FIELD OF THE INVENTION 
       [0001]    This invention relates to a power amplifier having an operating frequency in the radio frequency (‘RF’) or microwave or higher ranges, with a pre-distorter. Such a power amplifier is referred to below as a radio power amplifier. 
       BACKGROUND OF THE INVENTION 
       [0002]    Radio power amplifiers are used in telecommunications, for example, both for transmission and for reception. To operate a power amplifier efficiently, with maximum power and gain, it is desirable to run it with bias voltage and input signal amplitude conditions such that it functions close to saturation, with the consequence that the linearity of its transfer function is degraded or distorted relative to an otherwise similar amplifier running further from saturation. 
         [0003]    In order to compensate for the distortion, the amplifier is conveniently linearised by a pre-distorter, which should be simple to use and easy to integrate on the same semiconductor chip as at least initial stages of the main amplifier module, the so-called monolithic integrated circuit (‘MIC’) technology. Such a pre-distorter presents to the input signal of the amplifier a non-linear transfer function that tends to compensate the non-linearities in the operation of the power amplifier module. The pre-distorter comprises a scaled amplifier of the same physical characteristics, in particular the same technology and process of manufacture as the amplifier to linearise, whose component elements are easy to integrate and which will require a low number of tuning operations. 
         [0004]    Many ways of building pre-distorters have already been described and many of them use a “linear/non-linear path” topology. One linear/non-linear path topology is described in patent specification U.S. Pat. No. 4,992,754 and comprises a first path providing a copy of the input signal (linear path) and a second path in which some distortion is introduced through a non-linear device (non-linear path). The difference between the signals from the two paths contains a distortion that is arranged to be in opposition with the distortion of the power amplifier to linearise, in order to cancel out the distortion introduced by the power amplifier. 
         [0005]    One limitation of that system is that some delay must be introduced in the linear path to compensate for the delay in the non-linear path, and the relationship between the two paths in phase and amplitude is not easily maintained with frequency and from part to part. Another limitation is in the couplers, which may be well suited for hybrid technology but which are not easy to integrate, especially on semiconductor material such as Silicon where transmission lines are lossy. 
         [0006]    Patent specification U.S. Pat. No. 5,576,660 describes a pre-distorter that comprises a 90° input divider and a 90° output coupler in hybrid form, the circuits of the two channels (or paths) being interconnected by micro strip circuits or by hybrid techniques, and it cannot readily be implemented in integrated circuits. The two paths of the pre-distorter include microwave amplifiers, attenuators and variable phase shifters that are adjusted to adapt the transfer characteristics to a power amplifier of different technology. From the user point of view, numerous tuning operations are necessary in order to optimise the linearity of this system. Phase and gain need to be adjusted in each path to adjust the shape of the gain and phase pre-distortion curves. The bias voltages of all the active devices used in both paths need to be adjusted, and also the total gain of the pre-distorter to adjust its level characteristic to that of the power amplifier (the level at which the amplifier will start to introduce distortion). 
         [0007]    The present inventor&#39;s previous patent specification EP 022 913 305 describes a pre-distorter that is integrated in MIC technology and is more readily manufactured and tuned. However, it is still desirable to improve the characteristics relating to variability of resistors and their non-linearity when used at high RF level, to simplify further the tuning operations and to reduce further overall losses of the complete pre-distorter. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides amplifier apparatus having an operating frequency in the radio frequency (‘RF’) or microwave or higher ranges and comprising a pre-distorter, as described in the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic diagram of amplifier apparatus in accordance with one embodiment of the invention, given by way of example, 
           [0010]      FIG. 2  is a diagram of the variation with frequency of transfer characteristics of divider and combiner elements in a pre-distorter of the amplifier apparatus of  FIG. 1 , 
           [0011]      FIG. 3  is a more detailed schematic diagram of a pre-distorter in an embodiment of the amplifier apparatus of  FIG. 1 , 
           [0012]      FIG. 4  is a more detailed schematic diagram of a pre-distorter in another embodiment of the amplifier apparatus of  FIG. 1 , and 
           [0013]      FIG. 5  is a more detailed schematic diagram of an embodiment of the pre-distorter of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    The amplifier apparatus shown in the drawings comprises an input terminal  1  for a radio frequency input signal, a pre-distorter  2  receiving the signal from the terminal  1 , a variable attenuator  3  receiving an output signal from the pre-distorter  2  and a power amplifier  4  whose output signal is applied to an output terminal  5 . 
         [0015]    The pre-distorter  2  comprises an input divider  6  that divides the input signal from the terminal  1  into two signals of equal amplitude, which are applied respectively to a relatively high gain non-linear path  7  operating relatively close to saturation, similar to the operating conditions of said power amplifier, and a relatively low gain linear path  8  operating at substantially more linear conditions. The divider  6  introduces a phase difference of 90° between the two signals applied to the paths  7  and  8  respectively. The non-linear path  7  comprises an amplifier  9  and the linear path  8  comprises an amplifier  12 . The amplifiers  11  and  12  receive orthogonal signals from the divider  6  and have substantially identical physical characteristics. The paths  7  and  8  are connected to apply the orthogonal output signals of the amplifiers  11  and  12  to a combiner or coupler  13  that combines the signals from the two paths and introduces a further phase difference of 90° between the two signals from the paths  7  and  8  respectively before combining them in anti-phase, that is to say adding the signals together with 180° phase difference between the signals being combined. The coupler  13  applies the combined signal to the variable attenuator  3 . 
         [0016]    In one embodiment of the present invention, the divider  6  is arranged to have low pass characteristics and the coupler  13  is arranged to have high pass characteristics, as shown schematically in  FIG. 2  of the accompanying drawings. A pre-distorter  2  of this type is shown in  FIG. 3 . In another embodiment of the present invention, whose pre-distorter  2  is shown in  FIG. 4 , the divider  6  is arranged to have high pass characteristics and the coupler  13  is arranged to have low pass characteristics. 
         [0017]    The 90° combiners and dividers used in the embodiments shown in  FIGS. 3 and 4  comprise lossy lumped reactance elements, which have a natural tendency to be unbalanced, and advantage is taken of this unbalance to create a linear and non-linear path by having the two paths  7  and  8  of the otherwise balanced differential amplifier working at different levels. The unbalance between the input divider  6  and the output combiner  13  is arranged to be in the right direction so as to present transfer characteristics whose magnitudes and variations with frequency tend to compensate each other since, in the pre-distorter  2 , the signals from the two paths  7  and  8  are subtracted instead of being added and since one of the divider and the combiner is high pass while the other is low pass. The overall transfer characteristics of the pre-distorter are therefore substantially linear with frequency, as shown in  FIG. 2 . 
         [0018]    These topologies present a self-compensation effect reducing or avoiding need for gain and phase adjustment in each path  7  and  8  of the pre-distorter, and overall losses are minimized because there is no attenuator in the two paths  7  and  8  of the pre-distorter  2 . By using the same type of device and process technology in the pre-distorter amplifiers  9  and  12  as in the main amplifier  15 , the amplifiers present the same thermal behaviour, and there is no need to adjust the shape of the variation of the pre-distorter characteristics with temperature. The only adjustment required here is the attenuator  3  between the pre-distorter and the power amplifier that is to be linearised. It will be appreciated that for fine-tuning of the linearisation, the bias points of the amplifiers  7  and  8  can be adjusted but in most cases this is unnecessary. 
         [0019]    The pre-distorter shown in  FIG. 3  comprises a low pass input divider  6  comprising series inductive elements and a high pass output coupler  13  that comprises series capacitive elements. The input terminal  1  is connected to the path  7  at a connection point  102  and the path  8  starts at a connection point  104 . Capacitors  106  and  108  are connected between the connection points  102  and  104  respectively and ground. A resistor  110  is also connected between the connection point  104  and ground. An inductor  111  is connected between connection points  102  and  104 . Inductors  112  and  114  are connected in series in the paths  7  and  8  between the connection points  102  and  104  and the inputs of the amplifiers  9  and  12  respectively. An inductor  116  is connected across the inputs of the amplifiers  9  and  12 . Capacitors  118  and  120  are connected between the inputs of the amplifiers  9  and  12  respectively and ground. 
         [0020]    The output coupler  13  comprises inductors  122  and  124  that are connected between the outputs of the amplifiers  9  and  12  respectively and ground. A capacitor  126  is connected across the outputs of the amplifiers  9  and  12 . Capacitors  128  and  130  are connected in series in the paths  7  and  8  between the outputs of the amplifiers  9  and  12  and connection points  132  and  134  respectively. Inductors  136  and  138  are connected between the connection points  132  and  134  respectively and ground. A resistor  140  is also connected between the connection point  132  and ground. A capacitor  142  is connected between the connection points  132  and  134 . The pre-distorter output terminal  5  is connected to the connection point  134  in the path  8 . 
         [0021]    The pre-distorter shown in  FIG. 3  does not have especially wide band characteristics but its bandwidth is sufficient for use in many telecommunications applications and especially in cellular telephone systems like 2.5 G or 3 G, where a 10% bandwidth is adequate (UMTS band around 2.1 GHz is 2.8%). 
         [0022]    The embodiment of pre-distorter shown in  FIG. 4  resembles that shown in  FIG. 3  but the capacitive elements are replaced with inductive elements and vice-versa, so that the transfer characteristics of the input divider  6  are high pass and those of the output coupler  13  are low pass. In particular, the high pass input divider  6  comprising series capacitive elements and the low pass output coupler  13  comprises series inductive elements. 
         [0023]      FIG. 5  shows an example of the amplifiers  9  and  12  in an embodiment of the kind shown in  FIG. 3 . The inputs  200  and  202  of the amplifiers are connected through combinations  204  and  206  of capacitors and inductors in series to gate terminals of field effect transistors (‘FET’)  216  and  218 . The gate terminals of field effect transistors (‘FET’)  216  and  218  are supplied with gate bias voltage from terminals Vg 1  and Vg 2  through inductors  208  and  212 , with decoupling capacitors  210  and  214  connected between terminals Vg 1  and Vg 2  respectively and ground. The sources of the FETs  216  and  218  are connected to ground while the drains are connected through inductors  220  and  222  to a voltage supply terminal Vdd and to the amplifier outputs  232  and  234  through capacitors  228  and  230 . Feedback loops  222  and  224  extend from the connection of inductor  220  with capacitor  228  to the gate of FET  216  and from the connection of inductor  224  with capacitor  230  to the gate of FET  218 . 
         [0024]    The amplifiers  9  and  12  comprise transistors of smaller size than the transistors of the power amplifier  4  and, in particular, of smaller power capacity but otherwise of identical physical characteristics to the power amplifier transistors, being manufactured with the same technological process in the same manufacturing plant. In an embodiment of the present invention, the pre-distorter amplifiers  11  and  12  are integrated in the same integrated circuit as one or more initial stages of the power amplifier  4 . It will be appreciated that these features simplify tuning the pre-distorter  2 . 
         [0025]    The non-linear path amplifier  9  is operated under conditions of bias and signal voltage that are substantially equal to those of the power amplifier  4 . Accordingly, the distortion characteristics of the non-linear amplifier  9  are substantially identical to those of the power amplifier  4 . The linear amplifier  12  is operated at a similar bias voltage but with a smaller signal voltage, so that the linear amplifier  12  operates substantially with optimally linear characteristics, its gain being otherwise substantially identical to that of the non-linear path amplifier  9 . 
         [0026]    In an embodiment of the invention, all the components of the pre-distorter  2  are formed in or on a common semiconductor substrate. Moreover, components of the power amplifier  4  are also formed in the same semiconductor substrate and, in particular, all the components of the variable attenuator  3  and of the first stage  14 , the driver of the power amplifier, are formed in the same semiconductor substrate. All the components of the final stage  15  of the power amplifier are also formed in a common substrate, which can be the same as that of the pre-distorter if their relative sizes are compatible. The technology used in an embodiment of the invention for the common substrate(s) is monolithic microwave integrated circuit (‘MMIC’) technology. One embodiment of such an amplifier operates at a frequency of 2.14 GHz. 
         [0027]    In an embodiment of the invention, each of the paths  7  and  8  presents an impedance of 100 ohms and the divider  6  presents an impedance of 50 ohms to the input terminal  1  and 100 ohms to each of the paths  7  and  8 . 
         [0028]    An additional drawback of certain previous pre-distorters is that they use small devices to create the non-linearities; these devices have very high input and output impedances and are not easy to match to the standard 50 Ohms impedance. The pre-distorter of this embodiment of the present invention is readily matched to the standard 50 Ohms impedance; linear path  8 , and non-linear path  7 , are individually matched to 100 Ohms, which means that input divider  6  and output coupler  13  do not provide any impedance transformation and accordingly have a wider bandwidth 
         [0029]    Since the transistors of the amplifiers  9  and  12  have identical physical characteristics to the corresponding transistors of the power amplifier  4 , exactly the same kind of non-linearity is obtained, without phase or amplitude tuning in the two paths  7  and  8 . Also, the thermal behaviour of the pre-distorter is similar to that of the power amplifier  4  and no adjustment of the characteristics with temperature is required. 
         [0030]    It will be appreciated that the divider  6  and coupler  13  consist, basically, of low path and high path filters in the embodiment illustrated. Band path filters may be used instead if desired, provided complementary high and low pass characteristics are obtained in the operating bandwidth.
Amplifier apparatus comprising a power amplifier having an operating frequency in the radio frequency or microwave or higher ranges and a pre-distorter, the characteristics of the power amplifier comprising a distortion from a linear transfer function. The pre-distorter comprises a non-linear path and a linear path including amplifiers having substantially identical physical characteristics, an input divider responsive to an amplifier input signal for applying respective pre-distorter input signals to the paths, and an output coupler for combining the signals from the linear path and the non-linear path to produce a pre-distorted signal. The characteristics of the pre-distorter comprise a distortion relative to a linear transfer function that compensates for the distortion of the transfer function of the power amplifier.