Wave transmission lines and networks – Negative resistance or reactance networks of the active type – Having negative impedance
Reexamination Certificate
2000-10-02
2003-04-22
Shingleton, Michael B (Department: 2817)
Wave transmission lines and networks
Negative resistance or reactance networks of the active type
Having negative impedance
C330S053000, C330S12400D, C330S302000, C333S032000, C333S213000, C333S214000
Reexamination Certificate
active
06552634
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the field of amplifiers for audio, radio, and microwave frequencies, used in conjunction with filters, matching networks, and power combiners in systems such as transmitters, and more specifically to methods for enabling power amplifiers and their associated output networks and power combiners to operate safely and efficiently and with substantilly flat frequency responses over wider bandwidths than would otherwise be possible.
BACKGROUND ART
The production of high-power electronic signals is required in a variety of applications, including radio, microwave, audio, and servo amplifiers. Applications for radio and microwave power amplifiers include communications, broadcasting, and magnetic-resonance imaging. Typical applications for audio power amplifiers include communications, home entertainment, and sonar. Servo amplifiers are used for various control and positioning applications.
Electronic amplifiers are often required to operate over a wide bandwidth. An output filter is often required to prevent harmonics from reaching the load, and a matching network is often required between the load and the amplifier to provide the amplifier with a suitable load impedance. In many cases, multiple amplifiers operating in different frequency bands are employed to allow a system to operate over a large range of frequencies. It is also desirable for power amplifiers (amplifiers that produce a significant output power) to operate with high efficiency. The amplifier characteristics required to achieve these various performance goals often conflict with each other, hence designing such amplifiers using conventional techniques involves inherent trade-offs and limitations in achievable performance.
One example is an audio-frequency (AF) power amplifier (PA) driving a speaker. The resistance and inductance in the speaker form a low-pass filter, but it is desirable to maintain a flat frequency response to frequencies higher than the corner-frequency of the speaker. A second example is a radio-frequency (RF) power amplifier that drives an antenna through a matching network of limited bandwidth. The bandwidth of the matching network can prevent production of wideband signals such as spread-spectrum modulation. A third example is a modern solid-state radio transmitter that selects one output filter from a bank of filters that cover the operating frequency range of the transmitter. A relatively small filter bandwidth results in the need for a large number of filters. A fourth example is a transmitter system that employs separate HF and VHF PAs, but combines their outputs into the same antenna. A fifth example is the Meinzer modulator that uses a class-S amplifier to amplify the low-frequency components efficiently and a class-B amplifier to add the high-frequency components.
The prior art, reviewed in detail subsequently, has at least three significant limitations in these areas. Existing techniques for broadband filters and matching networks are based upon optimizing variation of gain or minimizing standing-wave ratio (SWR), but do not provide minimum voltage and current ratings for the the amplifiers that drive them. Signal-processing techniques that flatten the gain are typically based upon feedback and therefore have inherent trade-offs between the amount of gain flattening, stability, and bandwidth. Prior-art techniques for combining power amplifiers that operate on different frequencies are either lossy or require the bands to be noncontiguous.
Broadband Filters and Matching Networks
Output filters and matching networks are integral components of most amplifier systems. Filters are required to prevent the harmonics generated by power amplifiers from reaching the load. Matching networks are required because the load impedance for which the power amplifier can efficiently deliver the desired power is generally different from that of the load (e.g., antenna) into which the power must be delivered. Even small-signal amplifiers must generally be matched to the load to deliver the maximum output signal. However, the filter and matching networks impose limits on the bandwidth of the transmitter or amplifier system.
FIG. 1
,
FIG. 2
, and
FIG. 3
show an example of prior-art techniques for increasing the bandwidth. In
FIG. 1
, amplifier
2
drives load
9
through a series-tuned filter circuit consisting of inductor
4
and capacitor
5
with Q
1
=3.5 to provide adequate suppression of harmonics. As shown in the Smith-chart plot of
FIG. 3
, the PA-load impedance
3
exits the 2:1 SWR circle
12
at approximately 0.9041 and 1.1061 MHz, resulting in a passband ratio of 1.1061/0.9041=1.223 for a 2:1 SWR.
In this example, broadbanding is accomplished by adding a parallel-tuned circuit consisting of inductor
6
and capacitor
7
between the series-tuned circuit and the load. When the values L
2
and C
2
are properly chosen, the locus of the PA-load impedance
11
in
FIG. 3
now loops around within the 2:1 SWR circle and exits the circle
11
at 0.8673 and 1.1529 MHz, resulting in a larger passband ratio of 1.329.
The implementation of broadband filters and matching networks is a well-known technology that is taught In textbooks by P. L. D. Abrie (
The Design of Impedance
-
Matching Networks for Radio
-
Frequency and Microwave Amplifiers
, Dedham, Mass., Artech House, 1985) and W. K. Chen (
Broadband Matching: Theory and Implementation
, Teaneck, NJ, World Scientific, 1989) and numerous articles, for example T. R. Cuthbert, Jr., “Broadband impedance matching techniques”
R. F. Design
, vol. 17, no. 8, pp. 64-71, August 1994. A number of different methods for determining the values of the components have been developed, and the “real-frequency” technique developed by H. J. Carlin (“A new approach to gain-bandwidth problems,”
IEEE Trans. Circuits Syst
.,, vol. CAS-24, no. 4, pp. 170-175, April 1977) is one of the most popular. Applications (e.g., E. Franke, “Simple compensation of the single-section quarter-wave matching section,”
R. F. Design
, vol. 15, no. 1, pp. 38-46, January 1992) and calculation programs (e.g.,
R. J. Dehoney
, “Program synthesizes antenna matching networks for maximum bandwidth,”
R. F. Design
, vol. 18, no. 5, pp. 74-81, May 1995) abound in the literature.
All of the existing techniques for the design of broadband filters and matching networks are based upon controlling gain (attenuation) or SWR over a bandwidth. Since the gain of a linear, lossless network and the SWR are directly related, controlling one is equivalent to controlling the other. For example, the Butterworth characteristic maximizes the bandwidth with a smoothly decreasing gain or smoothly increasing SWR. The Chebyshev characteristic minimizes the ripple in the gain or maximum SWR within a given band with a given number of elements.
All of these techniques are subject to the famous limitation discovered by Fano and described in “Theoretical limitations on the broadband matching of arbitrary impedances,”
J. Franklin Inst.,
vol. 249, pp. 57-83, January 1950 and vol. 249, pp. 139-154, February 1950. The Fano limit basically states that regardless of the number of elements added to a network, it is not possible to obtain a perfect match at all frequencies and there is a fundamental trade-off between the bandwidth and the maximum gain ripple or SWR within the pass band. Thus increasing the bandwidth is achieved at the cost of increased gain ripple and increased SWR.
Delivery of power into a filter or matching network (hence the load) is, however, limited by the voltage and current ratings of the amplifier. The amplifier ratings are in turn directly related to the voltage and current that must be applied to the input of the filter or matching network to produce the desired amount of power at its output or load. In spite of the large number of publications in this area, no pre-existing techniques address the issue of of minimizing the input voltage and current so an amplifier can safely deliver maximum power to the load over a specified bandwidth.
Gain
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