Patent Publication Number: US-2017359031-A1

Title: A radio frequency power amplifier

Description:
The present invention relates to an improved balanced radio frequency power amplifier. 
     There is a desire for power amplifiers to be highly efficient to reduce the power they consume, which reduces the need for cooling. This is particularly important where the power amplifier is being used in applications with a limited power supply. 
     Techniques for designing efficient radio frequency power amplifiers (RFPAs) have been widely employed for many decades. Radio systems, however, introduce a more stringent efficiency requirement inasmuch as the efficiency of a conventional RFPA degrades very quickly as the output power is reduced (commonly termed ‘power back-off’ or PBO). This means that an amplitude modulated radio system will have much lower efficiency than the peak efficiency of the RFPA as the modulated transmissions cause low efficiency due to the power amplifier (PA) having low efficiency at the lower points in the modulation cycle. 
     Several partial solutions to this problem have been proposed, most notably the “Doherty” power amplifier (PA) that is in widespread use in modern mobile communications systems. A Doherty power amplifier comprises a main amplifier and a peaking amplifier, typically being of different operating classes, arranged such that the peaking amplifier operates together with the main amplifier over a defined input power range. 
     The Doherty PA, however, has limited operational bandwidth and as such is little used in military radar and electronic warfare (EW) systems. This limitation will also pose a problem in the development of new mobile communication systems that require greater bandwidths. 
     US2005134377 (Dent) describes an amplifier circuit having two amplifying devices run in quadrature and an auxiliary amplifier that generates an artificial reflection signal that is presented to an auxiliary port of the quadcoupler and thus to output ports of the amplifiers. The artificial reflection signal has a phasing offset by 180° from the phase of the output signals from the two amplifiers to provide a resistive load and thus appear to the amplifiers as a load impedance mismatch. The primary amplifiers will have transistor matching networks at their outputs to match the primary amplifiers to the system&#39;s impedance. However, because it is difficult to alter the impedance of transistor matching networks, this amplifier circuit will still suffer from limited operational bandwidth. 
     According to the invention there is provided a radio frequency power amplifier comprising: a balanced amplifier having an input coupler, an output coupler, and two amplifiers each comprising a transistor there-between; the radio frequency power amplifier further comprising means to modify and/or modulate the impedance presented to the output of the two amplifiers by presenting a signal to an isolated port of the output coupler; and further comprising means to modify the phase and/or amplitude of the signal presented to the isolated port of the output coupler to provide impedance matching and/or impedance modulation to tune for transistor parasitic effects. 
     This arrangement extends the efficiency of the radio frequency power amplifier over a wider input power range compared with a traditional balanced amplifier. Because impedance matching and/or modulation can be performed at the balanced device ports, the amplifiers need not comprise transistor matching networks thereby enabling the power amplifier of the invention to be physically smaller and operate over a wider bandwidth compared with a traditional Doherty PA and the circuit of US2005134377. 
     It is preferred that the radio frequency power amplifier includes an auxiliary amplifier the output of which is presented to the isolated port of the output coupler. 
     In a preferred embodiment, the signal presented to the isolated port of the output coupler has substantially the same frequency as a signal presented to the input port of the balanced amplifier but favourably has a prescribed relative phase and amplitude to the said input signal. 
     It is favourable that an input signal is presented to both the input of the balanced amplifier and the isolated port of the output coupler. This provides a convenient method of providing a signal to the isolated port of the output coupler that has the same characteristics as the signal inputted to the balanced amplifier. 
     It is preferable that the radio frequency power amplifier comprises means to modify the signal presented to the auxiliary amplifier. 
     It is preferred that the input coupler and output coupler are quadrature couplers. 
    
    
     
       The invention will now be described by way of example with reference to the following figures: 
         FIG. 1  is a schematic of a load modulated radio frequency balanced amplifier comprising a balanced amplifier having an auxiliary amplifier driving an isolated port of the output coupler; and 
         FIG. 2  is a circuit analysis schematic of the schematic  FIG. 1 . 
     
    
    
     Referring to the  FIG. 1  there is shown a balanced amplifier having an input quadrature coupler  1 , output quadrature coupler  2 , and two 3 W amplifiers  3 ,  4 . In this embodiment the two 3 W amplifiers  3 ,  4  are single transistors though more complex arrangements and/or different power levels may be used. Also illustrated is an auxiliary 1 W amplifier  5  that is connected to the output quadrature coupler  2  of the balanced amplifier to provide load balancing in a manner to be described. 
     The input quadrature coupler  1  has an input port  6  for receiving a signal  17  to be amplified, an isolated input port  7  terminated in a matched impedance  8 , and two outputs  9 ,  10 . The signals leaving the respective outputs  9 ,  10  have a ninety degree phase difference. This arrangement provides the benefit that signals reflected by the amplifiers  3 , 4  towards the input  6  cancel each other out. 
     The signals from the outputs  9 ,  10  are fed to the respective amplifiers  3 ,  4 . The outputs of the amplifiers  3 , 4  are in turn fed to input ports  11 ,  12  of the output quadrature coupler  2 . 
     The input signal  17  fed to input  6  of the input quadrature coupler  1  is also fed to the auxiliary amplifier  5 , optionally via signal modifying means  16  to be described later. 
     The output of the auxiliary amplifier  5  is presented to isolated port  14  of the output quadrature coupler  2 . The description below shows that through this arrangement, the load modulation presented by the auxiliary amplifier  5  to port  14  acts to modulate the impedances presented to the two amplifiers  3 , 4 . 
     The key properties of the load modulated balance amplifier are demonstrated using the schematic representation shown in  FIG. 2 . The transistors  3 , 4 , 5  of  FIG. 1  are represented as current sinks. As is convention, in  FIG. 2  the output port  13  is represented as {circle around ( 1 )}, ports  12  and  11  are represented as {circle around ( 2 )} &amp; {circle around ( 4 )} respectively and port  14  is represented as {circle around ( 3 )}. 
     The properties and actions of the load modulated balance amplifier can be determined by considering the 4-port impedance matrix for a 3 dB quadrature coupler: 
     
       
         
           
             
               [ 
               
                 
                   
                     
                       V 
                       1 
                     
                   
                 
                 
                   
                     
                       V 
                       2 
                     
                   
                 
                 
                   
                     
                       V 
                       3 
                     
                   
                 
                 
                   
                     
                       V 
                       4 
                     
                   
                 
               
               ] 
             
             = 
             
               
                 
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                 [ 
                 
                   
                     
                       
                         I 
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                         I 
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                         4 
                       
                     
                   
                 
                 ] 
               
             
           
         
       
     
     From this, the impedances at the balanced ports  12 {circle around ( 2 )} and  11 {circle around ( 4 )} can be shown to be: 
     
       
         
           
             
               Z 
               2 
             
             = 
             
               
                 Z 
                 4 
               
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                           I 
                           mod 
                         
                         
                           I 
                           bal 
                         
                       
                     
                   
                   ) 
                 
               
             
           
         
       
     
     where I bal  is the balanced device current, and I mod  is the current supplied by the auxiliary amplifier  5  to port  14 {circle around ( 3 )}. Thus as the value of I mod  varies, the impedance Z 2  Z 4  presented to respective amplifiers  4 , 3  by ports  11 {circle around ( 4 )}  12 {circle around ( 2 )} varies accordingly 
     By varying the current supplied by the auxiliary amplifier device  5  to port  14  it is possible to modulate I mod  so that optimum efficiency is maintained as the power of input signal  17  to port  6  decreases (power back-off). 
     Using the input signal  17  (modified or otherwise by means  16 ) to port  6  to control the output of the auxiliary device  5  provides a convenient method to vary the impedance presented to the amplifiers  3 , 4  to suit variation in the power of the input signal to port  6 . 
     In addition, because there is conservation of energy, the combined powers of the three amplifiers  3 , 4 , 5  will appear at the output port  13 {circle around ( 1 )}; e.g. in  FIG. 1  the final output at port  13 {circle around ( 1 )} of the output quadrature coupler will be 7 W, the sum of the three individual powers provided at ports  12 {circle around ( 2 )},  11 {circle around ( 3 )},  14 {circle around ( 4 )} of the output quadrature coupler  2 . 
     The system further comprises means  16  for controlling the amplitude and phase of the signal outputted by the auxiliary amplifier  5  to tune for transistor parasitic effects such as output capacitance. In this way it is possible to remove the need for transistor matching networks at the output of amplifiers  3 , 4 . Control of the amplitude and phase of the input signal to the auxiliary amplifier  5  can be done using commonly known apparatus and methods to persons skilled in the art, for example using a variable attenuator and/or a four quadrant phase shifter. 
     The power of amplifiers  3 , 4 , 5  may be varied from those described in relation to  FIG. 1  so long as the power of amplifiers  3 , 4  remains substantially the same. In most applications the power of the auxiliary amplifier  5  will be less than the power of the first and second amplifiers  3 , 4  though this does not always need to be the case. 
     It will be appreciated that the load modulated radio frequency balanced power amplifier described above may be applicable for use with signal frequencies not limited to those in the RF and microwave range. 
     Although it is preferred that the input signal fed to input  6  is also fed to auxiliary amplifier  5  and/or means  16  for controlling the amplitude and phase, in certain applications the signal fed to auxiliary amplifier  5  or means  16  may be generated independently from a different source. 
     In a variation, the auxiliary amplifier  5  may itself be or comprise a load modulated balanced amplifier in a recursive arrangement.