GB2396498A

GB2396498A - An amplifier arrangement linearised by predistortion and feedforward; adaptive bias for improved efficiency; thermal overload protection

Info

Publication number: GB2396498A
Application number: GB0325306A
Authority: GB
Inventors: Martin Neuhaus
Original assignee: N & L Ltd
Current assignee: N & L Ltd
Priority date: 2002-11-01
Filing date: 2003-10-30
Publication date: 2004-06-23
Also published as: WO2004040751A1; AU2003271959A1; GB2395077A; GB0325306D0; GB0225459D0

Abstract

An amplifier assembly comprises a pre-amplifier 002, power amplifier 012 and two distinct loops. The first loop is an arrangement and method of pre-distorting a modulated carrier in order to compensate for non-linearities present in power amplifier 012. The second loop is a feed-forward loop compensating for residual distortion after the amplified modulated carrier has passed through power amplifier 012. A third loop measures the amplifier system output signal and adjusts loop1 and loop2 to minimise the distortion component at the output of the amplifier system. The amplifier bias and the attenuation of the input signal are adjusted in accordance with a function of the amplifier power input integrated over time depending on the specific heat capacity of the amplifier semiconductor junction and the statistical probability of data occurrence in the modulated carrier input to the amplifier, particularly when the duration of amplifier power output exceeds a predetermined time limit which would induce thermal damage to the amplifier.

Description

1 - AMPLIFIER ASSEMBLY

5 This invention relates to power amplifiers and in particular to a multicarrier power amplifier system including pre-distortion and feed-forward to reduce intermodulation distortion and to a method of operating such an amplifier system.

In general, high power amplifiers are operated in the vicinity of the saturation region. In the case of a complex modulated input signal, the non-linearity of the amplifier will generate inter-modulation 15 distortion. BACKGROUND OF THE INVENTION

UK Patent Application GB 2318938 A filed 13 May 1997, Samsung Electronics Co Limited, describes an arrangement for linearizing an amplifier by a combination of two methods. In the first a pre 25 distorter supplies harmonics to the main amplifier in such a way as to counteract the distortion. In the second a feed-forward arrangement subtracts a signal derived from the input of the main amplifier from a signal derived from the output to produce a 30 distortion signal which is then subtracted from the main amplifier output to remove remaining distortion.

UK Patent Application GB 2352570 A filed 28 July 1999, Wireless Systems International Limited, 35 describes a linearizing arrangement including a digital signal processor. The digital signal processor implements a predistorter process which

- 2 - pre-distorts the input signal to the amplifier in such a manner as to counter distortion imposed by the amplifier. Feedback from the amplifier output is sampled to provide; feedback signal for controlling 5 the pre-distortion process. Additionally, a feed forward signal is combined with the amplifier output to further reduce distortion therein. The feed forward signal is derived by subtracting the input signal from feedback from the amplifier output (which 10 may contain residual distortion).

United States Patent 5770971 dated 23 June 1998, Northern Telecom Limited, describes a control arrangement making use of a reference signal, of 15 known frequency, which is amplified along with the desired carrier signals. The reference signal component of the amplified signal is isolated and a comparison is made, either with the actual reference signal, e.g. by a QAM demodulator, or alternatively 20 with the known frequency of the reference signal, e.g. by a FM discriminator, in order to determine the differences in gain and in phase of the reference signal component of the amplified signal in comparison with the reference signal.

SUMMARY OF THE INVENTION

30 According to one aspect of the present invention there is provided an amplifier assembly including: (a) a pre-amplifier having an input receiving an input modulated carrier and an output feeding an amplifier) and 35 (b) a first loop including a dynamically adjusted variable amplifier and a dynamically adjusted phase shifter in the output path of the first loop;

- 3 - wherein a portion of the input modulated carrier feeds into the first loop, and the first loop derives a distortion component from the output of the pre amplifier and sums this distortion component with the 5 modulated carrier output of the pre-amplifier before feeding the resultant summed signal to an input of the amplifier.

According to another aspect of the present invention lo there is provided an amplifier assembly including: (a) a pre-amplifier having an input receiving an input modulated carrier and an output feeding an amplifier; (b) a first loop including a dynamically adjusted 15 variable amplifier and a dynamically adjusted phase shifter in the output path of the first loop, wherein a portion of the input modulated carrier feeds into the first loop, and the first loop derives a distortion component from the output of the pre 20 amplifier and sums this distortion component with the modulated carrier output of the pre-amplifier before feeding the resultant summed signal to an input of the amplifier; and (c) a second loop including a dynamically adjusted 25 variable amplifier and a dynamically adjusted phase shifter in the output path of the second loop; wherein the second loop derives a residual distortion component from the output of the amplifier and sums this residual distortion component with the input 30 modulated carrier before feeding the resultant summed signal to an output of the amplifier assembly.

The system comprises two distinct loops. The first loop is an arrangement and a method of pre-distorting 35 the modulated carrier in order to compensate for the non-linearities present in the power amplifier. The second loop is a feed-forward loop that compensates

- 4 for the residual distortion present in the amplified modulated carrier after it has passed through the amplifier. 5 The first loop extracts the modulated carrier signal from the input feed to a pre- amplifier. The output of the pre-amplifier will comprise both the amplified modulated carrier signal and a distortion component induced by the pre-amplifier. This pre-amplifier lo output is also fed into the first loop and combined with the input modulated carrier. Feedback in the first loop dynamically reverses the phase and amplitude of the modulated carrier element relative to the output of the pre-amplifier. The resultant 15 combined signal, consisting primarily of the distortion component, is used as a pre-distortion signal and fed towards a power amplifier.

Preferably, in order to achieve the correct amplitude 20 and phase characteristics the pre-distortion signal is fed to the power amplifier through a variable amplifier and a phase shifter which are controlled on a feed-forward basis by the second loop, as described below. The output of the power amplifier may still contain a small residual distortion component derived from the first loop. The second loop extracts this output of the power amplifier and combines it with the original 30 modulated carrier signal input to the pre-amplifier.

Feedback in the second loop dynamically reverses the phase and amplitude of the modulated carrier element relative to the output of the power amplifier. The resultant combined signal, comprising primarily the 35 residual distortion component, is fed through a variable amplifier and a phase shifter before re combination with the power amplifier output. The

5 - aforementioned residual distortion component of the second loop is continuously measured or detected and controls the variable amplifiers and phase shifters in both the first and second loops on a feed-forward s basis so as to optimlse the distortion present in the pre-distortion signal and hence minimise the overall distortion present at the power amplifier output.

A particularly advantageous configuration of the amplifier assembly includes a third (control) loop summing a signal derived from the input modulated carrier with a signal derived from the amplifier output so as to minimise a distortion component of a resultant summed signal, wherein such summed signal 15 dynamically controls each variable amplifier and phase shifter. Preferably the control loop includes: (a) a summing circuit) and (b) a variable attenuator and a control phase shifter in the respective input paths of the summing circuit; 20 and such variable attenuator and control phase shifter are dynamically controlled by an output of the summing circuit. Substantially equivalent signals control both the input paths of the third loop and the respective output paths of the first and 25 second loops.

The complex modulated carrier contains both an amplitude and frequency component. The amplitude component, which might typically be 10 times greater 30 than the average power level, presents itself as a function of the modulating data stream. This might typically be 1 data bit per million. This amplitude component will cause an increase in temperature of the amplifier's semiconductor junction. If the 35 amplifier's bias is such that it allows the amplifier to deliver this higher than normally- rated average output power, then the time it is able to do this

- 6 - before destruction occurs will be a function of the amplifer's maximum semiconductor junction temperature and specific heat capacity.

5 In order to maximise efficiency of an amplifier, it is desirable to operate the amplifier as close as possible to its maximum power output level before the onset of thermal damage. Conventionally, the output circuit of the amplifier is rated to operate without lo damage at the expected peak power levels. For a complex modulated carrier signal, peak power inherently exceeds the average power level. For a multi-carrier signal, peak signal values may be superimposed causing sum excursions with large peak to-average values. Typically, peak power is approximately ten times the average power level but the amplifier circuits will be operating only at the average power levels more than 99% of the time.

20 According to another aspect of the present invention there is provided an amplifier wherein the bias is dynamically adjusted in accordance with a power component of a modulated carrier input to the amplifier. In particular, there is provided an 25 amplifier wherein an amplitude component of a modulated carrier input to the amplifier is integrated to generate a power level signal independently of amplification of the modulated carrier and the amplifier bias is dynamically 30 adjusted in accordance with the power level signal.

The amplitude component is integrated as a function of time. This will vary the DC operating conditions of the amplifier transistor in order to optimise the transistor load-line characteristic and efficiency.

The amplifier bias is adjusted in accordance with a power function of the modulated carrier integrated

- 7 - over time. This has a statistical relationship to the probability of data being present and hence peak power. In particular the amplifier bias is adjusted in accordance with a function of the specific heat 5 capacity of the output portion of the amplifier and of the statistical probability of data occurrence in the modulated carrier input to the amplifier. The bias is adjusted to maximise power output of the amplifier and, in particular, to optimise the power lo output and efficiency of the amplifier.

Additionally a variable attenuator may be included in the modulated carrier input to the amplifier. The modulated carrier is dynamically attenuated in 15 accordance with a function of the specific heat capacity of the output portion of the amplifier and of the statistical probability of data occurrence in the modulated carrier input to the amplifier. In particular the modulated carrier input is attenuated 20 when the duration of the amplifier power output exceeds a predetermined time limit that if exceeded would induce thermal damage to the amplifier.

The feedback and feed-forward parameters are adopted 25 dynamically to maximise linearity of the overall output of the amplifier assembly and to maintain output power levels without excessive reductions in efficiency. In summary: a) The variable attenuator

will reduce the power presented to the amplifier 30 should the duration of the power produced as a result of the modulated carrier exceed a predetermined time limit that would cause junction damage in the amplifier. b) The dynamic bias adjustment functions to increase the supply voltage and base bias to the 35 amplifier transistor when it is required to produce more output power. As the transistor is operating in a non linear mode, its output impedance (load-line

8 - characteristic) will vary according to signal level so at certain input power levels the transfer of power will be less efficient than at others. (For maximum power transfer the source impedance 5 (transistor output) should equal the load impedance.) By varying the bias conditions the efficiency may be kept at a more constant level.

An amplifier assembly and its method of operating as 10 described above are particularly useful in multi channel telecommunications networks, and especially for broadband and the so-called Third Generation (3G) networks. Typically, broadband networks such as 3G operate in the range 2.14 GHz to 2.17 GHz with a 15 channel bandwidth of 5 MHz. Typical power output of the amplifier is around 10 W per channel.

DESCRIPTION OF THE DRAWINGS

Figure 1 is an overall amplifier assembly block diagram according to one aspect of the present invention. Figure 2 is an overall block diagram of an amplifier assembly according to the present invention and incorporating a third (control) loop.

30 Figure 3 is a spectral distribution diagram of a modulated carrier as presented at the input to the amplifier system.

Figure 4 is a spectral distribution diagram of the 35 output of the preamplifier stage.

9 - Figure 5 is a spectral distribution diagram of the output of the summation circuit of the first loop.

Figure 6 is a spectral distribution diagram of the 5 output of the first loop.

Figure 7 is a spectral distribution diagram of the modulated carrier as presented at the input to the amplifier stage.

Figure 8 is a spectral distribution diagram of the output of the summation circuit of the second loop.

Figure 9 is a spectral distribution diagram of the 15 output of the second loop.

Figure 10 is a spectral distribution diagram of the modulated carrier at the output of the amplifier system shown in Figure 1.

Figure 11 is a spectral distribution diagram of the modulated carrier as presented at an input of the third loop summing circuit.

25 Figure 12 is a spectral distribution diagram of the modulated carrier at the output of the amplifier system shown in Figure 2.

Figure 13 is a block diagram showing the amplifier 30 stage split into its constituent sub-blocks.

DESCRIPTION OF THE INVENTION

The system comprises an arrangement and a method for reducing the distortion component of a complex

modulated carrier having been amplified by a non linear element. Modulated carrier refers to a complex one, having both an amplitude and frequency component. This typically might be a QAM / COMA type 5 signal. With reference to Figure 1; The system comprises two distinct loops. The first loop is an arrangement and a method of pre-distorting the modulated carrier in lo order to compensate for the non-linearities present in amplifier 012, that is the non-linear response of the transfer function of amplifier 012. The second loop is a feed-forward loop that compensates for the residual distortion present in the amplified 15 modulated carrier after it has passed through amplifier 012.

Loopl 20 The first loop comprises system components 001 to 011 and 021. A modulated carrier is presented at the input to the amplifier system, as shown in Figure 3.

The modulated carrier is then split by coupler 001 and splitter 005 into three paths. Two of these 25 three paths will be considered in the description of

loopl. Path 1. The modulated carrier passes through coupler 001 and preamplifier 002. The pre-amplifier 002 30 will naturally introduce a distortion component to the modulated carrier, as shown in Figure 4. The majority of the modulated carrier then passes through coupler 003 before being presented to a summation circuit 004 on input port (a). Considering the 3s modulated carrier split at coupler 003; the minority of the modulated carrier passes through variable attenuator 008 where it is presented to input port

- 11 (b) on summation circuit 007. Circuits 003 and 008 may be considered to be linear, that is they introduce an insignificant level of distortion with respect to the non-linear elements under 5 consideration. Path 2. The modulated carrier is split into two paths by coupler circuit 001, with the majority of the modulated carrier being presented to pre- amplifier 10 002 as described above, and the minority being further split by splitter circuit 005. From port (e) of splitter 005 the modulated carrier then passes through a variable phase shifter circuit 006 and is presented to input port (a) on summation circuit 007.

15 The variable attenuator 008 and variable phase shifter 006 are then adjusted by receiver circuit 011 so that the magnitudes of the signals at input ports (a) and (b) of summation circuit 007 are equal, and the phases of the signals at ports (a) and (b) of 20 summation circuit 007 are anti-phase. The resultant summed signal at the output of summation circuit 007 port (e) contains as little of the original modulated carrier as receiver circuit 011 can adjust for, and thus comprises mainly the distortion component 25 introduced by pre-amplifier 002, as shown in Figure 5. Circuits 001, 005, 006 and 007 may be considered to be linear.

The distortion component present at the output of 30 summation circuit 007 port (e), as shown in Figure 5, is then adjusted in phase and amplitude on passing through phase shifter circuit 009 and variable amplifier circuit 010 by common receiving circuit 021. Operation of phase shifter circuit 009 and 35 variable amplifier circuit 010 is described below with reference to the second loop and the resultant signal is shown in Figure 6. On arriving at port (b)

of summation circuit 004 the distortion component is then summed with the modulated carrier and distortion component on port (a) of summation circuit 004. The result at port (c) is a modulated carrier with a 5 distortion component of specific phase and amplitude as shown in Figure 7, which when passed through amplifier 012 will cancel the distortion component normally produced by amplifier 012 if amplifier 012 were presented with just the undistorted modulated carrier. Loop 2.

15 At the output of amplifier 012 the modulated carrier will still contain a small distortion component that is a function of the efficiency or effectiveness of dynamic cancellation in loop 1. The modulated carrier and the distortion component are split into 20 two paths by coupling circuit 013. The majority of the signal is transmitted to coupling circuit 019 and the minority of the signal is transmitted to attenuator 014. The modulated carrier and the distortion component are linearly reduced in 25 amplitude on passing through variable attenuator circuit 014 and presented to input port (b) of summation circuit 016. Attenuator 014 reduces the signal so on arriving at port (b) of summation circuit 016 the signal has a magnitude equal to the 30 signal appearing at port (a) of summation circuit 016. Input port (a) of summation circuit 016 also receives an undistorted modulated carrier from the input, via 35 splitter 005 port (b). Splitter 005 splits the modulated carrier that is then passed through variable phase shifting circuit 015. The modulated

- 13 carriers that are present on ports (a) and (b) of summation circuit 016 are presented to receiver circuit 020 and adjusted in magnitude (via variable attenuator circuit 014) and in phase (via variable 5 phase shifting circuit 015) to be in anti-phase with the signal at port (b) of summation circuit 016. The resultant signal at the output port (e) of summation circuit 016 contains a minimum modulated carrier component and a maximum distortion component, as 10 shown in Figure 8.

As shown in Figure 9 common receiving circuit 021 then adjusts the phase via variable phase shifting circuit 017 and magnitude via variable amplifier 018, IS of the resultant signal from port (e) of summation circuit 016. On re-combination via coupling circuit 019 of the signal at the output of amplifier 018 with the signal present at the output of amplifier 012, the resulting distortion component at the output of 20 the amplifier system is reduced further, as shown in Figure 10. Concurrently, common receiving circuit 021 adjusts the phase via phase shifter circuit 009 and magnitude via variable amplifier circuit 010, of the resultant signal from port (e) of summation 25 circuit 007, as shown in Figure 6, so that on re combination via summation circuit 004 with the signal present at the output of pre-amplifier 002 the distortion component is dynamically opposed in phase and amplitude to the distortion components inherent 30 in operation of the amplifier 012.

Figure 2 shows a modification of the amplifier assembly described above. The system is identical to that of Figure 1 except for the addition of a third 35 outlet port to splitter 005 and inclusion of a third (control) loop, as described below, with consequent alteration to the origin of control signals.

Loop3 measures the amplifier systems output signal and adjusts loopl and loop2 via receiver circuit 021 to ensure that the distortion component at the output 5 of the amplifier system is kept to a minimum.

The signal at the output of the amplifier system is split into two paths by coupling circuit 022. The majority of the signal is transmitted to the output lo of the overall amplifier system and the minority of the signal is transmitted to a variable attenuator 023. The resultant attenuated signal is presented to port (c) of circuit 024. Circuit 024 is functionally equivalent to the summation circuits 007 and 016 of 15 looper and 2 respectively. For convenience and to distinguish between the loops, circuit 024 is identified as a "summing circuit".

The signal at the input of the amplifier system is 20 split by coupler 001, split again by splitter 005 (port (d)), phase shifted by a variable phase shifter 026, and then presented to port (a) of summing circuit 024. Preferably the magnitudes of the signals at input ports (a) and (b) of summing circuit 25 024 are substantially equal and the phases of the signals at ports (a) and (b) of summing circuit 024 are anti-phase.

The resulting signal at output port (d) of summing 30 circuit 024 is optimised via receiver circuit 025 for a minimum distortion component. This is accomplished by receiver circuit 025 dynamically adjusting both the phase of the signal at input port (a) (via variable phase shifter 026) and the amplitude of the 35 signal at input port (c) (via variable attenuator 023). The resultant output of variable attenuator 023 (input to port (c)) is shown in Figure 11.

- 15 The resulting signal at output port (b) of summing circuit 024 is then directed to common receiving circuit 021. As described above receiving circuit 021 5 adjusts both phase shift circuits 009 and 017, and variable amplifier circuits 010 and 018, for a minimum distortion component at the amplifier systems output, as measured by receiving circuit 021 at output port (b) of summing circuit 024. For clarity lo receiving circuits 021 and 025 are shown in Figure 2 as separate modules, with output ports (b) and (d) of summing circuit 024 carrying the same information.

It is self-evident that receiving circuits 021 and 025 may be combined and presented with a signal from 15 a single output port of summing circuit 024.

The resulting output of the overall amplifier system is shown in Figure 12.

20 With reference to Figure 13; A composite modulated carrier signal fed from port (c) of summation circuit 004 is presented to port (a) of splitter 012a. Port (b) of splitter 012a transmits a proportion of the modulated carrier to an integrator circuit 012b, 25 which contains a method of detecting the amplitude component of the modulated carrier. This is then integrated and the bias of amplifier circuit 012d is adjusted to enable the amplifier 012 to produce the requisite power. Should the integrator 012b 30 determine that the duration of the power produced by amplifier circuit 012d exceeds a finite time limit that if exceeded will result in junction damage to amplifier circuit 012d, then the integrator 012b will reduce the power by attenuating the modulated carrier 35 via variable attenuator circuit 012c, possibly as well as adjusting the amplifier bias.

Claims

- 16 CLAIMS

1. An amplifier assembly including: 5 (a) a pre-amplifier having an input receiving an input modulated carrier and an output feeding an amplifier; and (b) a first loop including a dynamically adjusted variable amplifier and a dynamically adjusted phase lo shifter in the output path of the first loop; wherein a portion of the input modulated carrier feeds into the first loop, and the first loop derives a distortion component from the output of the pre amplifier and sums this distortion component with the 15 modulated carrier output of the pre-amplifier before feeding the resultant summed signal to an input of the amplifier.

2. An amplifier assembly including: 20 (a) a pre-amplifier having an input receiving an input modulated carrier and an output feeding an amplifier; (b) a first loop including a dynamically adjusted variable amplifier and a dynamically adjusted phase 25 shifter in the output path of the first loop, wherein a portion of the input modulated carrier feeds into the first loop, and the first loop derives a distortion component from the output of the pre amplifier and sums this distortion component with the 30 modulated carrier output of the pre-amplifier before feeding the resultant summed signal to an input of the amplifier; and (c) a second loop including a dynamically adjusted variable amplifier and a dynamically adjusted phase 35 shifter in the output path of the second loop; wherein the second loop derives a residual distortion component from the output of the amplifier and sums

- 17 this residual distortion component with the input modulated carrier before feeding the resultant summed signal to an output of the amplifier assembly.

5 3. An amplifier assembly as claimed in Claims 1

or 2 wherein the resultant summed signal of the or each loop comprises mainly the distortion component relative to the modulated carrier.

lo 4. An amplifier assembly as claimed in any one of the preceding claims wherein the first loop generates distortion signals dynamically opposed in phase and amplitude to the distortion components inherent in operation of the amplifier.

5. An amplifier assembly as claimed in any one of Claims 2 to 4 wherein the output of the second loop cancels distortion signals generated by the first loop.

6. An amplifier assembly as claimed in any one of the preceding claims wherein each variable amplifier and phase shifter is dynamically controlled in accordance with an output signal of the amplifier.

7. An amplifier assembly as claimed in Claim 6 including a third (control) loop summing a signal derived from the input modulated carrier with a signal derived from the amplifier output so as to 30 minimise a distortion component of a resultant summed signal, wherein such summed signal dynamically controls each variable amplifier and phase shifter.

8. An amplifier assembly as claimed in Claim 7 35 wherein the control loop includes: (a) a summing circuit; and

(b) a variable attenuator and a control phase shifter in the respective input paths of the summing circuit; and such variable attenuator and control phase shifter are dynamically controlled by an output of 5 the summing circuit.

9. An amplifier assembly as claimed in Claims 7 or 8 wherein substantially equivalent signals control both the input paths of the third loop and the lo respective output paths of the first and second 1 cops.

10. An amplifier assembly as claimed in any one of the preceding claims wherein an amplitude 15 component of the modulated carrier input to the amplifier is integrated to generate a power level signal independently of amplification of the modulated carrier and the amplifier bias is dynamically adjusted in accordance with the power 20 level signal.

11. An amplifier assembly as claimed in Claim 10 wherein the bias is adjusted in accordance with a function of the power input to the amplifier 25 integrated over time.

12. An amplifier assembly as claimed in Claim 11 wherein the bias is adjusted in accordance with a function of the specific heat capacity of the output 30 portion of the amplifier and of the statistical probability of data occurrence in the modulated carrier input to the amplifier.

13. An amplifier assembly as claimed in any one 35 of Claims 10 to 12 additionally including a variable attenuator in the modulated carrier input to the amplifier.

- 19 14. An amplifier assembly as claimed in Claim 13 wherein the modulated carrier is dynamically attenuated in accordance with a function of the 5 specific heat capacity of the output portion of the amplifier and of the statistical probability of data occurrence in the modulated carrier input to the amplifier. 15. An amplifier assembly as claimed in Claim 14 wherein the modulated carrier input to the amplifier is attenuated when the duration of the amplifier power output exceeds a predetermined time limit which would induce thermal damage to the amplifier.

16. A method of operating an amplifier assembly including: (a) a preamplifier having an input receiving an input modulated carrier and an output feeding an 20 amplifier; and (b) a first loop including a dynamically adjusted variable amplifier and a dynamically adjusted phase shifter in the output path of the first loops wherein a portion of the input modulated carrier 25 feeds into the first loop, and the first loop derives a distortion component from the output of the pre amplifier and sums this distortion component with the modulated carrier output of the pre-amplifier before feeding the resultant summed signal to an input of 30 the amplifier.

17. A method of operating an amplifier assembly including: (a) a preamplifier having an input receiving an 35 input modulated carrier and an output feeding an amplifier;

- 20 (b) a first loop including a dynamically adjusted variable amplifier and a dynamically adjusted phase shifter in the output path of the first loop, wherein a portion of the input modulated carrier feeds into 5 the first loop, and the first loop derives a distortion component from the output of the pre amplifier and sums this distortion component with the modulated carrier output of the pre-amplifier before feeding the resultant summed signal to an input of lo the amplifier; and (c) a second loop including a dynamically adjusted variable amplifier and a dynamically adjusted phase shifter in the output path of the second loop; wherein the second loop derives a residual distortion 15 component from the output of the amplifier and sums this residual distortion component with the input modulated carrier before feeding the resultant summed signal to an output of the amplifier assembly.

20 18. A method of operating an amplifier assembly as claimed in Claims 16 or 17 wherein the amplifier includes a third (control) loop summing a signal derived from the input modulated carrier with a signal derived from the amplifier output so as to 25 minimise a distortion component of a resultant summed signal, wherein such summed signal dynamically controls each variable amplifier and phase shifter.

30 19. A method of operating an amplifier assembly as claimed in any one of Claims 16 to 18 wherein an amplitude component of a modulated carrier input to the amplifier is integrated to generate a power level signal independently of amplification of the 35 modulated carrier and the amplifier bias is dynamically adjusted in accordance with the power level signal.

- 21 20. A method of operating an amplifier assembly as claimed in Claim 19 wherein the modulated carrier is dynamically attenuated in accordance with a 5 function of the specific heat capacity of the output portion of the amplifier and of the statistical probability of data occurrence in the modulated carrier input to the amplifier.

lo 21. A method of operating an amplifier assembly as claimed in Claim 20 wherein the modulated carrier input to the amplifier is attenuated when the duration of the amplifier power output exceeds a predetermined time limit which would induce thermal 15 damage to the amplifier.

22. An amplifier assembly substantially as described and with reference to the accompanying drawings. 23. A method of operating an amplifier assembly substantially as described and with reference to the accompanying drawings.