Amplifiers – With distributed parameter-type coupling means
Reexamination Certificate
2001-08-20
2003-02-18
Cunningham, Terry D. (Department: 2816)
Amplifiers
With distributed parameter-type coupling means
C330S12400D
Reexamination Certificate
active
06522196
ABSTRACT:
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'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'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'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 amplifi
Cabrera George
Moore Paul
Poggi Peter John
Cunningham Terry D.
Duane Morris LLP
Harris Corporation
Nguyen Long
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