GB2261784A

GB2261784A - An amplifier stage

Info

Publication number: GB2261784A
Application number: GB9122827A
Authority: GB
Inventors: Clive Button
Original assignee: TRACE ELLIOT Ltd
Current assignee: TRACE ELLIOT Ltd
Priority date: 1991-10-28
Filing date: 1991-10-28
Publication date: 1993-05-26
Anticipated expiration: 2011-10-28
Also published as: GB2261784B; GB9122827D0

Abstract

An audio amplifier stage, for example for a guitar output, has a push-pull driver M1, M2 driving a loudspeaker RL by way of a transformer T1. An input signal S is amplified by an amplifier A1 and fed directly to one MOSFET M1 of the push-pull driver, and by way of an inventor A2, R5, R6 to a second MOSFET M2. A negative voltage feedback is connected from the load circuit to the inventing input of the amplifier A1 by way of a resistor R3. Current feedback is fed by way of a resistor R4 to that same inventing input. The current feedback is adjustable by way of a potentiometer P1 and may be positive, to cancel the negative feedback, or negative to support and augment the negative feedback. Adjustment of the potentiometer P1 enables the distortion characteristics of the amplifier stage to be continuously varied. <IMAGE>

Description

AN AMPLIFIER STAGE The present invention relates to an amplifier stage, for example, to an audio frequency amplifier stage.

It is an object of the invention to provide an amplifier stage having continuously variable distortion characteristics.

According to a first aspect of the present invention there is provided an amplifier stage comprising a signal input, an amplifier circuit having an input and an output, the amplifier input being coupled to said signal input, and push-pull driver means coupled to said amplifier output, the amplifier stage further comprising a load circuit coupled to said push-pull driver means and arranged to respond to a signal applied to said signal input, and feedback means coupling the load circuit to said signal input, wherein said feedback means are adjustable to selectively provide either negative or positive feedback whereby distortion characteristics of the amplifier stage are variable.

It will be appreciated that the feedback means are sampling the load performance and feeding a representative of this to the signal input. If the feedback is negative, and hence acts to oppose the input, there will generally be an improvement of the load performance sampled, whilst if the feedback is positive and fed to support the input, the feedback introduces distortion. Accordingly, it will be appreciated that in an amplifier stage of the invention the level of distortion of the amplifier stage can be chosen as required.

Preferably, the amplifier stage is arranged for use at audio frequencies. Accordingly, the ability to choose the distortion characteristics of the amplifier stage may be utilised to provide special audio effects. The characteristic sound of an electric guitar, for example, arises because of distortion in its amplifier, and the amplifier stage of the invention can have distortion specifically introduced so as to replicate the sound desired. Additionally and/or alternatively, the choice of the distortion characteristics may be made to match the amplifier response to other variables. For example, the load circuit may be, or may incorporate a loudspeaker.

Although a loudspeaker has a nominal impedance, that impedance does vary with frequency, and the distortion characteristics of the amplifier stage may be used to match the amplifier response to the actual loudspeaker impedance.

In a preferred embodiment said feedback means comprises a potentiometer coupled to said load circuit, a wiper of said potentiometer being coupled to said signal input.

The potentiometer may be coupled across the load of the load circuit, for example across the loudspeaker.

Additionally and/or alternatively, the potentiometer may be coupled across the input to the load circuit.

Preferably, said load circuit comprises a load coupled to an input by way of resistance means, and said potentiometer is connected across said resistance means.

The resistance means may be one or more resistors connected in series between the input of the load circuit and said load.

In an embodiment said push-pull driver means comprises a push-pull circuit and an output transformer, a primary of said transformer being coupled to said push-pull circuit, and a secondary of said transformer being coupled to the load circuit.

Preferably said push-pull circuit is arranged to have an impedance to oppose the response of said load circuit.

The push-pull circuit may comprise first and second FETs having their sources connected to ground and their gates capacitively coupled to said amplifier output, the gate of the first FET being coupled directly to the amplifier output, and inverter means coupling the amplifier output to the gate of the second FET.

Generally, the FETs are biased on, for example, by the application of an appropriate bias voltage to their gates.

In a preferred embodiment, the FETs are MOSFETs for example, N-Channel MOSFETs.

The inverter means providing the inverted signal to the second FET may comprise any suitable means. In an embodiment, the inverter means comprises a unity gain amplifier, for example, an appropriately biased operational amplifier.

In a preferred embodiment, said amplifier circuit comprises an operational amplifier having a non-inverting input forming said amplifier input and coupled to said signal input, the operational amplifier also having an inverting input coupled to an output of the amplifier stage.

Where an operational amplifier is utilised, it will generally be provided with negative feedback as is conventional. For example, the output of the operational amplifier may be coupled to its inverting input.

Additionally and/or alternatively, any property of the amplifier stage may be appropriately sampled and fed back to the inverting input of said operational amplifier.

In an amplifier stage of the invention, the load circuit is already sampled by said feedback means to provide for variability of the distortion characteristics.

In this case therefore, it is preferred that negative feedback to the operational amplifier is from the load circuit to provide a mean level for the distortion characteristics. Thus, the inverting input of said operational amplifier is preferably coupled to the load circuit. This may be instead of, or together with a conventional negative feedback coupling the output of the operational amplifier to its inverting input.

According to a further aspect of the present invention, there is provided an amplifier stage comprising a signal input, an amplifier circuit having an input and an output, the amplifier input being coupled to said signal input, and driver means coupling said amplifier circuit to a load circuit such that said load circuit responds to a signal applied to said signal input, wherein said driver means is arranged to restrict the response of said load circuit, and further comprising feedback means coupling said load circuit to said signal input.

An embodiment of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawing which shows a circuit diagram of an amplifier stage of the invention.

The amplifier stage shown in the figure is particularly designed for use as the output stage for a guitar. However, it will be understood that the amplifier stage is not confined for use as a guitar output stage, and may be utilised as the output stage of other musical instruments and/or for other audio frequency applications.

Indeed, the amplifier stage may be used in other circumstances where an amplifier stage having variable distortion characteristics would be useful.

The amplifier stage illustrated comprises an operational amplifier Al whose non-inverting input is connected to a signal input S. The signal applied to the input S is arranged to be amplified by the operational amplifier Al and then applied to a load RL by way of a push-pull driver. The push-pull driver comprises a pair of N-Channel MOSFETs M1, M2 whose sources are grounded. A positive bias voltage is applied to the gate of each MOSFET M1, M2 by way of a respective bias resistor R7, R8.

Generally, the positive bias voltage applied to its gate is sufficient to bring each MOSFET to the point of conduction such that it will conduct when a positive going signal is applied to the gate, but does not conduct when a negative signal is applied. The gates of the two MOSFETs M1 and M2 are capacitively coupled by respective capacitors C1 and C2 to receive the output of the amplifier Al.

To enable the push-pull drive, the MOSFET M2 is arranged to receive the inverse of the signal applied to the MOSFET M1. In this respect, the operational amplifier Al is arranged to amplify the signal applied to the input S and its gain is determined by the ratio of resistors R1 and R2 which provide a conventional feedback path from the output to the inverting input of the operational amplifier Al. It will be seen that the output of the amplifier Al also is coupled by way of the capacitor C1 to the gate of the MOSFET M1. The output of the amplifier Al is also feed by way of a resistor R5 to the inverting input of a second operational amplifier A2. In known manner, the noninverting input of the operational amplifier A2 is connected to ground and its output is connected to the inverting input by way of a resistor R6.The resistors R5 and R6 are arranged to have the same resistance such that the operational amplifier A2 acts as a unity gain inverting amplifier and thereby applies the inversion of the output of the amplifier Al to the MOSFET M2 by way of the capacitor C2. It will therefore be appreciated that the two MOSFETs M1, M2 drive the primary of a transformer T1 in push-pull. The secondary of the transformer T1 is connected to drive the load RL in response to the signal applied at input S.

For small drain currents the MOSFETs M1 and M2 act as resistors, and in the particular embodiment illustrated each MOSFET is arranged to have an ON resistance of the order of 2 Ohms. If the load RL, for example a loudspeaker, has a low resistance, say 8 Ohms, then it will demand a large current from the secondary of the transformer T1 and a correspondingly large current will be demanded from the primary circuit by way of the MOSFETs M1 and M2. However, the relatively high internal ON resistances of the or both MOSFETs M1 and M2 will act to limit the actual current in the primary circuit and therefore reduce the output. The MOSFETs M1 and M2 will therefore oppose the demands of the load RL.

If the load RL has a relatively high resistance; for example of the order of 200 Ohms, then the internal resistances of the MOSFETs become negligible. The reduced current demanded by the load RL will be increased because of the low resistance in the primary circuit. In this respect, it will be recognised that a loudspeaker, for example, with a nominal impedance of 8 Ohms might have an impedance as high as 200 Ohms at certain frequencies and will therefore be looking to draw quite disparate currents at the different frequencies. It will also be appreciated that the push-pull driver circuit will act to restrict the limits of the response demanded by the loudspeaker.

The amplifier stage as described so far is substantially conventional. However, the stage is also provided with means for introducing a controllable level of distortion to the signal to be output. In this respect, the amplifier stage includes two feedback paths. The first is connected between the load circuit containing the load RL and the inverting input of the operational amplifier Al by way of a resistor R3. It will therefore be seen that this first feedback path provides negative feedback which is dependent upon the voltage of the load circuit. A second feedback path may provide positive or negative feedback and is responsive to the current of the load circuit.In this respect two low value resistors R9 and R10 are connected in the load circuit, the resistor R9 being connected in series with the secondary of the transformer T1 and the resistor R10 being connected in series to the load RL. The connection of the two resistors R9 and R10 is grounded. A potentiometer P1 is connected across the resistors R9 and R10, one end of the potentiometer track being connected to the resistor R9, and the other end of the track being connected to the resistor R10. The wiper of the potentiometer P1 is connected to a feedback resistor R4 which is connected to the inverting input of the operational amplifier Al. It will be appreciated that the position of the wiper along the potentiometer P1 determines whether positive or negative feedback is applied by way of the second feedback path.

The first, negative, voltage feedback applied by way of the resistor R3 is used to provide a mean level for the non-linearity of the amplifier stage. This then enables variations of the current feedback by way of the resistor R4 to act as either negative or positive feedback. If the wiper of the potentiometer P1 is moved to its end connected to the resistor R9, for example, positive feedback is applied to the amplifier Al, and this will be cancel the negative feedback applied by way of the resistor R3. Thus, the stage is subjected to maximum distortion and maximum non-linearity.Moving the wiper of the potentiometer P1 to the other end of its track adjacent the resistor R10 provides maximum negative current feedback and this adds to the negative voltage feedback by way of the resistor R3 and thereby reduces the distortion to a minimal level. It will be appreciated that the non-linear distortion produced by the amplifier stage can be continuously varied by appropriately positioning the wiper of the potentiometer P1 along the track.

The circuit described above and illustrated is given by way of example only, and variations and amendments thereto can be made as required. For example, the inversion of the signal applied to the gate of the second MOSFET M2 may be achieved in any suitable manner. Thus, any appropriate inverter means may be utilised.

Alternatively, the N-channel MOSFETs may be replaced by Pchannel devices and or the biasing of the MOSFETs may be altered to provide for the push-pull drive without the use of an inverter.

It is preferred that the push-pull driver be implemented using FETs, but bi-polar transistors could be utilised, if preferred. In this instance, the push-pull driver should also be provided with an impedance arranged to oppose the demands of the load.

It will be appreciated that other variations and modifications to the invention as described and illustrated may be made within the scope of the appended claims.

Claims

1. An amplifier stage comprising a signal input, an amplifier circuit having an input and an output, the amplifier input being coupled to said signal input, and push-pull driver means coupled to said amplifier output, the amplifier stage further comprising a load circuit coupled to said push-pull driver means and arranged to respond to a signal applied to said signal input, and feedback means coupling the load circuit to said signal input, wherein said feedback means are adjustable to selectively provide either negative or positive feedback whereby distortion characteristics of the amplifier stage are variable.

2. An amplifier stage as claimed in Claim 1, wherein said feedback means comprises a potentiometer coupled to said load circuit, a wiper of said potentiometer being coupled to said signal input.

3. An amplifier stage as claimed in Claim 2, wherein said load circuit comprises a load coupled to an input by way of resistance means, and said potentiometer is connected across said resistance means.

4. An amplifier stage as claimed in any preceding claim, wherein said push-pull driver means comprises a push-pull circuit and an output transformer, a primary of said transformer being coupled to said push-pull circuit, and a secondary of said transformer being coupled to the load circuit.

5. An amplifier stage as claimed in Claim 4, wherein said push-pull circuit is arranged to have an impedance to oppose the response of said load circuit.

6. An amplifier stage as claimed in Claim 5, wherein said push-pull circuit comprises first and second FETs having their sources connected to ground and their gates capacitively coupled to said amplifier output, the gate of the first FET being coupled directly to the amplifier output, and inverter means coupling the amplifier output to the gate of the second FET.

7. An amplifier stage as claimed in Claim 6, wherein said inverter means comprises a unity gain amplifier.

8. An amplifier stage as claimed in any preceding claim, wherein said amplifier circuit comprises an operational amplifier having a non-inverting input forming said amplifier input and coupled to said signal input, the operational amplifier also having an inverting input coupled to an output of the amplifier stage.

9. An amplifier stage as claimed in Claim 8, wherein the inverting input of said operational amplifier is coupled to said load circuit.

10. An amplifier stage comprising a signal input, an amplifier circuit having an input and an output, the amplifier input being coupled to said signal input, and driver means coupling said amplifier circuit to a load circuit such that said load circuit responds to a signal applied to said signal input, wherein said driver means is arranged to restrict the response of said load circuit, and further comprising feedback means coupling said load circuit to said signal input.

11. An amplifier stage as claimed in Claim 10, wherein said feedback means are arranged to provide either negative or positive feedback such that the feedback is arranged to support or oppose the response restriction on the load circuit.

12. An amplifier stage as claimed in Claim 10 or 11, and as claimed in Claim 2 or 3.

13. An amplifier stage as claimed in any of Claims 10 to 12, wherein said driver means comprises push-pull driver means, and as claimed in any of Claims 4 to 9.

14. An amplifier stage substantially as hereinbefore described with reference to the accompanying drawing.