GB2353646A

GB2353646A - A multistage amplifier with multistage predistortion and a predistorter

Info

Publication number: GB2353646A
Application number: GB9920408A
Authority: GB
Inventors: Hashem Madadi; Kambiz Madani
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1999-08-27
Filing date: 1999-08-27
Publication date: 2001-02-28
Also published as: GB9920408D0

Abstract

An amplifier circuit means is disclosed in which distortion introduced into an input signal in a multistage non-linear power amplifier circuit 20 is compensated for in advance by a multistage pre-distortion linearisation circuit 10 comprising a plurality of individual pre-distortion lineariser circuits. Each pre-distortion lineariser circuit stage corresponds to a respective one of the non-linea amplifier circuit stages, and pre-distorts the input signal in such a manner as to be the inverse of any distortion introduced into the input signal upon subsequent amplification in the respective amplifier circuit stage. The multistage pre-distortion technique has the advantage that each pre-distortion circuit stage can be closely matched to the corresponding amplifier circuit stage with respect to frequency characteristics and dynamic range, and hence large cancellation of intermodulation distortion is possible. A pre-distortion circuit suitable for use in the multistage pre-distortion linearisation circuit is also disclosed.

Description

2353646 An AWlifier Circuit with Multistage Pre-Distortion The present

invention relates to an amplifier circuit means for arnphfing an input signal in a substantially linear manner, and more particularly to an amplifier circuit means for amplifying an input signal which pre- distorts the input signal to account for any distortion which may be introduced into the signal by amplification in the amplifier stage. In addition, the present invention also relates to a pre-distortion circuit which may be used with the amplifier circuit means of the present invention.

The use of pre-distortion linearisers is known in the art for the cancellation of intermodulation distortion caused by the non-linear power transfer characteristic of power amplifiers. Figure I shows a typical power transfer characteristic of a class A amplifier, and from Figure I it will be seen that the characteristic of a typical amplifier is split into a first linear region 2 wherein the power of the signal output from the amplifier is in direct proportion to the power of the signal input to the amplifier, and a non-linear region 4 wherein the power output by the amplifier is no longer in direct proportion to the input power. In order to obtain maximuin power added efficiency the power amplifier should be operated in the non-linear region 4 to give the maximum power out XOuTfor the given input signal XIN. However, when a power amplifier is arranged to be operated in this manner the non-linear characteristic in the region 4 causes third order intermodulation products to be generated in the signal output from the amplifier, and which may cause problems fiuther along the signal path of a system in which the amplifier may be employed. In particular, the third order intermodulation products (inter-mods) may cause problems in wide band communication systems, wherein the generated third order intermodulation products may act as interference to other channels in the band.

In order to overcome this problem, a common technique in the prior art is to "back off' the amplifier from the most power efficient operating point in 2 the non-linear region 4 to another operating point of the characteristic within the linear region 2. This prevents the generation of third order intermodulation products at the expense of a substantial degradation in power amplifier efficiency.

For example, to obtain a typical 5 or 6dB of cancellation of intermodulation distortion in a class A power amplifier, a 3dB back off is required corresponding to a reduction in efficiency of the amplifier of 50%.

A further prior art technique for reducing intermodulation distortion is, as already mentioned above, the use of pre-distortion linearisers. In general, a pre-distortion lineariser circuit is a circuit which is arranged to have the inverse non-linearity characteristic to the power amplifier, and is cascaded in front of the power amplifier such that the input signal to the amplifier is pre- distorted in the pre-distortion lineariser circuit prior to being input into the amplifier. The signal is pre-distorted in such a manner such that when amplified in an amplifier operating at its point of maximum efficiency within the non-linear region of its power transfer characteristic, no intermodulation distortion is introduced into the signal. In ellect the non-linearity of the pre-distortion circuit is the inverse of the non-linearity of the power ampIffier, and therefore cancels the effect of the power amplifier non-linearity. An exemplary prior art arrangement of a pre- distortion lineariser as described above is given in US 5258722 (Jeffers), which is characteristic of the present state of the art prior to the present invention as described above, and which will, therefore, not be discussed finther herein.

Whilst it is possible to obtain 5 or 6dB of cancellation of intermodulation products using the prior art linearisation technique as described above, a number of severe disadvantages of the conventional technique prevent a better cancellation performance, and hence preclude the use of the technique in a number of applications, and in particular in wide band multi-carrier RF communications applications. More particularly, it has not previously been possible for conventional circuits to model the non-linearities introduced by the power amplifier accurately, and as a result the pre-distortion characteristics 3 introduced into the input signal by the pre-distortion circuit have not corresponded to those of the power amplifier, and hence good cancellation of intermodulation distortion has not been achieved. In particular, the pre-distortion characteristics of the pre-distorted circuit have generally varied in a different way to those of the power amplifier as the frequency and/or the input power level has been varied, and as a result cancellation of intermodulation distortion has not been achieved over a wide bandwidth and/or dynamic range. In addition, pre-distortion linearisers of the prior art have generally been designed to cancel the non-linear characteristics of the power stage only, as the driver stages of the power amplifier have previously been assumed to be highly linear. VVhilst it is true that most intermodulation distortion is caused by the power stage, for maximum power added efficiency the driver stages must also be operated in their nonlinear regions, and hence also require pre-distortion respective to each stage.

In order to overcome the above disadvantages of the prior art pre distortion techniques, it has been common practice to combine predistortion of the input signal with backing-off of the amplifier with the attendant degradation in amplifier efficiency as hereinbefore described.

The present invention overcomes the above-described disadvantages of the pre-distortion techniques of the prior art by presenting an amplifier circuit and associated method of amplification which employs multistage pre- distortion linearisation to cancel the non-linearities introduced into the signal to be amplified by a given multistage non-linear amplifier. More specifically, as all the stages of a power amplifier circuit contribute to the non-linearity of the whole amplifier, then pre-distortion must be applied to the input signal to the amplifier to account for the distortion introduced in each stage of the amplifier if substantial distortion cancellation is to be achieved.

In order to achieve this multistage pre-distortion, according to the present invention there is provided an amplifier circuit means for amplifying an input signal in a substantially linear manner, said circuit means comprising:

4 a plurality of pre-distortion circuit stages arranged in a first series, a first pre-distortion circuit stage in said first series being arranged to receive said input signal as an input; and a plurality of amplifier circuit stages arranged in a second series, a first amplifier circuit stage in said second series being arranged to receive the input signal output from a last pre-distortion circuit stage in said first series as an input; wherein each respective pre-distortion circuit stage is arranged to process the input signal in a predetermined manner to substantially cancel any distortion introduced into the input signal by a corresponding respective one of said plurality of amplifier circuit stages during subsequent amplification of the input signal in said plurality of amplifier circuit stages.

The plurality of pre-distortion circuit stages are preferably further arranged such that each of the remainder of the stages other than the first stage receive the input signal as an output of the immediately preceding circuit stage wherein the input signal passes through and is processed in each pre- distortion circuit stage in turn.

Furthermore, the plurality of amplifier circuit stages are preferably flirther arranged such that the processed input signal from the predistortion circuit stages passes through each of the amplifier circuit stages in turn and is ainplified in each in turn, thereby cancelling the pre-distortion applied to the input signal in the pre-distortion circuit stages.

The pre-distortion circuit stages may be placed in order with respect to the amplifier circuit stages such that the e pre-distortion stage cancels the distortion introduced into the input signal due to the n' amplifier circuit stage, or alternatively the pre-distortion circuit stages need not be arranged in order with respect to the amplifier circuit stages. In this case, the n' pre- distortion circuit stage cancels the distortion introduced into the input signal due to the j' amplifier circuit stage, wherein n is not equal to j.

Preferably, the pre-distortion characteristics of each pre-distortion circuit stage are identical to the corresponding respective amplified circuit stage such that almost ideal intermodulation distortion cancellation can be achieved.

The matching of the pre-distortion circuit stages to the amplifier circuit stages can be achieved by each corresponding amplifier stage and pre-distortion stage utilising the same device technology.

In a preferred embodiment each pre-distortion circuit stage preferably comprises a pre-distortion means arranged to apply pre- distortion to the input signal to the stage; and a linear amplifier means arranged to receive the processed input signal from the pre-distortion means and to amplify the signal in a substantially linear manner without introducing further distortion into the signal; whereby each of said pre-distortion circuit stages acts as an active stage thereby increasing the overall gain of the amplifier circuit means.

Furthermore, in the particularly preferred embodiment, the pre distortion means further comprise a first coupler arranged to receive the input signal to the pre-distortion circuit stage, and to split the input signal into a first circuit branch and a second circuit branch; a first phase adjuster arranged in the first circuit branch for varying the phase of the signal on the first circuit branch; an amplifier means arranged in the second circuit branch for amplifying the signal on the second circuit branch; at least one variable attenuator means arranged in series with the amplifier means in the second circuit branch for attenuating the signal on the second circuit branch; and a second coupler arranged to receive the respective signals from the first and second branches and subtract the signal from the second branch from the signal firom. the first branch to give a pre-distorted output signal; wherein said amplifier means has nominally substantially identical non-linearity characteristics to the respective amplifier circuit stage corresponding to the particular pre-distortion circuit stage of which the particular pre-distortion 6 means form a part.

In addition to the above, the present invention also provides a pre distortion circuit means for pre-distorting an input signal prior to amplification of said input signal in a non-linear amplifier, said circuit means comprising:

a first coupler arranged to receive the input signal and to split the input signal into a first circuit branch and a second circuit branch; a first phase adjuster arranged in the first circuit branch for varying the phase of the second circuit branch; an amplifier means arranged in the second circuit branch for amplifying the signal on the second circuit branch; at least one attenuator means arranged in series with the amplifier means in the second circuit branch for attenuating the signal on the second circuit branch; and a second coupler arranged to receive the respective signals from the first and second circuit branches and subtract the signal from the second branch from the signal from the first branch to give a pre-distorted output signal; wherein said amplifier means has nominally substantially identical non-linear characteristics as said non-linear amplifier.

Preferably the pre-distortion circuit means of the present invention further includes a linear amplifier means arranged to receive the output of the second coupler and to amplify the output in a substantially linear manner without introducing finther distortion into the pre-distorted output signal, wherein said pre distortion circuit means acts as an active stage thereby allowing said circuit means to act as a driver stage for the non-linear amplifier.

Both the amplifier circuit means and the pre-distortion circuit means of the present invention are particularly well suited to RF microwave applications, and in particular to wide band microwave communications applications wherein a good degree of intermodulation distortion cancellation is required.

Furthermore, from another aspect the present invention also provides 7 a method of amplifing an input signal in a substantially linear manner, comprising the steps of.

receiving said input signal in a first pre-distortion circuit stage, said first pre-distortion circuit stage being arranged to be first of a plurality of pre distortion circuit stages arranged in a first series; processing said input signal in said plurality of pre-distortion circuit stages; and receiving the processed input signal from a last pre-distortion circuit stage in said first series into a first amplifier circuit stage of a plurality of amplifier circuit stages arranged in a second series; and amplifying said processed input signal in said plurality of amplifier circuit stages; wherein said processing step is characterised in that each respective pre-distortion circuit stage is arranged to process the input signal in a predetermined manner to substantially cancel any distortion introduced into the input signal by a corresponding respective one of said plurality of amplifier circuit stages during said ainplit4ng step.

The processing step preferably further comprises the steps of pre distorting the input signal in turn in each pre-distortion circuit stage to apply pre distortion to said input signal corresponding substantially to the inverse of any distortion introduced into the input signal during amplification in each respective corresponding amplifier circuit stage; and linearly amplifying the processed input signal in each pre-distortion circuit stage prior to being output from each pre-distortion circuit stage; whereby each of said pre-distortion circuit stages acts as an active stage to amplify the processed input signal.

In the present invention, the preferred embodiment of the pre distortion step preferably comprises the steps of.

splitting said input signal into a first circuit branch and a second 8 circuit branch; adjusting the phase of the signal on the first circuit branch; amplifying the signal on the second circuit branch; attenuating the signal on the second circuit branch; and subtracting the signal from the second branch from the signal from the first branch to give a pre-distorted output signal; wherein said step of amplification of the signal on the second circuit branch is substantially nominally identical to said step of amplification of the processed input signal, whereby substantially nominally identical non- linearity characteristics are obtained from each respective amplification step.

Moreover, from a further aspect the present invention provides a method of pre-distorting an input signal prior to arnplification of said input signal in a non-linear amplifier, said method compiising the steps of.

splitting the input signal into a first circuit branch and a second circuit branch; adjusting the phase of the signal on the first circuit branch; amplifying the signal on the second circuit branch; attenuating the signal on the second circuit branch; and subtracting the signal on the second circuit branch from the signal on the first circuit branch to give a pre-distorted output signal; wherein the signal on the second circuit branch is amplified in a nominally substantially identical non-linear manner to that as would be achieved in said non-linear amplifier.

Preferably, the method of pre-distorting an input signal according to the present invention further comprises the steps of ampli4ring the pre-distorted input signal in a variable linear amplifier prior to outputting the processed signal from the pre-distortion circuit stage such that each pre-distortion circuit stage may act as an active stage and hence contribute to the overall gain of an amplifier system in which the pre-distortion circuit may be employed.

9 The use of multiple pre-distortion stages arranged in series to pre process the input signal prior to amplification in a multiple stage power amplifier presents a nurnber of significant advaptages. In particular, as each pre- distortion stage can use the same device technology with identical non-linearity characteristics to that of the corresponding power amplifier stage, then by careful adjustments and optimisation of the parameters of each pre-distortion stage it is possible to obtain superior cancellation of intermodulation distortion than was possible with prior art pre-distortion techniques.

Furthermore, for arnplifier linearisation applications, the multistage pre-distortion lineariser technique of the present invention is inherently wide band, since the non-linear frequency characteristics of the plurality of pre- distortion circuit stages are similar to the respective multistage amplifier devices that each respective pre-distortion stage is trying to cancel out. This allows the multistage pre-distortion technique of the present invention to be particularly suitable for use in wide band communication systems.

Furthermore, in addition to substantially identical frequency characteristics between each pre-distortion circuit stage and the corresponding amplifier circuit stage, the non-linearity of the pre-distortion stage also follows that of the power amplifier stage as the power of the input signal varies, resulting in a significant increase in the linear dynamic range of the applied pre- distortion, when compared with the prior art.

Finally, when compared with the prior art feefforward techniques, which also use active devices to cancel out the non-linearities of the power amplifier, the pre-distortion of the present invention takes place at much lower power levels, hence the power added efficiency of the amplifier is not significantly affected, and high amplifier efficiency can be combined with good linearity.

Further features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments thereof, presented by way of example only, and with reference to the accompanying drawings in which:

Figure 1 shows the power transfer characteristic of a typical class A amplifier; Figure 2 shows a system block diagram of the amplifier circuit means of the present invention incorporating multiple pre-distortion stages; Figure 3 shows a circuit block diagram of a preferred embodiment of the pre-distortion circuit of the present invention; and Figure 4 illustrates how two or more of the pre-distortion circuits of the present invention can be cascaded for use in the amplifier circuit means of the present invention.

A first embodiment of the amplifier circuit means of the present invention will now be described with reference to Figure 2.

In Figure 2, a power amplifier circuit 20 is formed from a number of individual amplifier stages NAI to NAn (25, 24, 21 respectively). Each ampIffier stage is arranged to operate in its most power efficient manner and hence the operating point of each amplifier stage is within the non-linear region of each stage's power transfer characteristic. As each stage of the power amplifier is arranged in series with the other stages, in that an input signal cascades through each stage in turn, each stage will add intermodulation distortion to the cascading input signal due to the non-linear characteristic of each amplifier stage at the chosen operating points. If the signal output from such a multistage amplifier were to be used directly from the output of the amplifier without any measures for compensating for the intermodulation distortion, then the signal would not be suitable for use in wide band communication systems due to the generated third order intermodulation products effectively interfering with other channels in the band.

In order to overcome the problem of intermodulation distortion, the present invention presents a multistage pre-distortion linearisation circuit 10 which consists of a number of individual pre-distortion circuit stages API, 1 to APLn (15, 11 14, and I I respectively) which are arranged in cascade such that the output of a preceding pre-distortion circuit stage is generally received by the following pre distortion circuit stage as an input. More particularly, the first pre- distortion circuit stage I I in the series receives the input signal 6 to the amplifier circuit means as an input, and processes the signal 6 in a predetermined manner to effectively pre-distort the signal. The output of the first predistortion circuit stage 11 is then fed to the second pre-distortion circuit stage in the series which finther processes the signal before outputting it to the next pre-distortion circuit stage for processing, and so on and so forth. The input signal 6 therefore cascades through each pre-distortion circuit stage in turn and is output from the last predistortion circuit stage 15 in the series (APLI) as a processed input signal 7. This processed input signal 7 is then fed to the first amplifier circuit stage in the amplifier 20.

Each individual pre-distortion circuit stage is arranged to cancel the non-linearities introduced into the input signal by amplification in a particular one of the amplifier circuit stages. More particularly, the parameters of each individual pre-distortion stage are arranged to produce a non-linear behaviour which is ideally the inverse of the corresponding front end amplifier stage non linearity. As an example, therefore, the pre-distortion. applied in the first pre distortion stage APLI (15) corrects the distortion introduced into the input signal by the corresponding amplifier stage NA 1 (25); the second pre-distortion circuit stage APL2 (14) corrects the second non-linear amplifier NA2 (24); and so on.

Please note, however, that there need not be a direct positional correlation between the position of a particular pre-distortion circuit stage in the series 10 and the position of the corresponding amplifier for which the particular pre- distortion stage accounts in the series 20 of amplifier circuit stages, as it may be the case that the particular pre-distortion circuit which accounts for the distortion introduced by a particular amplifier circuit stage is positioned at a different position in its respective series 10 than the amplifier circuit stage in its respective series 20. The only condition which must apply is that there are an equal number of pre- 12 distortion circuit stages to the number of amplifier circuit stages, and that each pre-distortion circuit stage introduces pre-distortion into the input signal which cancels the distortion which may be introduced into the input signal in a corresponding respective arriplifier stage.

By using a multistage pre-distortion linearisation circuit 10 which includes a number of individual pre-distortion circuit stages, and by ensuring that the pre-distortion introduced into the input signal by each pre- distortion circuit stage is ideally the inverse of the respective corresponding amplifier stage to each pre-distortion stage, much improved intermodulation distortion cancellation can be achieved when compared with the prior art.

Preferably, in order to obtain substantially identical non-linear characteristics between each pre-distortion circuit stage and its corresponding amplifier circuit stage, the pre-distortion circuit stage will utilise the same device technology as the amplifier circuit stage, thereby enabling optimisation of the non linear parameters of the pre-distortion circuit stage to substantially match those of the corresponding amplifier stage.

Furthermore, each pre-distortion circuit stage may incorporate a linear amplification element which allows the pre-distortion circuit stage to act as an active stage and hence increase the overall gain of the amplifier circuit means of the present invention. By introducing such an amplification element into the pre-distortion circuit stages, the pre-distortion stages can act as a driver stage for the primary power amplifier stages thus improving the power added efficiency of the whole amplifier system.

A pre-distortion circuit means according to the present invention and corresponding to a single pre-distortion circuit stage will now be described with reference to Figure 3.

In Figure 3, the input signal to the pre-distortion circuit stage is first fed to a first coupler 31 which is arranged to split the signal into a first circuit branch and a second circuit branch. The second circuit branch comprises a non- 13 linear amplifier module 36, which is arranged to be substantially identical in its non-hnear characteristics to that of the corresponding non-linear power amplifier stage for which the particular pre-distortion circuit is trying to correct. In addition to the non-linear amplifier 36, the second circuit branch also comprises at least one, but preferably two, variable attenuators 34 and 38 arranged to adjust the input and the output power of the non-linear amplifier 36. Where a single variable attenuator is employed, the attenuator may be positioned in front of the non-linear amplifier 36 whereby to adjust the power of the input signal to the non- linear amplifier, or at the output of the non-linear amplifier whereby it adjusts the output signal power of the amplifier. Where two variable attenuators are used, then one is placed before the amplifier and the second after the amplifier to adjust the power of both the input and the output signals.

The first circuit branch comprises a single variable phase adjuster 32 which is arranged to compensate for the phase delay caused by the variable attenuators and the non-linear amplifier in the second branch such that the phase difference between the signals on the first and second branches can be maintained.

Following processing in their respective branches, the respective signals from the first and second circuit branches are then fed to a second coupler 33 which acts to combine the signals in a manner to be described below, and the combined signal is then fed to the variable gain linear amplifier 35 for amplification prior to being output from the stage. Please note that the variable gain linear amplifier 35 does not introduce any additional distortion or pre distortion into the signal and it is merely used to adjust the drive power of the stage such that the stage can be used as a driver stage for the power amplifier stages. The linear amplifier 35 does not contribute to the pre-distortion characteristics of the pre-distortion stage, and may be omitted if it is only required that the pre-distortion circuit stage act to pre-distort the signal to cancel intermodulation distortion, and is not required also to act as a driver stage for the power amplifier. Please note that all the elements in the pre-distortion circuit 14 means and in each pre-distortion circuit stage of the present invention are preferably impedance matched at their respective input and output ports.

The first coupler 31 and the second coupler 33 are preferably 90 hybrid couplers, which act to split an input signal into two comparable signals, with 90' phase difference therebetween. The phase difference between the two signals ensure that the signal power out of the couplers is equal to the input signal power. When combining two signals the inverse operation is achieved ie. two identical signals with 90' phase difference therebetween are combined into a single signal of equal power to the two input signals. The effect obtain by the combination of the signals on the two circuit branches in the second hybrid coupler 33 is explained in more detail below.

The method of operation of the pre-distortion circuit will now be described. Prior to operation, each non-linear pre-distortion amplifier36 in each pre-distortion stage is adjusted so as to be nominally identical with respect to non linear characteristics to the corresponding amplifier circuit stage of the output power amplifier which the particular pre-distortion circuit stage is attempting to linearise. The pre-distordon applied to the input signal by each pre- distortion stage is produced by inverting the slope of the non-linear characteristics of the corresponding power amplifier stage. Good cancellation of the non- linearity is possible if all the circuit parameters and bias points of the respective pre-distortion stages are carefifly optimised.

The pre-distortion is applied to the input signal in each respective predistortion circuit stage as follows. With respect to Figure 3 the power of the input signal is split equally in amplitude but with 90' phase difference by the first coupler 3 1, and the split signals are each fed into a respective circuit branch. As described earlier, the second branch consists of a non-linear amplifier with adjustable attenuations at its input and output whereas the first branch has a linear variable phase shifter that can be adjusted to compensate for the phase delay of the components in the non-linear branch (ie. the second branch). The outputs of the linear (the first branch) and the non-linear (the second branch) branches are subtracted from each other by combining with a second coupler at the output. In this respect, the second coupler acts to subtract the signal on the second branch from the signal on the first.branch. to provide the pre-distorted output signal.

Finally, in the preferred embodiment, the linear amplifier 35 is placed after the second coupler to produce the required output power magnitude, without destroying the pre-distortion characteristics.

The appropriate pre-distortion characteristic is achieved by the pre distortion circuit as follows. Initially, as the input power is increased from a low value, the overall gain of the circuit is constant since the amplifier 36 on the second circuit branch is operating in its linear region. As the input power increases, however, the amplifier 36 begins to saturate in its non-linear region of operation and the output power of the amplifier starts to limit (c.f. region 4 of Figure 1). Thus, whilst the signal on the second non-linear circuit branch is limited by the characteristic of the power amplifier, the signal on the first linear circuit branch is not so limited, and hence the subtraction of the second signal from the first in the coupler 33 produces an increase in the overall gain of the pre distortion circuit. This gain expansion characteristic of the pre- distortion circuit is then adjusted by means of adjusting the gain of the amplifier 36 and the variable attenuators 34 and 38 to cancel out the gain compression of the corresponding non-linear amplifier stage of the output power amplifier. In other words, the gain expansion characteristic of the pre-distortion circuit is ideally adjusted to be the inverse of the gain compression characteristic of the amplifier power stage when operating in its respective saturation regions, and therefore the combination of gain expansion of the input signal in the pre-distortion stage together with the gain compression of the signal in the corresponding amplifier stage results in an overall linear amplification of the input signal, preventing the generation of third order intermodulation products.

As the non-linear amplifier 36 of the pre-distortion circuit is 16 nominally identical with respect to its non-linear characteristics to the corresponding power amplifier circuit stage, good matching between the pre distortion applied in the pre-distortion circuit and the distortion generated in the amplifier stage can be achieved, resulting in good cancellation of intermodulation distortion. Matching of the pre-distortion non-linear amplifier 36 and the corresponding power amplifier stage is preferably achieved by using the same device technology in each circuit.

A discussion of how the pre-distortion circuit means of the present invention can be used as the respective pre-distortion circuit stages of the amplified circuit means of the present invention will now be undertaken with reference to Figure 4.

In Figure 4 a first pre-distortion stage 41 and a second pre-distortion stage 42 each respectively comprise the pre-distortion circuit as previously described with respect to Figure 3. Although each pre-distortion stage 41 and 42 will have been adjusted and optimised individually to provide the inverse pre distortion characteristics to each stage's respective corresponding power amplifier stage, the operation of both of these stages is identical, and has already been described with reference to Figure 3. As a result, no further discussion of the operation of the pre-distortion stages will be undertaken here.

The important point to note from Figure 4 is that each pre-distortion stage is cascaded together in series such that an input signal input into the first stage of the series cascades in turn through each pre-distortion stage and is fiirther pre-distorted in each stage in turn. The fully pre-distorted input signal 7 is then fed to the amplifier circuit stages for amplification. As many pre- distortion stages are required as there are amplification stages, and each pre-distortion stage must be individually optimised to cancel distortion from its corresponding amplifier stage.

Whilst it has already been mentioned that each amplifier 36 in each pre-distortion stage has nominally identical distortion characteristics to the 17 corresponding amplifier stage, and that this is preferably achieved by use of the same device technology between the two corresponding stages, it is not necessarily the case that a particular pre-distortion stage must use the same device technology as another particular pre-distortion stage, and each pre-distortion stage may use difference device technology in accordance with the devices used in the multistage amplifying circuit. Obviously, where the multistage amplifying circuit is integrated, it is likely that the identical device technology is used between each amplifier stage, and hence each pre-distortion stage will also use the same type of devices.

Where the multistage amplifier is to be integrated, it will also be possible to integrate the various pre-distortion stages with the amplifier stages to give a My integrated amplifier with excellent linear amplification characteristics.

The input signal to be fed through the amplifier circuit means or pre distortion circuit of the present invention may be any signal which requires amplification with exceptional linearity and high power efficiency. In particular, the amplifier circuit means and pre-distortion circuit means of the present invention are particularly suitable for use with microwave RF signals as the input signal as the excellent modulation distortion cancellation characteristics of the present invention make it particularly suitable for use with wide band communications systems employing such microwave signals. However, the present invention is not to be limited to such RF applications, and the input signal may be any signal which requires linear amplification, such as an audio hi-fi signal, or indeed any other communications signal extending into any other part of the spectrum. In particular, it is envisaged that the present invention may find application in all existing cellular mobile multi-user and multi-carrier systems, as well as planned third generation base station and handset transmitters. in addition, other as yet unplanned but envisaged fiiture generations of terrestrial and satellite fixed or mobile communication systems will benefit from the multistage pre distortion technique of the present invention, and hence the present invention is 18 not to be limited to any particular use or range of uses.

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Claims

CLAIMS:

1. An amplifier circuit means for amplifying an input signal in a substantially linear manner, said circuit means comprising:

a plurality of pre-distortion circuit stages arranged in a first series, a first pre-distortion circuit stage in said first series being arranged to receive said input signal as an input; and a plurality of aniplifier circuit stages arranged in a second series, a first amplifier circuit stage in said second series being arranged to receive the input signal output from a last pre-distortion circuit stage in said first series as an input; wherein each respective pre-distortion circuit stage is arranged to process the input signal in a predetermined manner to substantially cancel any distortion introduced into the input signal by a corresponding respective one of said plurality of amplifier circuit stages during subsequent amplification of the is input signal in said plurality of amplifier circuit stages.

2. An amplifier means according to claim 1, wherein said plurality of pre-distortion circuit stages are flu-ther arranged such that each of the remainder of said pre-distortion circuit stages other than said first stage receive the input signal as an output of an immediately preceding pre-distortion circuit stage, wherein said input signal passes through and is processed in each pre- distortion circuit stage in turn.

3. An amplifier circuit means according to either of claims 1 or 2, wherein said plurality of amplifier stages are further arranged such that each of the remainder of said amplifier circuit stages other than said first stage receive the input signal as an output of an immediately preceding amplifier circuit stage, wherein said input signal passes through and is amplified in each amplifier circuit stage in turn.

4. An amplifier circuit means according to any of claims I to 3, wherein the e pre-distortion circuit stage in the first series cancels the distortion introduced into the input signal due to the ri' amplifier circuit stage in the second series.

5. An amplifier circuit means according to any of claims I to 3, wherein the n' pre-distortion circuit stage in the first series cancels the distortion introduced into the input signal due to the j' amplifier circuit stage in the second series, wherein wj.

6. An amplifier circuit means according to any of claims I to 5 wherein each of said plurality of pre-distortion circuit stages is matched to its respective corresponding amplifier circuit stage in that the two corresponding stages have substantially identical non-linear characteristics.

7. An amplifier circuit means according to claim 6, wherein each of said plurality of pre-distortion stages employs the same device technology to that of its respective corresponding amplifier circuit stage.

8. An amplifier circuit means according to any of the preceding claims, wherein each of said plurality of pre-distortion circuit stages comprises:

a pre-distortion means arranged to receive the input signal to the pre distortion circuit stage and to apply pre-distortion to said input signal to process said signal; and a linear amplifier means arranged to receive the output of said pre distortion means and to amplify said signal in a substantially linear manner without introducing further distortion into said signal; whereby each of said pre-distortion circuit stages acts as an active stage thereby increasing the overall gain of said amplifier circuit means.

21

9. An amplifier circuit means according to claim 8, wherein said pre distortion means finther comprises:

a first coupler arranged to receive the input signal to the pre distortion circuit stage, and to split the input signal into a first circuit branch and a second circuit branch; a first phase adjuster arranged in the first circuit branch for varying the phase of the signal on the first circuit branch; an amplifier means arranged in the second circuit branch for ampliring the signal on the second circuit branch; at least one variable attenuator means arranged in series with the amplifier means in the second circuit branch for attenuating the signal on the second circuit branch; and a second coupler arranged to receive the respective signals from the first and second branches and subtract the signal from the second branch from the signal from the first branch to give a pre-distorted output signal; wherein said amplifier means has nominally substantially identical non-linearity characteristics to the respective amplifier circuit stage corresponding to the particular pre-distortion circuit stage of which the particular pre-distortion means form a part.

10. An amplifier circuit means according to claim 9, wherein said first and second couplers are 90' hybrid couplers, whereby the signal on the first circuit branch is 90 0 out of phase with the signal on the second circuit branch.

11. An amplifier circuit means according to claim 9 or 10, further comprising a second variable attenuator means arranged in series with the amplifier means and the first variable attenuator means in the second circuit branch, wherein one of the first or second variable attenuator means is arranged to attenuate the signal input to the amplifier means, and the other of the first or 22 second variable attenuator means is arranged to attenuate the signal output from the amplifier means.

12. An amplifier circuit means according to any of claims 9 to 11, wherein the respective input and output ports of each of the respective elements of the pre-distortion means are substantially impedance matched.

13. An amplifier circuit means according to any of the preceding claims, wherein the input signal is a RF signal.

14. A pre-distortion circuit means for pre-distorting an input signal prior to amplification of said input signal in a non-linear amplifier, said circuit means comprising:

a first coupler arranged to receive the input signal and to split the input signal into a first circuit branch and a second circuit branch; a first phase adjuster arranged in the first circuit branch for varying the phase of the second circuit branch; an amplifier means arranged in the second circuit branch for amplifying the signal on the second circuit branch; at least one attenuator means arranged in series with the amplifier means in the second circuit branch for attenuating the signal on the second circuit branch; and a second coupler arranged to receive the respective signals from the first and second circuit branches and subtract the signal from the second branch firom the signal firom the first branch to give a pre-distorted output signal; wherein said amplifier means has nominally substantially identical non-linear characteristics as said non-linear amplifier.

15. A pre-distortion circuit means according to claim 14, wherein said 23 first and second couplers are 90' hybrid couplers, whereby the signal on the first circuit branch is 90' out of phase with the signal on the second circuit branch.

16. A pre-distortion circuit means according to claim 14 or 15, fiirffier comprising a second variable attenuator means arranged in series with the amplifier means and the first variable attenuator means in the second circuit branch, wherein one of the first or second variable attenuator means is arranged to attenuate the signal input to the amplifier means, and the other of the first or second variable attenuator means is arranged to attenuate the signal output from the amplifier means.

17. A pre-distortion circuit means according to any of claims 14 to 16, wherein the respective input and output ports of each of the respective elements of the pre-distortion circuit means are substantially impedance matched.

18. A pre-distortion circuit means according to any of claims 14 to 17, further comprising a linear amplifier means arranged to receive the output of said second coupler and to amplify the output in a substantially linear manner without introducing finther distortion into the pre-distorted output signal, wherein said pre distortion circuit means acts as an active stage thereby allowing said circuit means to act as a driver stage for the non-linear amplifier.

19. A pre-distortion circuit means according to any of claims 14 to 18, wherein said input signal is an RF signal.

20. A method of amplifying an input signal in a substantially linear manner, comprising the steps of.

receiving said input signal in a first pre-distortion circuit stage, said first pre-distortion circuit stage being arranged to be first of a plurality of pre- 24 distortion circuit stages arranged in a first series; processing said input signal in said plurality of pre-distortion circuit stages; and receiving the processed input signal from a last pre-distortion circuit stage in said first series into a first amplifier circuit stage of a plurality of amplifier circuit stages arranged in a second series; and amplifying said processed input signal in said plurality of amplifier circuit stages; wherein said processing step is characterised in that each respective pre-distortion circuit stage is arranged to process the input signal in a predetermined manner to substantially cancel any distortion introduced into the input signal by a corresponding respective one of said plurality of amplifier circuit stages during said amplifying step.

21. A method according to claim 20, wherein said processing step is further characterised by said input signal passing through and being processed in each pre-distortion circuit stage in turn.

22. A method according to either of claims 20 or 21, wherein said amplifying step is further characterised by said processed input signal passing through and being amplified in each amplifier circuit stage in turn.

23. A method according to any of claims 20 to 22, wherein the n' pre distortion circuit stage in the first series cancels the distortion introduced into the input signal due to the n' amplifier circuit stage in the second series.

24. A method according to any of claims 20 to 22, wherein the n' predistortion circuit stage in the first series cancels the distortion introduced into the input signal due to the j' amplifier circuit stage in the second series, wherein n is not equal to j.

25. A method according to any of claims 20 to 24, wherein said processing step flirther comprises the steps of..

pre-distorting the input signal in turn in each pre-distortion circuit stage to apply pre-distortion to said input signal corresponding substantially to the inverse of any distortion introduced into the input signal during amplification in each respective corresponding amplifier circuit stage; and linearly amplifying the processed input signal in each pre-distortion circuit stage prior to being output from each pre-distortion circuit stage; whereby each of said pre-distortion circuit stages acts as an active stage to amplify the processed input signal.

26. A method according to claim 25 wherein said pre-distortion step is further comprises the steps of splitting said input signal into a first circuit branch and a second circuit branch; adjusting the phase of the signal on the first circuit branch; ampli"g the signal on the second circuit branch; attenuating the signal on tile second circuit branch; and subtracting the signal from the second branch from the signal from the first branch to give apre-distorted output signal; wherein said further amplification step is substantially nominally identical to said amplification step of claim 20, whereby substantially nominally identical non-linearity characteristics are obtained firom. each respective amplification step.

27. A method of pre-distorting an input signal prior to amplification of said input signal in a non-linear amplifier, said method comprising the steps of.

26 splitting the input signal into a first circuit branch and a second circuit branch; adjusting the phase of the signal on the first circuit branch; amplifying the signal on the second circuit branch; attenuating the signal on the second circuit branch; and subtracting the signal on the second circuit branch from the signal on the first circuit branch to give a pre-distorted output signal; wherein the signal on the second circuit branch is arnplified in a nominally substantially identical non-linear manner to that as would be achieved in said non-linear amplifier.

28. A method according to claim 27, further comprising the step of linearly amplifying the pre-distorted output signal without introducing further distortion into the pre-distorted output signal, thereby allowing said pre-distorted output signal to act as a driver signal from said non-linear amplifier.