GB2439983A

GB2439983A - Frequency compensation for an audio power amplifier

Info

Publication number: GB2439983A
Application number: GB0613926A
Authority: GB
Inventors: Paul Martin Whatley
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-07-13
Filing date: 2006-07-13
Publication date: 2008-01-16
Also published as: GB0613926D0

Abstract

Improved frequency compensation for an amplifier enables much greater feedback to be applied to the output stage 16 of the amplifier, thereby reducing output signal distortion. In a conventional audio power amplifier, when frequency compensation is applied across the voltage driver stage 14 the available feedback that can be applied from the output stage to the input stage of the amplifier falls with increasing frequency, thus reducing the ability of the feedback loop to lessen output signal distortion. To solve this problem an RC network 50 is coupled to the compensating capacitor 30 so that local feedback from the output stage dominates at signal frequencies to improve linearity while feedback from the driver stage output dominates at high frequencies to ensure stability. Positive feedback from output node 18 to drive node 26 through network 50 provides a bootstrapping effect which reduces the load on the driver stage 14. Double pole compensation may be used (figures 6-8), with an optional phase offsetting lead network (55, figures 7,8) in the overall gain setting feedback path 20.

Description

1 2439983 Improvements in or relating to Amplifier Frequency

Compensation

Field of the Invention.

The present invention concerns improved amplifier frequency compensation. The invention relates to a technique and means for applying improved frequency compensation to an amplifier that enables much greater feedback to be applied to the output stage which significantly reduces output signal distortion. The present invention is applicable particularly, but not exclusively to audio power amplifiers which typically experience high order harmonic distortion at the output caused in part by cross-over distortion. Whilst the present invention is particularly applicable to audio power amplifiers, it can be implemented in any type of amplifier including operational amplifiers (op-amps).

Background to the Invention.

All power amplifiers have a power supply, an input portion and an output portion. The primary purpose of the power supply in a power amplifier is to take mains power and convert it to a direct current (DC) voltage usually having positive and negative voltage rails. Conversion from AC to DC is necessary because the semiconductor devices such as transistors, Field Effect Transistors (FETs), Metal Oxide Semiconductor FETs (MOSFETs), valves etc. used inside the equipment require DC voltage.

The general purpose of the input portion (sometimes called the "front end") is to receive and prepare the input signal for "amplification" by the output portion.

The output portion of the amplifier is the portion which actually converts the weak input signal into a much more powerful "replica" which is capable of driving a load such as a loudspeaker or speakers. This portion of the amplifier typically uses a number of "power transistors" such as MOSFETs. The output portion interfaces to the load.

In general, the purpose of a power amplifier is to take an input signal and make it stronger, i.e. increase its amplitude. Amplifiers find application in all kinds of electronic devices. There are many different types of amplifiers, each with a specific purpose in mind. For example, a radio transmitter uses a Radio Frequency (RF) amplifier. Such an amplifier is designed to amplify a signal so that it may drive an antenna. Audio power amplifiers on the other hand are those amplifiers which are designed to drive loudspeakers.

The purpose of an audio power amplifier is to take a signal from a source device such as a preamplifier or signal processor and make it suitable for driving a loudspeaker. Ideally, the only difference between the input signal and the output signal is the strength of the signal. However, no amplifier operates in an ideal manner. The output signals of all amplifiers contain additional unwanted signal components that are not present in the input signal. These additional signal components are generally known as distortion. There are many types of distortion, but the two most common types are known as harmonic distortion and intermodulation distortion.

Harmonic distortion comprises a signal component or components which are related to the original or fundamental wavelength of an input signal by an integer number. A pure tone signal has no harmonics. It consists of only one single frequency. When a pure tone signal is applied to the input of an amplifier, we find that the output signal of the amplifier is no longer pure. That is, several "new" frequencies or wavelengths in addition to the original appear in the output signal. These new frequencies are integer multiples of the original frequency, i.e. they are the harmonics of the original signal. The presence of the harmonic signal components distorts the sound quality of loudspeakers being driven by the amplifier, which is undesirable.

Intermodulation distortion comprises a signal component or components which are unrelated to any of the original or fundamental wavelength of the input signals by an integer number. When two pure tones that are not harmonically related to each other are applied to the input of an amplifier, one finds additional signal components that are not harmonics of the two pure tones.

These additional signal components are caused by the input signals modulating each other to produce additional signal components that occur at integer sum and difference frequencies of the original signals. The presence of these intermodulation signal components also undesirably distorts the sound quality of loudspeakers being driven by the amplifier.

In a conventional audio power amplifier such as that illustrated in schematic form in Figure la, when frequency compensation is applied across the voltage driver stage, the available feedback that can be applied from the output stage to the input stage of the amplifier reduces with increasing frequency thus compromising the ability to manage output signal distortion. The amount of feedback can typically reduce with increasing frequency by 6dB per octave, which results in increasing signal distortion for higher frequencies.

Distortion with rising frequency may increase by much the same rate, i.e. 6dB per octave.

To reduce output distortion, many techniques have been applied in conventional power amplifiers. These techniques include error correction and nested feedback loops. What these techniques have in common is that they greatly increase the complexity of the amplifier design and thus its cost.

Object of the Invention.

It is an object of the invention to mitigate and/or obviate problems associated with conventional power amplifier arrangements. This may be achieved by including the output stage within the frequency compensation loop for as large a part as possible of the frequency spectrum of the amplifier according to the invention.

Summary of the Invention.

In a first main aspect, the present invention provides a power amplifier comprising: an input stage feeding a voltage driver stage which, in turn, feeds an output stage; a feedback stage connecting an output of the output stage to an input of the input stage; a frequency compensation stage providing frequency compensation to the voltage driver stage; and a frequency selection means for controlling a source signal of the frequency compensation stage.

Preferably, the frequency compensation stage provides frequency compensation directly to the voltage driver stage.

Preferably, the frequency compensation stage provides a frequency compensation feedback signal to the voltage driver stage.

Preferably also, the frequency compensation stage provides a negative frequency compensation feedback signal to the voltage driver stage.

Preferably further, the frequency selection means is arranged to control the source signal of the frequency compensation stage by providing a feedback signal to said stage mainly from the output of the output stage for a low frequency part of the amplifier's frequency spectrum and to provide a feedback signal to said stage mainly from an output of the voltage driver stage for a high frequency part of the amplifier's frequency spectrum.

The change in source between the output stage at low frequency to the voltage driver at high frequency may be of the same order as the closed loop bandwidth of the amplifier and may be in the range of 100KHz to 2MHz.

Preferably, the frequency selection means comprises a resistive element connecting the output of the output stage to an input of the frequency compensation stage and a capacitive element connecting the output of the voltage driver stage to the input of the frequency compensation stage.

Preferably, the capacitive element is a capacitor having a value of equal to or less than one nano-Farad.

Alternatively, the frequency selection means comprises other active and/or passive components or a digital signal processor arranged to provide a feedback signal to said frequency compensation stage indicative of the output signal of the output stage for a low frequency part of the amplifier's frequency spectrum and to provide a feedback signal to said stage indicative of an output signal of the voltage driver stage for a high frequency part of the amplifier's frequency spectrum.

Preferably, the amplifier is an audio power amplifier.

In a further main aspect, the present invention provides a method in a power amplifier of controlling a source signal of a frequency compensation stage, said amplifier comprising an input stage feeding a voltage driver stage which, in turn, feeds an output stage; a feedback stage connecting an output of the output stage to an input of the input stage; a frequency compensation stage providing frequency compensation to the voltage driver stage; and a frequency selection means, the method comprising utilising the frequency selection means to control the source signal of the frequency compensation stage.

Brief description of the drawings.

A description of the present invention will follow with reference to the accompanying drawings, of which: Figure la is a block schematic diagram illustrating one arrangement of a conventional audio power amplifier; Figure lb is a simplified discrete component circuit diagram for the conventional audio power amplifier arrangement with a single ended driver stage of figure la; Figure 2 is a simplified discrete component circuit diagram for a conventional audio power amplifier with a balanced or push/pull driver stage; Figure 3 is a simplified discrete component circuit diagram for a conventional double pole compensation audio power amplifier; Figure 4a is a block schematic diagram illustrating a first preferred embodiment of an audio power amplifier in accordance with the invention; Figure 4b is a simplified discrete component circuit diagram for the audio power amplifier arrangement of figure 4a; Figure 5 is a simplified discrete component circuit diagram illustrating a second preferred embodiment of an audio power amplifier in accordance with the invention; Figure 6 is a simplified discrete component circuit diagram illustrating a third preferred embodiment of an audio power amplifier in accordance with the invention; Figure 7 is a simplified discrete component circuit diagram illustrating a fourth preferred embodiment of an audio power amplifier in accordance with the invention; and Figure 8 is a simplified discrete component circuit diagram illustrating a fifth preferred embodiment of an audio power amplifier in accordance with the invention.

Detailed description of preferred embodiments.

The foregoing and further features of the present invention will be more readily understood from a description of preferred embodiments, by way of example thereof, with reference to the accompanying drawings.

Figure la is a block schematic diagram illustrating one arrangement of a conventional audio power amplifier generally denoted as 10. The amplifier 10 comprises a power supply (not shown), an input stage 12 feeding a voltage driver stage 14 which in turn feeds an output stage 16, an output 18 of which interfaces with a load (not shown) such as a loudspeaker. A negative voltage feedback loop 20 links the output 18 of the output stage 16 to an input 22 of the input stage in a known manner. A frequency compensation stage 24 also of a known arrangement provides a negative frequency compensation loop linking an output 26 of the voltage driver stage 14 to its input 28. The frequency compensation stage 24 may comprise a Miller capacitor 30 or the like.

Figure lb is a simplified discrete circuit diagram for the known audio power amplifier arrangement 10 of figure la illustrating the power supply comprising DC positive and negative voltage rails 32a,b together with ground 32c. The input stage 12 comprises a differential input stage such as a long tailed pair of transistors Ql, Q2 with a current source 34, bias resistor Rbias and current mirror 36 in a typical arrangement. The negative voltage feedback ioop comprises a series connected to ground 32c pair of resistors RF1, RF2 linking the output 18 of the amplifier to the base of one of the transistors Qi, Q2 of the long tailed pair. The voltage driver stage 14 comprises a current source 38 and a bias circuit 40 of known form (not shown) connected to the collector of a single transistor Q3. The frequency compensation stage 24 comprises the capacitor CCOMP 30 connected across the collector to base of the voltage driver stage transistor Q3. The output stage 16 comprises two output circuits 16a,b shown only in schematic form in the figure. Each of the output circuits may comprise any known arrangement of bipolar transistors, MOSFETs, Insulated Gate Bipolar Transistors (IGBTs), valves or any combination thereof.

Whilst the frequency compensation stage 24 does provide frequency stability over much of the frequency range of the amplifier 10, it itself also causes frequency variation whereby the available feedback on the negative voltage feedback loop 20 that can be applied from the output stage 16 to the input stage 12 of the amplifier reduces with increasing frequency.

Figure 4a is a schematic block diagram of a first preferred embodiment of an audio power amplifier 100 according to the invention. The amplifier arrangement of the first embodiment has generally the same arrangement as that of the conventional arrangement depicted by figures la and lb and thus like numerals are employed herein to denote like parts. The arrangement of the present invention differs, however, in that it includes a frequency selection means 50 for varying with frequency the source signal of the frequency compensation stage. In its simplest form as shown in figure 4a, the frequency selection means 50 comprises a resistive element R such as a resistor connecting the output 18 of the amplifier to an input 48 of the frequency compensation stage 24 and a capacitive element Cs such as a capacitor connecting the output 26 of the voltage driver stage 14 to the input 48 of the frequency compensation stage 24. The arrangement of the resistive and capacitive elements is such that for low frequencies in the amplifier frequency range a source signal for the frequency compensation stage is drawn mainly from the output 18 of the amplifier 100, which greatly reduces distortion at the output stage 16, whereas for higher frequencies the source signal for the frequency compensation stage 24 is mainly drawn from the output 26 of the voltage driver stage 14 to ensure that sufficient stability is achieved.

The addition of the frequency selection means 50 to the standard frequency compensation method allows a low frequency signal to be routed from the output 18 of the output stage 16 and a high frequency signal to be routed from the output 26 of the voltage driver stage 14 to the input 48 of the frequency compensation stage 24. The frequency of the transition is determined in part by components R5 and C (in the simplest form shown in figures 4a and 4b). The routing of the high frequency signal from the output 26 of the voltage driver stage 14 ensures the high frequency stability of the amplifier. The routing of the low frequency signal from the output 18 of the output stage 16 ensures that the feedback signal through the frequency compensation stage 24 also includes the output stage within its loop for low frequencies. This additional feedback will lower the distortion of the output stage 16 by the extra amount of feedback being applied through the frequency compensation stage 24 when compared with the conventional amplifier arrangement of figures la and lb. In effect, the frequency selection means 50 allows the output stage 16 to be included within the frequency compensation loop (of the frequency compensation stage 24) and benefit from the extra feedback while also ensuring high frequency stability of the amplifier.

Whilst the frequency selection arrangement 50 of the first embodiment is illustrated as comprising a CR circuit arrangement, it will be understood that any passive, active or digital processing technique could be employed to provide equivalent operation. Further, whilst the first embodiment of the invention is presented as an audio power amplifier 100, it will be understood that the present invention can be applied to amplifiers of many different arrangements and configurations and for different functions. It will also be understood that the amplifier arrangement of the present invention need not be limited to a transistor based arrangement, but is equally applicable to amplifiers comprising other components such as valves, etc. In some other conventional amplifier arrangements (not shown), the output of the frequency compensation stage is connected to the input of the input stage rather than directly to the input of the voltage driver stage in contrast to the conventional amplifier arrangement depicted by figures Ia and lb. This does not affect operation of such other conventional amplifiers as regards frequency compensation. The frequency compensation stage still provides frequency compensation to the voltage driver stage regardless of whether the frequency compensation signal from the frequency compensation stage is applied to the input of the input stage or directly to the input of the voltage driver stage. For such other conventional amplifier arrangements, the present invention provides an improvement in the form of a frequency selection means as described with respect to figures 4a and 4b for varying with frequency the source signal of the frequency compensation stage.

Referring now to figure 2, illustrated is a simplified discrete component circuit diagram of yet another conventional audio power amplifier arrangement 60. This amplifier arrangement is of similar form to that depicted by figures la and lb as will be understood by a person skilled in the art and like numerals will be used to denote like parts. However, the amplifier arrangement 60 of figure 2 has resistors R1, R2 in its input stage 12' rather than a current mirror and comprises a balanced or push/pull voltage driver stage 14' comprising two transistor elements Q4, Q5 together with their associated resistors R3, R4 and a current mirror 38' in a typical balanced arrangement.

The conventional amplifier arrangement depicted by figure 2 serves to illustrate that the present invention is equally applicable to such other conventional arrangements as will be apparent from figure 5 of the drawings.

Figure 5 is a simplified discrete component circuit diagram of a second preferred embodiment of the present invention. It comprises a circuit configuration for an amplifier 200 identical to that of the conventional arrangement of figure 2 save for the inclusion in a similar manner to the first embodiment of figures 4a and 4b of a frequency selection means 50 for controlling the source signal of the frequency compensation stage 24. The frequency selection means 50 comprises a resistive element R connecting the output 18 of the amplifier 200 to the input 48 of the frequency compensation stage 24 and a capacitive element C5 connecting the output 26 of the voltage driver stage 14' to the input 48 of the frequency compensation stage 24. The arrangement of the resistive and capacitive elements is such that for low frequencies in the amplifier frequency range the source signal for the frequency compensation stage 24 is drawn mainly from the output 18 of the amplifier 200 whereas for higher frequencies the source signal for the frequency compensation stage 24 is mainly drawn from the output 26 of the voltage driver stage 14'. As with the first embodiment, any suitable means for implementing the frequency selection means 50 may be employed in the circuit arrangement of the second embodiment.

Figure 3 is a simplified discrete component circuit diagram of yet another conventional arrangement of audio power amplifier 70. This has a similar configuration to that of the conventional arrangement of figures la and lb so like numerals are employed to denote like parts. It differs in that it employs a double pole frequency compensation stage 24" comprising a first capacitor CD connecting the collector of the single transistor 03 of the voltage driver stage 14 and a second capacitor CCOMP connecting the negative side of the first capacitor to the base of the single transistor 03. A resistor R0 is connected from both the capacitor CD and the capacitor CCOMP to the negative rail 32b. The purpose of CD and RD is to form a simple first order high pass filter which reduces the compensation being applied through CCOMP below a predetermined frequency by 6dB per octave. When the response of this filter is combined with the first order response produced by CCOMP an overall second order response results which has a slope of -12dB per octave, i.e. a double pole compensation is produced over a limited frequency range. The double pole frequency compensation stage 24" of the conventional arrangement 70 depicted by figure 3 can assist in reducing signal distortion in both the input and output stages of the amplifier when compared to the conventional arrangement of figures Ia and lb. In some amplifiers the input stage can be a significant source of signal distortion. However, the double pole frequency compensation stage of the conventional arrangement of figure 3 suffers from much the same drawbacks as the arrangement of figures la and lb and in addition it increases the load on the voltage driver stage at higher frequencies which in turn increases the distortion produced by the voltage driver stage. Due to the variations in circuit parameters, the range of the double pole is not clearly defined. In many cases this does not present a serious problem but by adding an additional capacitor C1 (not shown) in series with element RD in figure 3 it is possible to accurately limit the range of the double pole compensation. When the additional capacitor CL is added the range of the double pole compensation is limited to (C1 + CD + CCOMp)/(CD + CCOMP). For example, if CCOMP is lOOpF, CD is 470pF and CL is 4.7nF, the range of the double pole compensation is a maximum of 9.75.

The conventional amplifier arrangement depicted by figure 3 serves to illustrate that the present invention is equally applicable to such other conventional arrangements as will be apparent from figure 6 of the drawings.

Figure 6 is a simplified discrete component circuit diagram of a third preferred embodiment of an amplifier 300 in accordance with the present invention. It comprises a circuit configuration identical to that of the conventional arrangement of figure 3 save for the inclusion in a similar manner to the first embodiment of figures 4a and 4b of a frequency selection means 50 for controlling the source signal of the frequency compensation stage 24". The frequency selection means 50 comprises a resistive element R5 connecting the output 18 of the amplifier 300 to the input 48 of the frequency compensation stage 24" and a capacitive element Cs connecting the output 26 of the voltage driver stage 14 to the input 48 of the frequency compensation stage 24". The arrangement of the resistive and capacitive elements is such that for low frequencies in the amplifier frequency range the source signal for the frequency compensation stage is drawn mainly from the output of the amplifier whereas for higher frequencies the source signal for the frequency compensation stage is mainly drawn from the output of the voltage driver stage.

As well as the advantages provided by including the frequency selection means already described with reference to the first and second embodiments of the invention, the inclusion of such means as illustrated by figure 6 in the conventional arrangement of figure 3 provides the advantage of considerably reducing the load on the voltage driver stage by way of the output stage bootstrapping the load on the voltage driver stage. As with the first and second embodiments, any suitable means for implementing the frequency selection means may be employed in the circuit arrangement of the third embodiment.

As illustrated in a fourth embodiment of an amplifier 400 in accordance with the invention depicted by figure 7, other problems associated with double pole compensation can be eliminated by an additional frequency compensation network 55 connected from the output of the amplifier to an inverting input of the input stage to ensure that frequency compensation as a whole for the amplifier reduces at a certain, i.e. constant rate, for example 6dB per octave. Elimination of such problems enables the transition frequency defined in part by the first capacitor CD and resistor R0 of the frequency compensation stage to be made higher than in the conventional arrangement which is beneficial.

Standard frequency compensation reduces the compensated gain of an amplifier by 6dB per octave and limits the phase shift to -90 . A problem with double pole compensation is that the compensation reduces by 12dB per octave for part of the frequency spectrum of the amplifier and this is associated with a phase lag of up to 180. The proportion of the part of the compensation that causes a -12dB per octave slope must be limited to ensure adequate amplifier stability. One solution is to limit the frequency range of the -12dB per octave slope to a frequency range of one decade. This will ensure a maximum phase lag of 145 0 As compensation returns to a -6dB per octave slope, the phase lag will reduce and the phase margin will approach that of the standard frequency compensation. The increase in phase lag causes resonant peaks in the frequency response of the amplifier and overshoots in the transient response before the feedback loop is closed. After the loop is closed, these problems will be reduced by negative feedback but not eliminated. The maximum phase lag reduces amplifier stability and may result in a number of problems. For example, when the output is limited by the available voltage rails, the amplifier will saturate and during recovery may produce bursts of oscillation. The purpose of adding additional frequency compensation network in the embodiment depicted by figure 7 is to remove the foregoing limitations and problems. Network 55 returns the overall compensation to a -6dB per octave slope by providing standard compensation over that part of the spectrum that double pole compensation operates.

Additional frequency compensation network 55 achieves' its purpose by having a predetermined transition frequency that is designed to be identical to the predetermined transition frequency of the simple high pass filter of the frequency compensation stage 24" that was previously described for the conventional audio power amplifier 70 of figure 3. In effect, the zero produced by network 55 and RF1 cancels the additional pole of the frequency compensation stage 24" to ensure that the overall frequency compensation is standard rather than double pole.

The additional frequency compensation network 55 and the frequency compensation stage 24" work together to produce an overall standard frequency compensation for the amplifier. The components of frequency compensation stage 24" are modified in effective value by the frequency selection means 50 and vice-versa. Network 55 works at low frequency and the frequency compensation stage 24" and frequency selection means 50 work at high frequency. Because the low frequency signals are routed through the network 55, the complete amplifier benefits from the additional feedback of the network 55 at lower frequency while compensation is provided at higher frequency by the frequency compensation stage 24" and frequency selection means 50 which ensures amplifier stability at high frequency.

Disadvantages of adding network 55 are that the amplifier bandwidth is reduced and there is a requirement for a corresponding increase in the gain margin of the amplifier. For example, if the double pole compensation operates over a frequency range of a decade then the closed loop bandwidthof the amplifier is reduced by a factor of ten. This may be unacceptable in many practical designs and limiting the range of the double pole to a frequency range of say three would be more practical. This can be accomplished by adding a capacitor CL (not shown) in series with element RD as previously described for figure 3. It might be considered that limiting the double pole compensation to a range of about three would provide little improvement, but because the feedback around the input stage is increased and the loading on the input stage by the compensation network is simultaneously reduced the distortion of the input stage could be reduced by about an order of magnitude which may be very beneficial in some designs. For example, assume that a double pole compensation based on compensation network 24" is selected and that double pole compensation operates over the range of 100kHz to 300kHz. Selecting Rc of network 55 to be half the value of RF, of the feedback loop 20 and selecting C and R of network 55 to produce a transition frequency of 300kHz would reduce the gain of the amplifier between 100kHz and 300kHz at 6dB per octave ensuring that overall compensation for the amplifier is effectively standard.

The frequency selection means 50 provides additional feedback around the output stage 16 which further reduces distortion of the output stage 16. It also reduces distortion that would otherwise be produced by the voltage driver stage 14 being loaded by C0 and R0 because the load of the voltage driver stage 14 is bootstrapped by the output stage 16 via the frequency selection means 50.

It is possible to add a phase advance circuit (not shown) across component RF, of negative voltage feedback loop 20 to improve transient performance. This can be combined with network 55 such that no additional components are required and thus no additional cost incurred.

As illustrated by a fifth embodiment of an amplifier 500 in accordance with the invention depicted by figure 8, an additional gain stage or stages 65 can be added to the amplifier circuit to further reduce signal distortion and stabilised by using nested frequency compensation feedback around the additional stage or stages. The additional gain stage or stages 65 may comprise transistors Q6, Q7 with a current source 134 and current mirror 136 in a typical arrangement.

Nested feedback is often used in op-amps to reduce the signal distortion of the voltage driver stage, but in the embodiment depicted by figure 8 it will also reduce signal distortion produced by the output stage. It is theoretically possible to reduce distortion levels below detectable levels for the complete audio frequency spectrum allowing this technique to be considered, effectively, as distortionless in an ideal implementation. In a practical implementation, the distortion will depend on circuit wiring layout and is unlikely to achieve the theoretical characteristic, but the distortion can be reduced to below the noise floor level for the audio spectrum in practical designs.

The frequency selection means of the invention bootstraps the load on the voltage driver stage which can produce regenerative feedback around the output stage. However, with the output stage arranged to have approximately unity gain, no stability problems are encountered.

The loading of the capacitive and resistive elements of the frequency selection means on the voltage driver stage can also be a problem. Although this is bootstrapped by the output stage, the load on the voltage driver stage is not entirely eliminated because the bootstrapping is not entirely effective.

However, when the value of the capacitive element is kept small, say less than or equal to one nano-Farad, then the load on the voltage stage is negligible.

It will be understood that in the foregoing description of preferred embodiments, any transistor component depicted in the figures can be replaced by any of a FET, MOSFET, Darlington, other composite transistor arrangement or any other amplifying device.

Claims

Claims 1. An amplifier comprising: an input stage feeding a voltage

driver stage which, in turn, feeds an output stage; a feedback stage connecting an output of the output stage to an input of the input stage; a frequency compensation stage providing frequency compensation to the voltage driver stage; and a frequency selection means for controlling a source signal of the frequency compensation stage.

2. The amplifier of claim 1, wherein the frequency compensation stage is arranged to provide frequency compensation directly to the voltage driver stage.

3. The amplifier of claim 1 or claim 2, wherein the frequency compensation stage is arranged to provide a frequency compensation feedback signal to the voltage driver stage.

4. The amplifier of claim 3, wherein the frequency compensation stage is arranged to provide a negative frequency compensation feedback signal to the voltage driver stage.

5. The amplifier of any preceding claim, wherein the frequency selection means is arranged to control the source signal of the frequency compensation stage by providing a feedback signal to said stage mainly from the output of the output stage for a low frequency part of the amplifier's frequency spectrum and to provide a feedback signal to said stage mainly from an output of the voltage driver stage for a high frequency part of the amplifier's frequency spectrum.

6. The amplifier of claim 5, wherein the frequency selection means is arranged to control the source signal of the frequency compensation stage from the output stage at low frequency to the voltage driver at high frequency such that a change in said source signal is in the same order as the closed loop bandwidth of the amplifier.

7. The amplifier of any preceding claim, wherein the frequency selection means comprises a resistive element connecting the output of the output stage to an input of the frequency compensation stage and a capacitive element connecting the output of the voltage driver stage to the input of the frequency compensation stage.

8. The amplifier of claim 7, wherein the capacitive element is a capacitor having a value of equal to or less than one nano-Farad.

9. The amplifier of any one of claims 1 to 6, wherein the frequency selection means comprises any suitable active and/or passive components or a digital signal processor arranged to provide a feedback signal to said frequency compensation stage indicative of the output signal of the output stage for a low frequency part of the amplifier's frequency spectrum and to provide a feedback signal to said stage indicative of an output signal of the voltage driver stage for a high frequency part of the amplifier's frequency spectrum.

10. The amplifier of any preceding claim, wherein the amplifier has an additional frequency compensation stage connected from the output *f the amplifier to an inverting input of the input stage to ensure that frequency compensation as a whole for the amplifier reduces at a constant rate.

11. The amplifier of any preceding claim, wherein the amplifier is an audio power amplifier.

12. A method in an amplifier of controlling a source signal of a frequency compensation stage, said amplifier comprising an input stage feeding a voltage driver stage which, in turn, feeds an output stage; a feedback stage connecting an output of the output stage to an input of the input stage; a frequency compensation stage providing frequency compensation to the voltage driver stage; and a frequency selection means, the method comprising utilising the frequency selection means to control the source signal of the frequency compensation stage.

13. The method of claim 12, wherein the frequency compensation stage provides frequency compensation directly to the voltage driver stage.

14. An amplifier substantially as hereinbefore described with reference to figures 4a and 4b of the drawings.

15. An amplifier substantially as hereinbefore described with reference to figure 5 of the drawings.

16. An amplifier substantially as hereinbefore described with reference to figure 6 of the drawings.

17. An amplifier substantially as hereinbefore described with reference to figure 7 of the drawings.

18. An amplifier substantially as hereinbefore described with reference to figure 8 of the drawings.