WO1993018582A1

WO1993018582A1 - Controlling amplifiers

Info

Publication number: WO1993018582A1
Application number: PCT/GB1993/000489
Authority: WO
Inventors: Keith Jones
Original assignee: Keith Jones
Priority date: 1992-03-11
Filing date: 1993-03-09
Publication date: 1993-09-16
Also published as: GB9205295D0

Abstract

The specification describes a method of operating an amplifier, and an amplifier, having an output stage receiving an input signal and generating an output signal, which further includes monitoring the output stage quiescent current and controlling the input to the output stage in response to the monitored level to prevent the quiescent current falling below a predetermined level. Current monitoring is effected in the current path of the output stage remote from the signal path and provides that a voltage is fed negatively to the stage input whilst the stage quiescent current is less than a second predetermined level to maintain the quiescent current in the stage at a pre-set level. A described amplifier includes a pair of emitter-coupled transistors the collectors of which are coupled to respective DC supply rails, the collector circuit of one or both transistors including a shunted pair of linear (resistive) and non-linear (diode) elements across which the feed back voltage is developed. The input signal is fed to the transistor bases across a resistor driven via the base-collector path or paths of one or more further transistors to the emitter (of each) of which is applied the feed back voltage. The or each path carrying the feed back voltage includes a resistor and a capacitor is provided coupling the emitter of the or each further transistor to a respective supply rail to effect AC isolation of the arrangement.

Description

CONTROLLING AMPLIFIERS

DESCRIPTION

The invention relates to methods and apparatus for controlling amplifiers, in particular but not exclusively for reducing distortion in electronic audio amplifiers.

One particular area of distortion in electronic amplification is caused by temperature effects upon the quiescent current in the amplifier, that is to say changes in ambient temperature (or even heat generated by elements within the amplifier itself) may cause alterations in the operational characteristics of the amplifier. Typically an increase in the temperature of an active component, for example a transistor, in an amplifier will mean that for a given voltage thereacross the component will carry more current. The increase in current passing through the component will lead to a further increase in temperature in the device again increasing the current which can be taken by the device leading, in a cyclical manner, to the effect known as Thermal Run Away.

Proposals have been made to reduce this effect in amplifiers which include the incorporation of resistive elements in the output stage of the amplifier which elements act to increase the total resistance of the stage and thus limit current flow through the active elements of the stage. Such an arrangement is shown in Figure 1 which shows the output stage of a class A-B amplifier to include transistors 10 and 12 emitter coupled to provide an output at 14 to a load 16. The output stage, comprising transistors 10 and 12, is driven by a driver 18 passing a signal at 20 across a resistor 22 coupled to the bases of transistors 10 and 12.

In the quiescent or steady state where the signal provided at 20 does not vary, resistor 22 provides a quiescent bias for transistors 10 and 12 maintaining them conductive.

To reduce the effects of alteration of this quiescent current, in particular to reduce the chances of Thermal Run Away, the circuit further includes resistors 24 and 26 coupled in the emitter paths of transistors 10 and 12. The resistors 24 and 26 are shunted by capacitors 28 and 30 as shown in an attempt to keep the impedance of the stage as low as possible. Such an arrangement protects the output stage from the effects of quiescent current variation and in particular from Thermal Run Away effectively by limiting the quiescent or steady state current passing through transistors -10 and 12. However, this proposal introduces difficulties in the operation of the amplifier and in particular introduces distortion to the output signal provided by the amplifier.

Whilst there is a practical limit on the size of the capacitors 28 and 30 the circuit, as shown, must provide some low frequency signal distortion.

Additionally, the resistors 24 and 26 will react to increase other forms of distortion - particularly that known as Cross Over Distortion (one of the transistors 10 and 12 stops acting as an amplifier before the other transistor 12 or 10 has started to amplify).

Objects of the present invention include the provision of a method of operating an amplifier, and an amplifier, in which flow of quiescent current through the output stage of the amplifier is controlled.

In one aspect the present invention provides a method of operating an amplifier with an output stage to which an input signal is fed to generate an output signal for a load, which method comprises the steps of monitoring current level in the output stage of the amplifier, generating a signal in response to the monitored level and applying the generated signal so as to modify the signal input of the output stage thereby to prevent the quiescent current in that stage falling below a predetermined level. The generated signal may be applied to modify the signal input to the output stage whilst the quiescent current in that stage is above said minimum level will act to maintain said quiescent current at a pre-set level.

In a second aspect the invention provides an amplifier having an output stage for providing an output signal to a load in response to an input signal fed thereto, and means for monitoring current in the output stage and generating a control signal which is used to modify the signal to input to the output stage , so as to prevent the quiescent current in the output stage falling below a predetermined level.

The generated control signal may be fed to the input of the output stage.

The monitoring means is preferably located in the current path of the output stage remote from the signal path.

The monitoring means is desirably operable to modify the quiescent current in the output stage whilst that current is below the predetermined level to maintain the quiescent current at said desired pre-set level.

The monitoring means is with advantage arranged to be substantially disabled if current in the output stage of the amplifier exceeds a second predetermined level.

Means may be provided for isolating the control signal generated by the monitoring means from alternating signals.

Advantageously, the monitoring means includes a resistive or other linear element in the current path of the output stage across which a control voltage is developed, which control voltage is used to modify the signal" input to the output stage so as to prevent the quiescent current in the output stage falling below said second predetermined level.

The control voltage is preferably used to effect negative feedback and maintain the quiescent current in the output stage at said desired pre-set level.

The monitoring means may further include, in parallel with the resistive or other linear element, an element having a non-linear response to an applied potential which non-linear element is operable to provide substantially zero resistance to current flow should the voltage across the resistive or other linear element exceed a predetermined value.

One amplifier embodying the invention comprises a pair of amplifying elements coupled together and to respective potential voltage supplies, and wherein the output of the amplifier is taken from the coupled amplifying elements, wherein the circuit coupling at least one of the amplifying elements to a potential voltage supply includes a shunted pair of linear and non-linear elements across which is generated the control voltage applied to the input signal fed to said amplifying elements.

This embodiment may be implemented by a pair of transistors the emitters of which are coupled together and the collectors of which are coupled to respective potential voltage supplies, wherein the output of the amplifier is taken from the coupled emitters of the two transistors, and wherein the collector circuit of at least one of the transistors includes a shunted pair of linear and non-linear elements across which is generated the control voltage applied to the input signal fed to the bases of said transistors.

The or each of shunted pair of linear and non- linear_elements respectively may comprise a resistor and a diode.

The bases of the emitter-coupled transistors are preferably coupled across biasing means to which the input signal is applied; the biasing means may comprise a resistor. The input signal is advantageously applied to the resistor coupling the bases of the emitter-coupled transistors via the: base-collector path of a further transistor to the emitter of which is applied the control voltage developed across the said at least one shunted pair of linear and non-linear elements.

The input signal may also advantageously be applied to the resistor coupling the bases of the emitter- coupled transistors via the base-collector paths of two further transistors to the emitters of which are applied control voltages developed across shunted pairs of linear and non-linear elements in the respective collector circuits of each of the emitter-coupled transistors.

With advantage the or each path carrying a control voltage to the emitter of the or each further transistor includes means for reducing AC feedback; said means for reducing AC feedback may be provided by a resistor in the emitter path of the or each further transistor and a capacitor coupling the or each emitter to a respective supply rail.

The control voltage is preferably operable to effect change in the generating conditions of the output stage.

The above and other aspects, features and advantages of the present invention will become apparent from the following description of embodiments of the invention now made to reference to the accompanying drawings, in which:-

Figure 2 schematically illustrates the method and apparatus of the present invention.

Figure 3 illustrates a first amplifier embodying the invention.

Figure 4 illustrates a further amplifier embodying the present invention, and

Figure 5 illustrates another amplifier embodying the present invention. Figure 2 schematically shows the output stage of an amplifier coupled between positive and negative voltage supply rails 100 and 102, in particular having coupled thereacross two output devices 104 and 106. As illustrated in Figure 2 devices 104 and 106 are Bi-polar transistors (although any other form of active amplifying device e.g. Field Effect Transistors S.C.R or the like may be used). The emitters of the devices 104 and 106 are coupled as shown and provide an output at 108 to a load 110. A signal input to the output stage, as shown at

112, is fed to the base of a transistor 114 the collector of which is coupled to positive supply rail 100 by a voltage divider 116, 118; whilst its emitter is coupled to a current monitoring device 120 in the collector path of the transistor 106.

Current monitoring device 120 monitors the quiescent current in the collector path of transistor 106 and provides a voltage which is fed to the emitter of the transistor 114. In the steady state - with no signal or an invarient signal fed to the base of transistor 114 - a stable voltage is be developed across resistor 118. This acts to provide the quiescent or bias current required by transistors 104 and 106 to maintain their operation. This current is monitored by device 120 and any increase in the collector current of transistor 106 will cause the current monitoring device 120 to increase the voltage at the emitter of transistor 114. As the signal 112 fed to the base of transistor 114 is steady the increase in the emitter voltage of the dev»ice will have the effect that 0 the collector voltage of transistor 114 will rise reducing the voltage across resistor 118 and acting to reduce the collector current in transistor 106.

Whilst the input signal 112 applied to the base of transistor 114 is steady the current monitoring device ' -^■ will, in response to changes in the collector current of transistor 106 act to generate a voltage fed to the emitter of 114 which will maintain the quiescent current in the output stage of the amplifier at a generally desired, pre-set, level.

20 Additionally the current monitoring device 120 is arranged, should an alternating current signal be applied to input 112, to be effectively disabled.

Figure 3 illustrates one implementation of the device 120 in accordance with the invention. Parts of the

25 circuit shown in this Figure which correspond to parts shown in Figure 2 are given the same reference numerals.

In this arrangement the signal input to the amplifier at 112 is fed to the base of transistor 114 via a capacitor 122 coupled to the negative supply rail 102 by

30 resistor 124 and battery 126.

It is to be noted that any constant current source could be used at this position to ensure that a steady quiescent or steady state voltage is applied to the base of transistor 114 to cause this transistor to develop

35 the desired quiescent bias potential across resistor 118. The monitoring circuit 120 (shown within the dotted lines) comprises a resistor 128 and diode 130 coupled in parallel in the collector path of transistor 106.

It is to be noted that with the particular configuration shown the current monitoring device 120 presents an impedance to current flow in the collector path of transistor 106 which is essentially linear when that current flow is near or below a first predetermined level. It will be seen that as current flow through transistors 104 and 106, and therefore in the collector path ^"of transistor 106 increases, the voltage across resistor 128 will increase driving the voltage at the emitter of transistor 114 up. As the steady state voltage at the base of transistor 114 is constant, the collector current through transistor 114 reduces and the voltage across resistor 118 also reduces. This in turn acts to reduce current flow through transistors 104 and 106.

Similarly, in the steady state, should the current in the collector path of transistor 106 fall the potential voltage applied to the emitter of transistor 114 by device 120 will reduce and the potential voltage across resistor 118 will increase causing transistors 104 and 106 to conduct more current.

In the steady state therefore the current monitoring device 120 acts to provide negative feedback controlling the quiescent current in the output stage of the amplifier.

It is to be noted that no elements (linear or non-linear) are in the signal path of the output stage of the amplifier making use of this arrangement.

When an alternating signal is applied to the input 112, the effect of resistor 128 in the current monitoring device 120 is over-ridden as diode 130 is driven into full conduction.

With the particular configuration shown it is envisaged that a rise in voltage on the base of transistor 114 of only 0.25 volts will be sufficient to drive diode 130 into for conduction, thereby effectively disabling the negative feedback aspects of the current monitoring device 120. The dynamic performance of the arrangement may be improved if the signal on the negative feedback path to the emitter of transistor 114 is isolated. Such an arrangement is shown in Figure 4 in which a further resistor 132 and a capacitor 134 are provided, as shown. In the arrangement of Figure 4 a further additional resistor 136 is shown provided, to generate a DC bias for transistor 114.

The additional components shown in Figure 4, resistors 132 and 136 and capacitor 134, provide alternating current isolation of the signal from the current monitoring device 120 to the emitter of transistor 114. These components, however, will also reduce, to an extent, the effectiveness of the arrangement in controlling quiescent current changes caused by temperature variations.

This control may be restored by modifying the amplifier such as is shown in Figure 5.

In Figure 5 the signal input at 112 to the amplifier is fed to the centre point of a potential divider string incorporating resistors 140, 142, 144 and 146 as shown.

Between resistors 144 and 146 a tap is taken to drive the base of transistor 114. The emitter circuit of transistor 114 includes the arrangement shown in Figure 4 - that is to say the current monitoring device 120 and AC isolation elements 132 and 134 and emitter bias resistor 136 as shown. Between resistors 140 and 142 a further tap is taken to the base of a transistor 114' the emitter circuit of which includes a current monitoring device 120* including a resistor 128* and diode _.,130', AC isolation elements 132' and 134* as shown and an emitter bias resistor 136'. In this arrangement the collectors of the transistors 114 and 114' are coupled, as shown, to the resistor 118 providing the quiescent bias for the output transistors 104 and 106. It will be seen that the elements of the arrangement of Figure 5 identified with reference numerals which are prime present a "mirror image" of the remaining elements of the circuit.

As any variation in the collector current of transistor 106 will be mirrored by an equivalent variation in the collector current in transistor 104 the variation in potential voltages applied to the emitter of transistors 114 by monitoring device 120 will be substantially equal to the variation in potential applied to the emitter of transistor 114' by monitoring device 120'.

Thus an increase, (or decrease) in the current in the collector paths of transistors 104 and 106 will have the effect of reducing (or increasing) the potential voltage across resistor 118 and thereby the quiescent current in the output stage.

In this way the voltage across resistor 118 will be modified, by the devices 120 and 120*, to prevent thermal runaway effects. i will be appreciated that various modifications may be made to the arrangement described without departing from the scope of the invention.

For example transistor(s) 114 may be replaced by any amplifying or voltage level shifting device operable to achieve the desired effects. Such devices may comprise thermionic valves. Unipolar or Bipolar devices; for example including FET's, MOSFET's and transistor SCR's.The resistor 118 may be replaced with any suitable device, which will generate sufficient potential voltage across its terminals to provide bias for the desired transistors 104 and 106. The resistor may be replaced by any suitable combination of resistive, resistive/semiconductor or semiconductor/semiconductor elements or by a constant current source.

Various particular elements of the circuit illustrated may be replaced by other functionally equivalent elements or devices. Typically all components would be implemented within a monolithic (integrated) circuit by use of resistor/transistor or transistor/transistor elements. It will be seen from the above description that embodiments of the invention provide a current monitoring device operable to maintain the quiescent current level in the output stage of an amplifier above a predetermined level - and which moreover provides feedback tending to drive that quiescent current level to a pre-set desired level.

The monitoring means furthermore has a reduced, substantially zero, effect when a varying signal is applied to the amplifier. Thus the current monitoring circuit effectively provides two stage control of current flow in the output stage of the amplifier.

It will further be seen that the arrangements of the invention enable the direct interconnection of the two output devices (transistors 104 and 106 of Figure 3) without need for interposition of any linear or non-linear elements in the signal path which would have a deleterious and distorting effect on output signal.

Claims

1. A method of operating an amplifier with an output stage to which an input signal is fed to generate an output signal for a load, which method comprises the steps of monitoring current level in the output stage of the amplifier, generating a signal in response to the monitored level and applying the generated signal so as to modify the signal input of the output stage thereby to prevent the quiescent current in that stage falling below a predetermined level.

2.— A method as claimed in Claim 1 , wherein it is provided that the generated signal applied to modify the signal input to the output stage whilst the quiescent current in that stage is above said minimum level will act to maintain said quiescent current at a pre-set level.

3. An amplifier having an output stage for providing an output signal to a load in response to an input signal fed thereto, and means for monitoring current in the output stage and generating a control signal which is used to modify the signal to input to the output stage, so as to prevent the quiescent current in the output stage falling below a predetermined level.

4. An amplifier as claimed in Claim 3, wherein the generated control signal is fed to the input of the output stage.

5. An amplifier as claimed in Claim 3 or Claim 4, wherein the monitoring means is located in the current path of the output stage remote from the signal path.

6. An amplifier as claimed in any one of Claims 3, 4 and 5, wherein the monitoring means is operable to modify the quiescent current in the output stage whilst that current is above the predetermined level to maintain the quiescent current at said desired pre-set level.

7. An amplifier as claimed in any one of Claims 3 to 6, wherein the monitoring means is arranged to be substantially disabled if current in the output stage of the amplifier exceeds a second predetermined level.

8. An amplifier as claimed in any one of claims 3 to 7, wherein means are provided for isolating the control signal generated by the monitoring means from alternating signals.

9. An amplifier as claimed in Claim 6, Claim 7 or Claim 8, wherein the monitoring means includes a resistive or other linear element in the current path of the output stage across which a control voltage is developed, which control voltage is used to modify the signal input to the output—stage so as to prevent the quiescent current in the output stage falling below said second predetermined level.

10. An amplifier as claimed in Claim 9, wherein the control voltage is used to effect negative feedback and maintain the quiescent current in the output stage at said desired pre-set level.

11. An amplifier as claimed in Claim 9 or Claim 10, wherein the monitoring means further includes, in parallel with the resistive or other linear element, an element having a non-linear response to an applied potential which non-linear element is operable to provide substantially zero resistance to current flow should the voltage across the resistive or other linear element exceed a predetermined value.

12. An amplifier as claimed in any one of claims 9, 10 and 11, and which comprises a pair of amplifying elements coupled together and to respective potential voltage supplies, and wherein the output of the amplifier is taken from the coupled amplifying elements, wherein the circuit coupling at least one of the amplifying elements to a potential voltage supply includes a shunted pair of linear and non-linear elements across which is generated the control voltage applied to the input signal fed to said amplifying elements.

13. An amplifier as claimed in any one of claims 9, 10 and 11, and which comprises a pair of transistors the emitters of which are coupled together and the collectors of which are coupled to respective potential voltage supplies, wherein the output of the amplifier is taken from the coupled emitters of the two transistors, and wherein the collector circuit of at least one of the transistors includes a shunted pair of linear and non¬ linear elements across which is generated the control voltage applied to the input signal fed to the bases of said transistors.

14-.- An amplifier as claimed in Claim 11 or Claim 12 or Claim 13, wherein the or each of shunted pair of linear and non-linear elements respectively comprise a resistor and a diode.

15. An amplifier as claimed in Claim 13, or Claims 13 and 14, wherein the bases of the emitter-coupled transistors are coupled across biasing means to which the input signal is applied.

16. An amplifier as claimed in Claim 15, wherein said biasing means comprises a resistor.

17. An amplifier as claimed in Claim 16, wherein the input signal is applied to the resistor coupling the bases of the emitter-coupled transistors via the base-collector path of a further transistor to the emitter of which is applied the-control voltage developed across the said at least one shunted pair of linear and non-linear elements.

18. An amplifier as claimed in Claim 16, wherein the input signal is applied to the resistor coupling the bases of the emitter-coupled transistors via the base-collector paths of two further transistors to the emitters of which are applied control voltages developed across shunted pairs of linear and non-linear elements in the respective collector circuits of each of the emitter-coupled transistors.

19. An amplifier as claimed in Claim 17 or Claim 18, wherein the or each path carrying a control voltage to the emitter of the or each further transistor includes means for reducing AC feedback.

20. An amplifier as claimed in Claim 19, wherein said means for reducing AC feedback is provided by a resistor in the emitter path of the or each further transistor and a capacitor coupling the or each emitter to a respective supply rail.

21. An amplifier as claimed in Claim 17 and either one of Claims 18 and 19, wherein the control voltage is operable to effect change in the generating conditions of the output stage.

23. A method of operating an amplifier as claimed in Claim 1 and substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

24. An amplifier as claimed in Claim 3 and substantially as hereinbefore described with reference to Figure 2, or Figure 3, or Figure 4, or Figure 5 of the accompanying drawings.