BACKGROUND OF THE INVENTION 
   The present invention relates generally to power amplifiers and, more particularly, to electronically tuned power amplifiers. 
   Power amplifiers boost a low-power signal to a higher power level, to be delivered to a load. The load determines the gain, linearity, and efficiency of the amplifier. Having the ability to dynamically vary this load impedance allows the properties of the amplifier to be dynamically varied. 
   Current techniques for tuning power amplifiers include mechanical tuning, tuning using electrically variable capacitors or inductors, and injecting a carrier signal using a circulator. Mechanical tuning is employed in “cavity” amplifiers, which include mechanical cavity tuning controls. Here tuning is carried out by mechanically adjusting a cavity dimension. Mechanical tuning is usually slow and cumbersome. 
   The above-noted electrical tuning techniques (variable capacitors or inductors and injecting a carrier signal using a circulator), in general, allow for more rapid amplifier tuning than the mechanical adjustment method. However, these methods are typically suitable only for narrow band applications. Circulators, for example, have relatively narrow bandwidths and are therefore unsuitable for tuning in broad band applications. 
   Thus, there is a need for a power amplifier that includes a tuning system that is suitable for broadband applications. 
   SUMMARY OF THE INVENTION 
   A power amplifier apparatus that includes a main power amplifier circuit and an injection power amplifier circuit is provided. The main amplifier includes a pair of quadrature coupled amplifiers. Similarly, the injection power amplifier also includes a pair of quadrature coupled amplifiers. The output signal from the injection amplifier is connected to the isolation port of the quadrature coupler (or quadrature combiner) of the main amplifier. The input signal is split and applied to the inputs of both the main and injection amplifiers. The gain and phase through the injection amplifier is adjustable in order to vary the magnitude and phase of the signal injected into the isolation port of the quadrature combiner of the main amplifier. By varying the magnitude and phase of the injected signal, the impedance seen by the individual amplifiers within main amplifier can be controlled. Controlling these impedances allows the properties of the main amplifier to be dynamically varied. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic illustration of a power amplifier apparatus in accordance with an embodiment of the present invention. 
       FIG. 2  is a plot illustrating the impedances presented to the individual amplifiers within the main amplifier by adjusting the phase and amplitude of the injected signal using the apparatus of  FIG. 1 . 
       FIG. 3  is a diagrammatic illustration of a power amplifier apparatus in accordance with another embodiment of the present invention. 
       FIG. 4  is a plot illustrating the impedances presented to the individual amplifiers within the main amplifier by adjusting the phase and amplitude of the injected signal using the apparatus of  FIG. 3 . 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention relates, in general, to electronically tuned power amplifiers. More specifically, the present invention relates to power amplifier systems that employ quadrature hybrid circuits (described further below) that can be designed to provide a large bandwidth over which the power amplifier can be electronically tuned. 
     FIG. 1  is a diagrammatic illustration of a power amplifier apparatus  100  in accordance with an embodiment of the present invention. The primary elements of power amplifier  100  are a main power amplifier circuit  102  and an injection power amplifier circuit  104 . In general, main power amplifier circuit  102  includes components that boost a low-power signal to a higher power level, to be delivered to a load (such as  106 ). The impedance of the main amplifier  102  is normally passively matched to the load  106 . This means of matching is normally fixed. Therefore apparatus  100  employs injection amplifier  104 , which can provides impedance matching to the load by introducing a phase and amplitude adjustable injection signal into main power amplifier  102 . 
   Main power amplifier  102  includes a main power amplifier input quadrature hybrid circuit  108 - 1 , a first large signal amplifier  110 - 1 , a second large signal amplifier  112 - 1  and a main power amplifier output quadrature hybrid circuit  114 - 1 . 
   As can be seen in  FIG. 1 , each of main power amplifier quadrature hybrid circuits  108 - 1  and  114 - 1  include an isolation port (ISO), an input port (IN), and two output ports (0 and −90). In the FIGS., these same reference characters and numbers are used to denote the four port types of the various quadrature hybrid circuits. However, this does not require that the quadrature hybrid circuits be of identical construction. In general, ports 0 and −90 output first and second versions, respectively, of a signal received at the IN port, with the first version of the signal and the second version of the signal having a phase difference of 90 degrees. Further, the ISO port and the IN port are electrically isolated and therefore cannot receive signals from each other. It should be noted that, in accordance with the present invention, a quadrature hybrid circuit (or quadrature combiner) is any circuit, combination of circuits, or component(s) that include(s) the above-noted four ports and provide the above-described general functionality. 
   As can be seen in  FIG. 1 , the IN port of main power amplifier input quadrature hybrid circuit  108 - 1  serves as an input for the main power amplifier. The ISO port of circuit  108 - 1  is connected to ground via an isolation resistor  116 - 1 . A first of the two output ports (port 0) is coupled to an input of first large signal amplifier  110 - 1  and a second of the two output ports (port −90) is coupled to an input of second large signal amplifier  112 - 1 . An output of first large signal amplifier  110 - 1  is connected to a second output port (port −90) of main power amplifier output quadrature hybrid circuit  114 - 1  and an output of second large signal amplifier  112 - 1  is coupled to a first output port (port 0) of circuit  114 - 1 . The IN port of circuit  114 - 1  is connected to load  106  and the ISO port of circuit  114 - 1  is coupled to injection power amplifier circuit  104 . 
   As mentioned above, injection power amplifier  104  is configured to provide an impedance matching load by introducing a phase and amplitude adjustable injection signal into main power amplifier  102 . Other than a phase shifter circuit  120  and an attenuator circuit  122  (which are used to adjust a phase and an amplitude, respectively, of an input signal to injection power amplifier  104 ), the remaining components ( 108 - 2 ,  110 - 2 ,  112 - 2 ,  114 - 2  and  116 - 2 ) of circuit  104  can be in exemplary embodiments substantially similar to corresponding components ( 108 - 1 ,  110 - 1 ,  112 - 1 ,  114 - 1  and  116 - 1 ) of main power amplifier circuit  102 . It should be noted that components/circuits  110 - 2  and  112 - 2  are also large signal amplifiers. 
   In general, in an exemplary embodiment, components ( 108 - 2 ,  110 - 2 ,  112 - 2 ,  114 - 2  and  116 - 2 ) of injection power amplifier circuit  104  are connected in a similar configuration to the earlier-described connection configuration of the components ( 108 - 1 ,  110 - 1 ,  112 - 1 ,  114 - 1  and  116 - 1 ) of main power amplifier circuit  102 . However, as can be seen in  FIG. 1 , injection power amplifier input quadrature hybrid circuit  108 - 2  is not directly coupled to, or does not directly receive, an injection power amplifier input signal, but instead receives a version of the injection power amplifier input signal, which is output from attenuator  122 . Further, injection power amplifier output quadrature hybrid circuit  114 - 2  has its ISO port connected to ground, via isolation resistor  118 , and its IN port connected to the ISO port of main power amplifier output quadrature hybrid circuit  114 - 1 . In embodiments of the present invention, phase shifter circuit  120  and attenuator circuit  122  are software controlled (using a digital controller  128 , for example, that includes a suitable software application  130  for carrying out the necessary phase and/or amplitude control). While in exemplary embodiments circuits  102  and  104  can have substantially similar components and circuit configurations, this need not be the case in all embodiments. For example, other configurations of circuit  104  can be used to generate the injection signal to control impedance seen by amplifiers  110 - 1  and  112 - 1  of circuit  102 . 
   Also included in (or used in conjunction with) apparatus  100 , and shown in  FIG. 1 , are a source  124 , which can provide a suitable source signal of a desired frequency (high frequency (HF), very high frequency (VHF), ultra high frequency (UHF), etc.), and a splitter circuit  126 , which is configured to receive the source signal and responsively output a main power amplifier input signal and an injection power amplifier input signal. It should be noted that, in  FIG. 1 , circled arrows  132 ,  134 ,  136 ,  138 ,  140 ,  142 ,  144  and  146  specifically represent phases of different signals output by different components of circuit  100 , but in the following description of the operation of circuit  100 , for simplification, they are used to generally represent the respective component input/output signals. 
   In operation, a signal from source  124  is split, by splitter circuit  126 , into a main power amplifier input signal  132  and an injection power amplifier input signal  134 . Signals  132  and  134  are typically in phase with each other. Main power amplifier input quadrature hybrid circuit  108 - 1  receives main power amplifier input signal  132  at its IN port and responsively provides, for amplification, a first version  136  of main power amplifier input signal  132  to first large signal amplifier  110 - 1  and a second version  138  of main power amplifier input signal  132  to second large signal amplifier  112 - 1 . As indicated in  FIG. 1 , first version  136  and second version  138 , of main power amplifier input signal  132 , have a phase difference of substantially 90 degrees. Amplified versions  140  and  142 , of signals  136  and  138  are output from amplifiers  110 - 1  and  112 - 1  and fed to main power amplifier output quadrature hybrid circuit  114 - 1 , which, in turn, provides, via its IN port, a main power amplifier output signal  144  to load  106 . 
   The injected signal is split by quadrature hybrid  114 - 1  creating waves traveling toward the output ports of amplifiers  112 - 1  and  110 - 1 . Signals traveling toward the amplifiers combined with signals generated by the amplifiers to produce the presented impedances Z LOAD1  and Z LOAD2  at the output reference planes of amplifiers  110 - 1  and  112 - 1 . The phase relationship between the forward waves and the injected waves is such that the impedances Z LOAD1  and Z LOAD2  are equal. 
     FIG. 2  is a Smith&#39;s Chart that includes plots illustrating a range of injection signal amplitude and phase adjustments that can be carried out in power amplifier apparatus  100  of  FIG. 1  to vary the impedance seen by amplifiers  110 - 1  and  112 - 1 . In  FIG. 2 , Smith&#39;s Chart plots  200 - 1  represent possible impedance matches for amplifier  110 - 1  (Z LOAD1  ( FIG. 1 )) and Smith&#39;s Chart plots  200 - 2  represent impedance matching for amplifier  112 - 1  (Z LOAD2  ( FIG. 1 )). In general, if no injection signal is present, there is an initial match between impedances Z LOAD1  and Z LOAD2  and load  106 . In this case, the attenuator circuit  122  is adjusted such that injection power amplifier input signal  134  is attenuated completely, resulting in no injection signal  146  being provided to main power amplifier circuit  102 . Under a 50 ohm resistance standard or resistance plane, a setting of attenuator  122 , which results in no injection signal  146  being output, is represented by 50 ohm points  202 - 1  and  202 - 2  in  FIG. 2 . Circles  204 - 1  and  204 - 2  represent an opposite condition in which impedances Z LOAD1  and Z LOAD2  and load  106  are adjusted using the injected signal to be substantially different from the load  106 . In this case, the injection power amplifier input signal is adjusted to produce injection signal  146  which creates waves traveling toward the amplifiers which are equal in magnitude to the waves traveling away from the main amplifiers. Concentric circles  206 - 1  and  206 - 2 ,  208 - 1  and  208 - 2 , and  210 - 1  and  210 - 2  represent different signal attenuation levels that lie between the two opposite scenarios discussed above. Different points on each of circles  204  through  208  represent different phases (0 to 360 degrees) that can be obtained through phase adjustments carried out with the help of phase shifter circuit  120 . 
     FIG. 3  is a diagrammatic illustration of a power amplifier apparatus  300  in accordance with another embodiment of the present invention. Other than including additional impedance transformers  304 - 1  and  304 - 2  in main power amplifier circuit  300 , the remaining components of power amplifier apparatus  300  are substantially similar to, and are connected together in a manner similar to, the components of power amplifier apparatus  100  ( FIG. 1 ). As can be seen in  FIG. 3 , impedance transformers  304 - 1  and  304 - 2 , which are typically substantially similar to each other, are connected between amplifiers  110 - 1  and  112 - 1  and the respective ports of main power amplifier output quadrature hybrid circuit  114 - 1 . The introduction of impedance transformers  304 - 1  and  304 - 2  altars Z LOAD1  and Z LOAD2  based on impedance ratios of the transformers. For example, if each of transformers  304 - 1  and  304 - 2  has an impedance ratio of 9:1, the 50 ohm reference plane is altered to a 5.56 (50/9) ohm reference plane. Amplitude and phase adjustment ranges for a 5.56 ohm reference plane power amplifier apparatus are shown in Smith&#39;s Cart plots  400 - 1  (Z LOAD1  ( FIG. 3 )) and  400 - 2  (Z LOAD2  ( FIG. 3 )). As can be seen in  FIG. 4 , when no injection signal is applied, 5.56 ohm points  402 - 1  and  402 - 2  represent complete attenuation of injection power amplifier input signal  134 . Concentric circles  404 - 1  and  404 - 2 ,  406 - 1  and  406 - 2 , and  408 - 1  and  408 - 2 , with 5.56 ohm points  402 - 1  and  402 - 2  as their respective centers, represent different signal attenuation levels when injection signals are required. 
   Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
A power amplifier apparatus that includes a main power amplifier circuit and an injection power amplifier circuit is provided. The main amplifier includes a pair of quadrature coupled amplifiers. Similarly, the injection power amplifier also includes pair of quadrature coupled amplifiers. The output signal from the injection amplifier is connected to the isolation port of the quadrature coupler of the main amplifier. The input signal is split and applied to the inputs of both the main and injection amplifiers. The gain and phase through the injection amplifier is adjusted in order to vary the magnitude and phase of the signal injected into the isolation port of the quadrature combiner of the main amplifier. By varying the magnitude and phase of the injected signal, the impedance seen by the individual amplifiers within main amplifier can be controlled. Controlling these impedances allows the properties of the main amplifier to be dynamically varied.