of the resulting

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Difficulties regarding the stabilization of the individual stages and the measurement of the signal propagation times. In addition, that components, which at low frequencies as components with "complementary" properties match sufficiently Show behavior - such as bipolar transistors in NPN or PNP technology - at frequencies show deviations in the megahertz range, which idealized the structure of an amplifier with such Affect properties. This has essentially been the case with broadband operational amplifiers Made use of successive differential amplifier stages, which are intended to keep the voltage drift small. However, operational amplifiers that are internally asymmetrical also meet the requirements in this context There are no requirements for signal processing.

The invention specified in claim 1 is based on the object of an amplifier of the type mentioned at the beginning indicate that has a large gain-bandwidth factor at a high input resistance and above in addition, can also be carried out with low noise. The amplifier should be potentiometric in the manner of an operational amplifier be wired so that a signal feed at a signal point located on virtual ground potential can take place, which is high-resistance and potential-free.

Rather, the invention is based on the knowledge that at the present application for the construction of an amplifier with potentiometric properties a completely new path has to be taken in order to

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quality requirements to be met. In the solution according to the invention, the signal amplification practically continuously carried out twice in phase by amplifying stages of complementary structure in each case, with the output signals of the individual stages are reassembled into a common signal at the amplifier output be, so that the inequalities in terms of signal processing by transistors of different Compensate conductivity type and additionally through successive, in turn, complementary stages for it it is ensured that the unbalanced signal influence is also balanced out over the successive stages.

The quality of a measuring device depends on the type and structure of the broadband DC voltage measuring amplifier addicted.

A list of the requirements simultaneously met by such an amplifier is as follows:

The amplifier should have a low inherent noise. This results in the requirement for as few as possible and as low-noise active and passive components as possible.

The amplifier should have an extremely high bandwidth. This results in the requirement for as much as possible fast active components and after a low-resistance circuit, as well as small capacities in the entire circuit network.

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With this bandwidth, the amplifier must have a short signal propagation time in order to make the transient response as dimensionally true as possible without overshooting in the feedback mode.
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In relation to its supply voltages, the amplifier should have the highest possible modulation capability have (crest factor) and even when overdriven no signal distortion or signal reflections have in the normal dynamic range.

The amplifier should have a high input resistance so that when connected with measuring resistors Amplifier input to allow precision gain.

The measuring amplifier must have a small input capacitance, especially because the input resistances im Case of the preamplifier are extremely high and also with the follower amplifier for reasons of power and power savings Resistance values that are too low cannot be used. For the same reasons the capacities within the amplifier must be kept small.

The amplifier should cover its entire control range, i.e. for both negative and positive Amplitude values have the same speed properties (transit time and bandwidth) for the signals to be able to reinforce with the greatest possible dimensional accuracy.

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The amplifier should have a high no-load gain in order to be able to increase the input resistance by means of negative feedback and to be able to achieve a high level of measurement accuracy.
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The amplifier should require little component and assembly effort in order to meet the device-specific task cost-effective to take into account.

The list shows that the amplifier simultaneously has the properties of a low-noise amplifier, a broadband amplifier, a large-signal amplifier, a phase-compensated amplifier, a measuring amplifier, a have long-term stable amplifier and should be inexpensive, with the dimensional accuracy of a small-signal amplifier is assumed.

The advantageous inventive solution consists in the consistent one Use of two symmetrical, parallel running amplifier trains, formed from alternately connected in series bipolar PNP and NPN transistors in a kind of complementary push-pull circuit with in-phase Control of the two transistors of a stage or corresponding FET transistors, if the noise properties very high demands are made.

This extremely unusual structure has a number of advantages. First, through the double parallel switched to the zero point symmetrical structure achieves that a signal modulation with positive as with negative

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Amplitude in this push-pull structure prevents that Signal as with an amplifier built on one side With a small signal amplitude, small currents, and with a large signal amplitude Large currents flow through the individual semiconductors and that, as a result, have amplitude-dependent transit time properties appear. One encountered this circumstance be! one-sided built up amplifiers by a high quiescent current so that the relative signal modulation does not cause such high percentage current differences. That is here LO avoided according to the invention. It is true, for example, for an NPN emitter follower in the amplifier train at Modulation with a positive signal of one polarity in a decreasing current. At the same time, however, applies in the symmetrical Amplifier train for the PNP emitter follower present there the opposite condition, its current consumes for the corresponding modulation at the amplifier input. Since both amplifier trains have a summation point there, where both collectors of two transistors work together, the different properties in on this double amplifier constructed like a bridge circuit. This principle of compensation of the current-dependent Modulation properties of the amplifier train built up as a bridge allows it to be with extremely little Power requirement to build an amplifier that is also insensitive to levels with large signal amplitudes.

This push-pull bridge circuit according to the invention combines further advantageous properties. For example In this way it is possible to use bipolar transistors to build an amplifier that has an almost has ideal input impedance behavior. This is true

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probably for the purely ohmic component as well as for the capacitive one Component of the input resistance. For this purpose, the two should first - according to an advantageous embodiment - Input emitter followers mutually connected to the base connections and their connection to consider the base connections of the following two transistors connected together via resistors at the emitters will.

It is assumed that there are two transistors with essentially the same current gain (h _fe ). Since both emitter followers have symmetrically identical emitter resistances at symmetrically identical supply voltages (here, for example, + 12 V and - 12 V), the same current flows through their emitters. Since the same base current, but with a different sign, flows through both bases with the same or almost the same gain, one input transistor generates the input current for the complementary input transistor of the same amplifier stage. As a result, the signal source does not need to deliver any current, which results in a very high input resistance. This input resistance is increased again many times over by the additional negative feedback. For paired transistors, input resistances in the teraohm range could be measured in the laboratory. Such an amplifier can, for example, be connected to a 10 MOhm precision series resistor without a measurable error occurring.

The same applies to the input capacitance as for the input resistance of the amplifier. If we take a normal operational amplifier input, e.g. that of a differential

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stronger, then the base-emitter capacitance must be charged by the signal current. This input capacitance works is particularly disadvantageous at the input of the amplifier, namely when the source resistance is relatively high. at The solution according to the invention applies that this input capacitance is extremely small, in the ideal case even zero.

Advantageous further developments of the invention are set out in the subclaims are identified below along with the description of the preferred embodiment the invention illustrated in more detail with reference to the figures. Show it:

Figure 1 shows a first embodiment of the invention Amplifier as well

FIG. 2 shows a variant of the exemplary embodiment according to FIG. 1.

In the embodiment shown in Figure 1, the input signal is fed to the input E, where through the resistor and capacitor combination Rl / Cl and R2 / C2 a potentiometric circuit for setting the gain of the "operational amplifier" is formed.

The input stage consists of a complementary bipolar transistor pair T1 and T2, with the base connections are connected together with the merged connections of the resistors / capacitors R1 / C1 and R2 / C2. This Point represents the "virtual earth", whereby the resistor / capacitor R2 / C2 because of the

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stronger achieved phase rotation of 180 ° generates both a DC and AC voltage negative feedback will.

The amplifier has the properties because of its high overall gain and the overall phase rotation of 180 ° of an operational amplifier. Since the bases of the complementary input transistors T1 and T2 are connected to each other and the transistors have different polarity, the base current is compensated so that its DC voltage component the resistors Rl and R2 are loaded. The complementary first amplifier stage is designed as an emitter follower, a voltage division via the resistors R3 / R5 or R4 / R6 to the transistors T3 and T4 of the following Stage is made. The free connections of the resistors R5 and R6 are on fixed DC voltage potentials, which of the symmetrical voltage supply potentials "+" or "-" over in the direction of flow switched diodes Dl and D2 or D3 and D4 is derived.

The voltage supply potentials "+" and "-" are smoothed by capacitors C5 and C7 or C6 and C8, with capacitors C7 and C8 being electrolytic capacitors are formed, while the capacitors C5 and C6 screen out high frequency components as wound capacitors. The current flowing through the diodes connected in the forward direction forms the collector current for the Transistors of the input stages. An increasing collector current of the input stage causes a DC voltage Negative feedback for the subsequent stages, which

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are connected to this point with their bases, so that the amplifier has more direct voltage despite the fact that it is continuously connected Coupling has high stability. The emitter resistors R9 and RIO of the operated in the emitter circuit Transistors T3 and T4, which are also designed to be complementary, lead with their common connection point via a resistor RlI to the reference potential. Because here too the flowing in and out of the two transistors If the currents compensate each other, their common connection is when the resistors R9 and RIO are of the same size usually at the reference potential.

Should a compensating current occur due to an asymmetry caused by specimen variance, this will be generated Across the resistor RlI a voltage drop which causes one of the negative feedback generated Currents are increased and the other is decreased, so that almost compensation occurs. The voltage drop on the collector resistors R13 and R14 assigned to the transistors T3 and T4 form the input voltage for the Transistors T5 and T6 of the following stage.

These transistors are operated in a base circuit, the base being connected to the DC voltage by the currents is influenced by the transistors of the input stage potentials, so that in this respect also a DC voltage Control effect occurs. The collectors of transistors T5 and T6 are across a common collector resistor R15 connected. The voltage on the collectors goes directly to the bases of the subsequent complementary Stage forming transistors T7 and T8. These

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Transistors are in turn operated in the emitter circuit, the output via the connection point of the the two emitter resistors R17 and R18 is balanced. The latter two resistors form an additional one DC negative feedback. The successive transistor stages T3 and T5 or T4 and T6 form what are known as complementary cascode circuits which are also complementarily symmetrical with one another.

To save components and to initiate the reinforcement with the second stage, the second stage is used used as an inverting push-pull amplifier. Because the potential at the point between the emitters of the transistors fluctuates following the input signal, it then draws a different one proportional to the signal Current through the resistor between the center of the emitter and ground. This current is also passed through the Collectors of the second transistors flow. This current then flows through the between the collectors of the second Transistors and the respective collector resistances to the positive - or negative - supply voltage such that the following two amplifier transistors are current-controlled from the emitter, since their bases in turn are supplied with a constant voltage via the bias voltages formed by each of the two diodes.

Due to the 180 ° phase shift generated by the overall amplifier, it acts as an operational amplifier with a high voltage gain.
30th

The following explanations on the mode of operation are intended to clarify the special properties of the amplifier:

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First of all, in the equivalent circuit diagram of the input, it can be imagined that, as in a bridge, a capacitance over the Base-emitter path of the PNP transistor Tl and a similar one via which the NPN transistor T2 is present. When the polarity of the signal current is reversed, these bridge capacitances cause mutual compensation. Farther the emitters of the input transistors T1 and T2 follow the common base input potential in parallel in one Direction in such a way that the bases and thus the emitters of the following transistors also have a potential change at the same time experienced in the same direction. This fact occurs in the transistors connected downstream of the input transistors T1 and T2 T3 and T4 is of great importance, since any capacitance on the transistors of the second stage and on their Emitter resistors do not have to be reloaded. That is, the first four transistors can take an input signal follow at extreme speed.

As a result, the four input transistors Tl to T4 represent an extremely fast isolating amplifier that with very high input resistance, very small input capacitance has speeds that can only be achieved by this push-pull double emitter follower can be realized. One can consider this push-pull emitter follower bridge to be almost ideal Designate power amplifiers in terms of shape fidelity, in terms of speed and signal propagation time. He offers also the possibility, if one measures at the midpoint between the two second emitters, the extreme one Control capability up to the positive or negative supply voltage.

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The following constituting a differential current amplifier Transistors T5 and T6 work with their high-resistance collectors directly into one another, so that they are once extremely generate high amplification and, on the other hand, at the common collector point the previously separate ones Sum amplifier strips at this point. With this configuration a multiple is achieved. On the one hand, the collectors are mutually exclusive against each other high collector resistance, on the other hand play at this single high-ohmic internal point of the amplifier again the capacities of the subsequent push-pull emitter followers no decisive importance, because these again are not capacitive because of the synchronization of their potentials Represent load.

This amplifier made up of a total of 2x4 transistors has other interesting advantages based on its symmetry. It is well known that NPN transistors are used faster components than similar components because of the better mobility of the charge carriers in the N-doped zones constructed PNP transistors. It can be seen immediately from the circuit that there are PNP and NPN transistors in the upper amplifier train follow constantly alternating. Since in the lower booster train the complementary types alternate Sequence are arranged in the same way, differences in transit time become apparent once in the amplifier trains and only right at the summation when merging both amplifier trains. This compensation is another Advantage of the bridge-like structure.

Furthermore, there is also the requirement for a high level of controllability Taken into account. Since in the first two amplifier

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step the potential fluctuates around the zero point, there is a symmetrically equal modulation to both "+" and after "-" given. Since also in the emitter-driven base-base circuit of the third amplifier stage the constant base voltages are formed by two diodes, so that at the emitters of this third amplifier stage T5, T6 the constant held potential at the emitter is only 0.6 V, this stage, which is called Differential current amplifier acts, practically in each case close to respective battery voltages work so that a modulation of up to about 1 V to the collectors Battery voltages is possible. The same naturally applies to the downstream emitter followers T7 and T8. To this Art was achieved that e.g. with a power supply of + 12 / -12 V a linear modulation of the signal of 22 V peak / peak could be measured without signal distortion or the well-known signal mirroring took place. Of course, the feedback improves these properties considerably.

As already mentioned, it is important for a high-speed amplifier that the capacitance is as small as possible are present and that they are not effective due to low resistance or low resistance sources can. The amplifier takes this into account in a very special way. As already explained, the input capacitance of the amplifier is compensated by the first amplifier stage. Your output, the emitter of the first both emitter followers are low-resistance voltage sources.

The base-emitter capacitances of the next stage cannot take effect because the emitters are mutually dependent on the

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Synchronization of voltages, no reloading of capacities must make the emitter of the second push-pull amplifier stage T3, T4 in turn low-resistance voltage sources represent and also provided with relatively low resistance in the emitter resistor T-circuit are. The same applies to the collectors of this second stage, since they are kept constant at the potential of the Differential current amplifiers are switched, so there are again kept low resistance. That’s up to At this stage hardly any capacities are effective in the two amplifier trains. But now it can be because of the required high no-load gain cannot avoid a high-resistance point in the circuit. But that is the point of the two collectors working together. There the connections have to be kept very short and it has to open Capacities against ground potential are respected. However, there again a push-pull output emitter follower from two transistors is formed, which is hardly a capacitive load because of the synchronous properties to see that all points of the amplifier except for the sum point are emitter-controlled and that thereby the Time constants within the amplifier can be kept low.

The low resistance of almost all network points also has the advantage that interfering signals can hardly speak; provided that the high-resistance sum point in the dimensions small and protected by surrounding screened low-resistance potentials.

It can be shown that the measuring amplifier is thus minimized with regard to its semiconductor components. This way of looking at things allows the following conclusion:

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The first two push-pull bridge successors serve the Separation of a high-ear input for low-resistance signal processing. The second push-pull amplifier stage is used for repeated low-capacitance separation and the inversion of the signals to the respective supply voltage level. The third push-pull stage is used for the actual gain to form the no-load gain and the summation of the bridge amplifier trains initially running separately. The last two transistors serve as' Low-capacitance isolation amplifiers to form the low-impedance amplifier output source. To increase its efficiency, silicon transistors with a high current gain are selected.

It is advantageous that this broadband DC voltage amplifier is used to build this a minimum of components can be used. This fact optimizes this circuit at the same time with regard to the value analytical aspects and also with regard to the noise can be extreme when used low-noise transistors offer an unexpected optimum.

It should be noted that this amplifier also has excellent properties as a DC voltage amplifier, because with the help of the already described double strategy - the use of complementary transistors in a consistent Push-pull circuit and, thanks to the bridge-like structure, the temperature drift of the individual stages is also precisely compensated will.

In the experiment it has been found that the amplifier has a constant DC output voltage over a wide temperature range having.

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Through additional measures, such as the use of low-noise, high-speed silicon transistors and through the Construction in SMD technology with extremely short conductor tracks, an amplifier could be created that all meets the requirements mentioned at the beginning.

While the amplifier, made up of eight bipolar transistors, has very good noise properties, especially in the Has high-frequency range, the properties in the low-frequency range are particularly sufficient for frequencies below 10 Hz (and especially when it is driven with a very high resistance) does not meet all requirements.

The reasons for this lie in the conduction mechanisms of the bipolar transistors and the high requirements be made here by connecting two amplifiers in series and in the circumstance of the high-impedance Control at the entry point of the entire channel amplifier train.

In one embodiment variant, which is shown in FIG. 2, a particularly low-noise behavior of the input stage is achieved. The transistor Tl is replaced by an n-channel junction field effect transistor, which is operated as a source follower, with the transistor Tl ¹ having an (analogously) reversed polarity compared to the bipolar transistor Tl due to the potential ratios present in field effect transistors. While the reference electrode was facing away from the reference potential in the purely bipolar circuit, the reference electrode of the field effect transistor faces the reference potential. as

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Source resistors are used for R4 ' _f R5 ¹ and R7, the resistor R7 being designed as a trimmer and being adjustable to compensate for sample variations. The resistor R4 'generates the voltage drop between the base terminals of the transistors T3 and T4 of the subsequent stage. The diodes D1 and D2 are connected to the drain electrode. However, the drop across these diodes voltage is a measure of the current flowing through the transistor Tl ¹ current in this circuit and not - as is the case in purely bipolar circuit - a measure of the collector current through the transistor T2.

For example, the input amplifier of an oscilloscope is preferred with such an input isolating stage Circuit provided. Field effect transistors have the advantage that in the extremely low frequency range because of the Field effect control mechanisms, twinkle and scintillation noise are far lower than with bipolar transistors. On the other hand, require field effect transistors because of their temperature drift properties and the fact that they are not readily considered complementary types are available, possibly a different circuit structure for the input stage. As shown in figure, two identical field effect transistors are switched so that the currents through both field effect transistors are the same. With the help of the two potentiometers, the Operating point can be set so that for the actual source follower the current from the second field effect transistor, whose gate is closed to ground, is supplied and that thereby over wide temperature ranges

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the operating point voltages remain constant due to the compensation circuit built in this way for the input-source follower, in particular the bias voltage at its gate, which should be 0 V here.
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Since the field effect transistor has the properties of a has a high input resistance, it also fulfills the high resistance required at this point. One could the input capacitance by a push-pull circuit with complementary field effect transistors according to the same Set up the principle as shown for the bipolar transistors.

In the circuit according to FIG. 2, the input stage is asymmetrical executed because complementary field effect transistors can only be produced with a great deal of effort and expense are accordingly expensive. The transistor T2 'forms a constant current source via resistors R6' and R8 is set with respect to its operating point.

The current through the transistor T2 ¹ corresponds to that which flows through the resistor R4 ', the voltage drop at the ^{diodes D3 and D4, which are also connected in series with the transistor T2 1} , serving to generate the direct voltage potential of the subsequent stages.

By providing a second field effect transistor T2 ¹ of identical polarity, the gate terminal of which is not connected to the input, the voltage gain is reduced compared to a completely complementary circuit, by inserting an identical component without the effect of the

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Control electrode is achieved, however, that with regard to the control of the subsequent stages, a quas! Complementary Behavior.

In this way, the advantages of the invention Amplifier without the use of expensive components can also be used for applications in which on value is placed on a high-resistance input.

The structure of the following stages corresponds approximately to the embodiment according to FIG. 1, so that for matching Components identical reference numerals could be used.

The embodiment of the invention is not limited to the preferred exemplary embodiment specified above. Rather, a number of variants are conceivable, which of the solution shown also in principle make use of different designs.

Claims (13)

subsequently submitted I. CREATEC March 25, 1985 Gesellschaft für Elektrotechnik mbH
D-1000 Berlin Broadband signal amplifier Expectations lj Multistage DC-coupled broadband signal amplifier with transistors of different conductivity types, in particular for amplifying the input analog signal in an oscilloscope, / 2 Nachgerek
CR35.6 page 2 REPORTEDJ characterized, that at least two successive amplifier stages between the supply potentials symmetrically in pairs have wired transistors of different conductivity types, the respective successive Transistors also of different conductivity types are and the two transistors of a stage each have the complete useful signal in the same phase position received and processed. 2. Broadband signal amplifier according to claim 1, characterized in that at least one of the successive stages is designed as a symmetrical emitter follower. 3. Broadband signal amplifier according to claim 2, d a characterized in that two consecutive Stages as a symmetrical cascode circuit, i.e. as successive transistors in emitter and basic circuit, with transistors of different conductivity types are formed. 4. Broadband signal amplifier according to claim 3, characterized in that
that the symmetrical cascode circuit is connected upstream and / or downstream of a symmetrical emitter follower circuit. CR35.6 page 3 SUBMITTED 5. Broadband signal amplifier according to one of the preceding claims, characterized in that that the mean DC voltage level of the signals at the input or output connection with respect to a reference potential is zero. 6. Broadband signal amplifier according to one of the preceding claims, characterized in that that the control electrodes of the input transistors are directly connected to one another. 7. Broadband signal amplifier according to one of the preceding claims, characterized in that that the input and output signals of the amplifier have a phase shift of 180 ". 8. Broadband signal amplifier according to one of the preceding claims, characterized in that that field effect transistors are provided in place of the bipolar in the respective equivalent circuit. 9. Broadband signal amplifier according to one of the preceding claims, characterized in that that two field effect transistors of matching polarity are provided in series as the input stage, from one as a source follower and the other as a constant current source is connected, the gate terminal CR35.6 page 4 I submitted later of the source follower forms the amplifier input and the control electrodes of the complementary transistors of the following Stage to which the terminals of a resistor are connected between the source resistance of the Source follower and the constant current source is switched on. 10. broadband signal amplifier according to claim 9, d a characterized in that the source resistors of the field effect transistors connected as source follower and constant current source have the same values. 11. Broadband signal amplifier according to claim 10, characterized in that the source resistors designed as trimming resistors to compensate for sample variations in the field effect transistors are. 12. Broadband signal amplifier according to one of the preceding claims, characterized in that that the transistors are made in silicon technology. 13. Broadband signal amplifier according to one of the preceding claims, characterized in that that input and output connections and the voltage CR35.6 FOLLOWED Power supply connections of the amplifier form a bridge circuit. / 6