Abstract:
A system and method for improving the input return loss in RF amplifiers is disclosed. One embodiment of the present invention amplifies only one of the two output quadrature signals of 3 dB coupler in an amplifier module while substantially maintaining a constant impedance at the input to the 3 dB coupler. This removes one of the design constraints for designing the input network for an amplifier module thereby allowing for more flexible amplifier designs and ease of cascading amplifier modules. One embodiment of the present invention improves the input return loss of an RF amplifier pallet in a cascaded-stage power amplifier circuit for a television transmitter including a 3 dB coupler by replacing one of the two amplifiers connected to the output of the 3 dB coupler with an electrical circuit of substantially equivalent impedance to the input impedance of the non-replaced amplifier.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to improving the input return loss in RF amplifiers. More specifically, the impedance of an amplifier module including a 3 dB coupler at the input is maintained substantially constant while removing one of the two amplifiers normally connected to the output terminals of a 3 dB coupler. A preferred embodiment replaces the removed amplifier with an electrical circuit with an impedance that is substantially equivalent to the input impedance of the non-removed amplifier. Another preferred embodiment uses the amplifier module including a 3 dB coupler with an electrical circuit in place of one of the amplifiers in a cascaded-stage power amplifier circuit for a television transmitter. 
     RF amplifier input network designs must typically satisfy a number of constraints: (a) maintain required operational performance, such linearity, gain flatness, and maintaining sufficient signal with a specified slope at the active device input terminal, (b) maintain operational stability over the design operating range while compensating for incidental effects, such as am to am and am to pm distortion, (c) satisfy overall physical constraints for the amplifier, such as size, shape, weight, and cost considerations, and (d) provide a match to some nominal system interface impedance, which is typically 50 ohms for an RF system. These design criteria are typically used in television transmitter amplifier networks. 
     The requirement to match the nominal system interface impedance with sufficient accuracy is typically the hardest to achieve. Matching the interface impedance becomes an even more daunting task for amplifier networks comprising cascaded stages. For such systems, it is very difficult to match the interface impedance of each stage while efficiently achieving, in terms of amplifier efficiency, size, shape, weight, and cost, the necessary amplification at each stage. Overall performance of the cascaded amplifier networks, when compared with the results expected from the sum of individual stage performances, degrades rapidly when interstage impedance is not maintained. 
     One prior art solution to the problem of maintaining interstage impedance is the use of a quadrature hybrid combined amplifier for each stage of a cascaded-stage amplifier system. Quadrature hybrid combined amplifiers are known in the art and are described in detail in Anaren&#39;s 1997 Product Catalog, pp. 60-73, Anaren Microwave, Inc., which is hereby incorporated herein by reference. These quadrature hybrid combined amplifiers are used in cascaded-stage power amplifier networks for television transmitters. 
     An example of a prior art quadrature hybrid combined amplifier is shown diagrammatically in FIG.  2 . The prior art quadrature hybrid combined amplifier  200 , also referred to herein as an “amplifier pallet” or “pallet”, comprises the 3 dB coupler  210  at the input of the device acting as a divider, an amplifier for each of the output terminals of the 3 dB coupler  210 , and the 3 dB coupler  240  at the output of the pallet  200  acting as a combiner. 
     A typical 3 dB coupler, as is known in the art, may input a signal at one input terminal and produce, as a function of the input signal, an in-phase and a quadrature signal, relative to the input signal, each at a separate output terminal and each at approximately one-half of the power of the input signal. Generally, for example, when used at the input of the pallet  200 , the input 3 dB coupler  210  receives the input signal  201  on one input terminal while the other input terminal is terminated by the input terminator  215 . The 3 dB coupler  210  produces an in-phase signal and a quadrature signal, which are sent to the amplifier circuits  220  and  230 , respectively. The amplifier circuits  220  and  230  produce an amplified version of the in-phase and quadrature signals, respectively, which are combined in the output 3 dB coupler  240 . The output 3 dB coupler  240  produces an amplified, recombined input signal on one output terminal while hi the other output terminal is terminated. While this description provides a general idea of the signal flow paths through the pallet  200  in FIG. 2, a more complete description of FIG. 2 will be provided below. 
     3 dB hybrids have the desirable property of high input return loss at the common driven input port, provided the load impedances at the in-phase and quadrature output terminals are identical. Taking the example of the input 3 dB coupler  210  of FIG. 2, the impedance of the amplifiers  220  and  230  at the in-phase and quadrature output ports, respectively, is typically different than the nominal system impedance. However, as long as the impedance of each of the amplifiers is identical, essentially all of the energy reflected by the amplifiers at the in-phase and quadrature output ports of the input 3 dB coupler is absorbed at the terminated port of the 3 dB coupler  210 . This results in nominal system impedance at the non-terminated input port of the 3 dB coupler. Return losses of better than 20 dB are typically achieved over the two to one and greater bandwidths of commercially available 3 dB hybrids. 
     While placing two amplifiers of identical impedance at the in-phase and quadrature output ports of the input 3 dB coupler of an amplifier stage effectively matches the impedance of the stage with the nominal system impedance, such a solution may be inefficient in terms of the stage&#39;s cost, complexity, size, and overall efficiency if the amplification capacity with two amplifiers is more than is needed. In a cascaded-stage amplifier network, the amplification capacity of two amplifiers are not always needed in every stage and there is not an infinite gradation of available active to semiconductor devices at a corresponding cost gradation to allow a convenient scaling of the two amplifier approach to any required design capacity. In that regard, the elimination of one of the two amplifiers can supply the appropriate scaling to match the required design capacity. Typically, some of the amplifier modules in the initial stages of a cascaded-stage amplifier network, such as a driver stage, do not need the two amplifier capacity. Using an amplifier module with two amplifiers in the driver stage may not be cost effective, may render the driver stage too large physically to fit into a desired space, and may underutilize the amplification capacity available thereby reducing the overall efficiency of the cascaded-stage amplifier network while unnecessarily increasing the complexity of the system. 
     The present invention solves the above-mentioned drawbacks of the prior art by replacing one of the amplifiers at either the in-phase or quadrature output port of the input 3 dB coupler with an electrical circuit (“dummy network”) that emulates the input impedance of the non-replaced amplifier. A preferred embodiment matches the impedance of the dummy network with the input of the input network of the non-replaced amplifier, thereby maintaining the impedance balance between the in-phase and quadrature output ports of the 3 dB coupler. Another preferred embodiment uses the above-described matching dummy network configuration for television transmitters that may be used to transmit COFDM and/or 8VSB signals that may be in the 470 MHz to 860 MHz frequency range. 
     The dummy network that replaces one of the amplifiers may be the same as, or similar to, the input network of the non-replaced amplifier and may comprise a simple reactive network with a resistive/reactive termination to simulate the amplifier&#39;s active device load. The dummy network may replace either the amplifier at the in-phase output terminal or the amplifier at the quadrature output terminal. So long as the impedance of the dummy network substantially matches the input impedance of the non-replaced amplifier, the input impedance of the 3 dB coupler, and therefore the impedance of the amplifier stage, will remain at the nominal interstage impedance. Typical interstage impedance values for cascaded-stage amplifier networks in television transmitters is 50 ohms but it is to be understood that the effectiveness of the present invention is not limited to systems with 50 ohm impedance. The present invention is also effective in systems where the nominal interstage impedance is 75 ohms, such as for a CATV (cable television) system, and 300 ohms (balanced) such as for television receiver antenna circuits. It is to be understood that the above examples are not limiting and that the applicability of the present invention is not limited to any particular interstage impedance value. 
     The prior art amplifier circuit design method required that the circuit designer faced with the task of designing a circuit or an amplifier stage with only a single amplifier, had to design the amplifier with the capacity to amplify the input signal the desired amount as well as design an input network for that amplifier that satisfied all of the multiple criteria mentioned above: flat signal gain with frequency, stability, low am to am and am to pm distortion, and good input impedance relative to the nominal system impedance. By far the most difficult criteria to achieve is matching the input impedance with the nominal system impedance. 
     The present invention allows for the power amplifier circuit designer to design an amplifier stage with a single amplifier without having to worry that the impedance of the stage will not match the nominal system impedance. The use of a 3 dB coupler with a dummy network attached to either the in-phase or quadrature output terminal where the impedance of the dummy network substantially matches the impedance of the single amplifier greatly relieves the design burden of the amplifier circuit. It is much easier to design an amplifier input network that satisfies all of the design constraints but the constraint of matching nominal system impedance and a dummy network to mimic the input impedance of that amplifier than it is to design a single amplifier with an input network that satisfies all of the design constraints. 
     While the present invention sacrifices the amplification capacity of the replaced amplifier by replacing the amplifier with a dummy network, the amplification capacity that is lost is typically underutilized. Therefore, the overall performance of the cascaded-stage amplifier network does not suffer. The present invention also sacrifices 3 dB of gain compared to prior art systems with two amplifiers. However, the loss of 3 dB of gain is not a detriment to the operation of the cascaded-stage amplifier network provided that the overall design gain requirements of the network are met. 
     Accordingly, it is an object of the present invention to obviate many of the above problems in the prior art and to provide a novel system and method for improving the input return loss in RF amplifiers. 
     It is another object of the present invention to provide a novel system and method for amplifying only one of the two output quadrature components of an input signal to a 3 dB coupler in an amplifier module while substantially maintaining a constant input impedance for the 3 dB coupler. 
     It is yet another object of the present invention to provide a novel system and method for improving the input return loss of an RF amplifier pallet including a 3 dB coupler by replacing one of the two amplifiers connected to the output of the 3 dB coupler with an electrical circuit of substantially equivalent impedance to the input impedance of the non-replaced amplifier. 
     It is still another object of the present invention to provide a novel system and method of operating a hybrid combined amplifier module comprising a 3 dB coupler by replacing one of the two amplifiers connected to the output of the 3 dB coupler with an electrical circuit of substantially equivalent impedance to the input impedance of the non-replaced amplifier. 
     It is a further object of the present invention to provide a novel system and method for amplifying either a COFDM or an 8VSB signal in the 470 MHz to 860 MHz range in a cascaded-stage power amplifier where each stage includes a 3 dB coupler by replacing one of the two amplifiers connected to the output of the 3 dB coupler with an electrical circuit of substantially equivalent impedance to the input impedance of the non-replaced amplifier. 
     It is yet a further object of the present invention to provide a novel system and method for improving the input return loss of an RF amplifier pallet in a cascaded-stage power amplifier circuit for a television transmitter including a 3 dB coupler by replacing one of the two amplifiers connected to the output of the 3 dB coupler with an electrical circuit of substantially equivalent impedance to the input network of the non-replaced amplifier. 
     It is still a further object of the present invention to provide a novel system and method for maintaining, at a predetermined value, the impedance of an amplifier module comprising a 3 dB hybrid coupler with two output terminals each passing one of the two quadrature output signals to a separate amplifier, by replacing one of the amplifiers with electrical circuit with an impedance that substantially matches the input impedance of the non-replaced amplifier. 
     These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram in block form illustrating a prior art amplifier circuit with a single amplifier. 
     FIG. 2 is a circuit diagram in block form illustrating a prior art amplifier module (“pallet”) with a first amplifier at the in-phase output terminal and a second amplifier at the quadrature output terminal. 
     FIG. 3A is a circuit diagram in block form illustrating a modified amplifier module (“modified pallet”) with an amplifier at the in-phase output terminal and a dummy network at the quadrature output terminal. 
     FIG. 3B is a circuit diagram in block form illustrating a modified amplifier module (“modified pallet”) with an amplifier at the quadrature output terminal and a dummy network at the in-phase output terminal. 
     FIG. 4 is a circuit diagram in block form illustrating a three-stage cascaded network of amplifiers with a modified pallet in the pre-driver stage and pallets in the driver in output stages. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to the drawings, like numerals represent like components throughout the several drawings. 
     FIG. 1 is a circuit diagram in block form illustrating a prior art amplifier circuit  100  comprising a single amplifier. The input signal  101  is received by the amplifier input network  102 , is passed to the active device  104  and then to the amplifier output network  106 . The amplifier  100  is powered by the power supply  150 . The output signal  160  is output from the amplifier output network. If a designer were to try to design the amplifier circuit  100  to be used in a cascaded-stage amplifier network, the designer would be faced with the difficult task designing the input network  102  to satisfy a number of constraints: (a) maintain required operational performance, such linearity, gain flatness, and maintaining sufficient signal with a specified slope at the active device input terminal, (b) maintain operational stability over the design operating range while compensating for incidental effects, such as am to am and am to pm distortion, (c) satisfy overall physical constraints for the amplifier, such as size, shape, weight, and cost considerations, and (d) provide a match to some nominal system interface impedance, which is typically 50 ohms for RF systems. As discussed above, the most difficult criteria to meet is matching the input impedance of the amplifier circuit  100  with the system impedance. For a single amplifier circuit such as that shown in FIG. 1, much time and care must be taken by the designer to ensure that the design of the amplifier input network  102  meets the impedance criteria while maintaining the other three criteria within acceptable limits. 
     With reference now to FIG. 2, one prior art solution to the impedance matching problem is shown. The amplifier pallet  200  comprises the two amplifiers  220  and  230  and two 3 dB hybrid couplers, the 3 dB coupler  210  at the input of the pallet  200  and the 3 dB coupler  240  at the output of the pallet  200 . The 3 dB coupler  210  receives the input signal  201  via a first input terminal. A second input terminal is connected to the terminator  215 . The terminator is typically a resistor of nominal system impedance, which may be 50 ohms to match the nominal system impedance of an RF system, which is typically 50 ohms. The 3 dB coupler  210  derives an in-phase signal and a quadrature signal from the input signal  201 . The in-phase signal and the quadrature signal are output from the 3 dB coupler via separate terminals. As shown in FIG. 2, the in-phase signal (denoted by 0° in the FIGS.) and the quadrature signal (denoted by 90° in the FIGS.) in the amplifier pallet  200  are sent to the amplifier circuit  220  and the amplifier circuit  230 , respectively. The power supply  250  supplies the necessary power for the amplifier circuits  220  and  230 . The output of the amplifier circuits  220  and  230  are combined in the 3 dB coupler  240  and the output signal  260  is output via a first output terminal of the 3 dB coupler  240 , and hence is output from the pallet  200 . A second output terminal from the 3 dB coupler  240  is connected to the terminator  245 , which may be a resistive  20  load of nominal system impedance, which may be 50 ohms to match the nominal system impedance of an RF system, which is typically 50 ohms. 
     With continued reference to FIG. 2, each of the amplifier circuits  220  and  230  may contain an input network  222  and  232 , respectively, an active device  224  and  234 , respectively, and an output network  226  and  236 , respectively. While the overall configuration of the amplifier circuits  220  and  230  may be the same as the amplifier circuit  100  of FIG. 1, the design of the amplifier circuits  220  and  230  is simpler due to the presence of the 3 dB couplers in the pallet  200 . As discussed above, the 3 dB couplers have the desirable property of high input return loss. The impedance of the amplifiers  220  and  230  at the in-phase and quadrature output ports, respectively, is typically different than the nominal system impedance. However, so long as the impedance of each of the amplifiers is identical, essentially all of the energy reflected by the amplifiers at the in-phase and quadrature output ports of the input 3 dB coupler is absorbed at the terminated input port of the coupler. This results in nominal system impedance at the non-terminated input port of the 3 dB coupler, which is typically 50 ohms for an RF system, but may be, for example, 75 ohms for a CATV (cable television) system, and 300 ohms (balanced) for television receiver antenna circuits. 
     Since the 3 dB coupler acts to isolate the impedance of the amplifier circuits  220  and  230 , the designer is faced with the much easier task of designing the input network  222  and the input network  232  of the amplifier circuits  220  and  230 , respectively, without regard to the nominal system impedance, so long as the input impedance of amplifier circuit  220  is substantially identical to the input impedance of the amplifier circuit  230 . If the input impedances of the amplifier circuits  220  and  230  are substantially similar, the 3 dB coupler will substantially maintain the nominal system impedance thereby relieving the designer of the burden of meeting an impedance criteria. 
     While the prior art solution is practical, it is not always a very efficient solution. In the case of a cascaded-stage amplifier network, the driving amplifiers may not require the amplification capacity of two amplifiers. Therefore, the cost, size, cooling requirements, and excess amplification capacity of the second amplifier in the pallet  200  may not be an attractive solution. However, the use of a single amplifier circuit  100  may present too many design problems and may also be an unattractive option. Up until now, the prior art was limited to one of the two above solutions. 
     With reference now to FIGS. 3A and 3B, a modified pallet  300  is shown, where components in FIGS. 3A and 3B represent like components in FIG. 2 where the last two digits in the identification numbers are the same. 
     The modified pallet  300  comprises dummy network  370 , which replaces an amplifier circuit similar to the amplifier circuit  230  in the pallet  200  in FIG.  2 . The dummy network  370  is designed so as to have an input impedance identical to, or at least substantially similar to, the input impedance of the amplifier circuit  320 . The dummy network  370  may be comprised of components that are identical to or substantially similar to the components of the amplifier input network  322 . By substantially matching the impedance of the dummy network  370  to the input impedance of the amplifier circuit  320 , the 3 dB coupler  310  is able to substantially match the nominal system impedance at its input terminal. Furthermore, the modified pallet  300  eliminates the excess cost, size, cooling requirements, and amplification capacity of a second amplifier circuit. Therefore, the modified pallet  300  relieves the designer of the burden of meeting the impedance criteria while designing the amplifier as well as resulting in a more efficient design by only using one amplifier circuit in the modified pallet. 
     FIG. 3A shows the amplifier circuit  320  connected to the in-phase component terminal of the 3 dB coupler  310  and the dummy network  370  is connected to the quadrature terminal of the 3 dB coupler  310 . It is to be understood that the present invention contemplates that the terminals to which the amplifier circuit  320  and the dummy network  370  are connected can be switched. The amplifier circuit  320  may be connected to the quadrature terminal and the dummy network  370  may be connected to the in-phase terminal of the 3 dB coupler  310  as shown in FIG.  3 B. 
     The modified pallet  300  only amplifies one of the two components of the input signal  301 , while the pallet  200  amplifies both of the components of the input signal  201 . The loss of one of the components of the input signal  301  in the modified pallet  300 , and the concomitant loss in signal power, is a relatively minor drawback of the cascaded-stage amplifier network, as discussed above. 
     The 3 dB coupler  340  has only one input signal from the amplifier circuit  320  and therefore differs from the 3 dB coupler  240  in FIG. 2, which has two input signals, one from each of the amplifier circuits  220  and  230 . The 3 dB coupler  340  has the terminator  345  connected to the second input terminal and outputs the output signal  361 , which may be the in-phase component of the input signal to the 3 dB coupler  340 , and the output signal  362 , which may be the quadrature component of the input signal to the 3 dB coupler  340 . 
     It should be noted that the pallet  200  is capable of receiving one input signal and delivering one output signal. The modified pallet  300  is capable of receiving one input signal and delivering two output signals and therefore may be useful in a cascaded-stage amplifier network that fans-out, i.e., has more parallel amplification paths in a given stage than in a previous stage, without the need of a divider circuit. 
     In a preferred embodiment of the present invention, the modified pallet in FIG. 3 may also be configured and operated without the 3 dB coupler  340  and the output terminator  345 . In such a configuration, the modified pallet  300  would produce only one output signal rather than the two output signals shown in FIG.  3 . Otherwise, the configuration and operation of the modified pallet  300  would be the same as shown in FIG.  3  and as described above. 
     With reference now to FIG. 4, where like numerals represent like components with FIGS. 2 and 3A and  3 B, a cascaded-stage amplifier network is depicted in block form with three stages: a pre-driver stage, a driver stage, and an output stage. It is to be understood that FIG. 4 is merely a depiction of one embodiment of the invention and that the invention should not be limited to this particular configuration of amplifiers or to the number of amplifiers and/or dividers and combiners shown. 
     According to one embodiment of the present invention, the pre-driver stage is comprised of one modified pallet  300 , as described above, receiving an input signal  401 . Each of the two outputs from the modified pallet  300  are received by one of the pallets  200 - 1  and  200 - 2 , each similar to pallet  200  described above. The output of each of the pallets  200 - 1  and  200 - 2  are directed to a 4-way divider,  481  and  482 , respectively. The outputs of divider  481  are directed to pallets  200 - 3  through  200 - 6  and the output of divider  482  are directed to pallets  200 - 7  through  200 - 10 , all in the output stage shown in FIG.  4 . The outputs of pallets  200 - 3  through  200 - 6  are combined in the 4-way combiner  483 . Similarly, the outputs of pallets  200 - 7  through  200 - 10  are combined in the 4-way combiner  484 . The outputs of the 4-way combiners  483  and  484  are combined in the 2-way combiner  485 , which outputs the output signal  460 . 
     The 4-way dividers  481  and  482 , the 4-way combiners  483  and  484 , and the 2-way combiner  485  may be typical divider or combiner components known in the art. 
     In operation, the input signal  401  enters the modified pallet  300  in the pre-driver stage, is amplified and sent to the pallets  200 - 1  and  200 - 2  in the driver stage, where additional amplification is added. Similarly, the pallets in the output stage each amplify a portion of the signal entering the 4-way dividers  481  and  482 . The 4-way combiners  483  and  484  and the 2-way combiner  485  combine the outputs of the pallets  200 - 3  through  200 - 10  to form the output signal  460 , which is function of the input signal  401 . The above-described cascaded-stage amplifier network may be used, e.g., in a television transmitter. Other uses of the amplifier network shown in FIG. 4 would include, but would not be limited to, a cellular base station power amplifier. 
     While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and that the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence, many variations and modifications naturally occurring to those of skill in the art from a perusal hereof.