Amplifiers – With semiconductor amplifying device – Including gain control means
Reexamination Certificate
2000-09-28
2003-02-18
Shingleton, Michael B (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including gain control means
C330S12400D, C330S096000, C330S284000, C330S302000
Reexamination Certificate
active
06522201
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to RF amplifiers operable at full power and at a lower back-off power, and more particularly the invention relates to improving the amplifier efficiency at a back-off power level.
FIG. 1
is a schematic of an RF/microwave amplifier including a bipolar transistor
2
having a grounded emitter, a DC biased RF input connected to the base, a power terminal (+V) connected through inductor
4
to the collector, and an output coupled to the collector through capacitor
6
.
An RF and microwave amplifier can achieve high efficiency at full output power. However, in many wireless communication applications such as CDMA, the amplifier is operated at a back-off power level much more frequently than at full power level. Unfortunately, at back-off power level the efficiency of an amplifier usually suffers. The efficiency degrades as a result of not fully using the available voltage and current, as illustrated in the current versus voltage plot of
FIG. 2
for a RF power amplifier operating at various input levels. Line
10
(solid) is the load line for the class A RF amplifier operating at full voltage, while line
12
(wavy) is the load line for the amplifier operating at a back-off power level. Changing from class A bias to class B bias helps the efficiency as shown by load line
14
(wavy), but efficiency at the back-off power level is still poor.
One method for achieving a higher efficiency at a back-off power level is to change the bias voltage with power level as shown in the plot of FIG.
3
. At the back-off voltage as well as at full power, neither voltage nor current utilization is wasted. However, at the back-off voltage, the knee of the characteristic current/voltage curves degrades the amplifier efficiency.
The present invention is directed to improving the operating efficiency of an RF/microwave amplifier when operating at a back-off power level.
SUMMARY OF THE INVENTION
In accordance with the invention, efficiency of a power amplifier is increased at a back-off power level by increasing the load impedance of the amplifier at the reduced output power level as compared to load impedance at a higher power level including full output power.
In a preferred embodiment, an RF/microwave amplifier which is operable at two or more output power levels comprises a first signal amplification unit having an input terminal for receiving an input signal, an output terminal for producing an amplified output signal, and a power terminal for receiving electrical power. A load is coupled to the output terminal and has a first impedance value for a full power output and a second impedance value for a back-off power level, the second value being greater than the first value. The signal amplification unit may comprise at least one transistor such as a bipolar transistor or a field effect transistor.
In one embodiment of the invention the RF amplifier includes a second signal amplification unit in parallel with the first signal amplification unit with a second load coupling the output terminal of the second signal amplification unit to the RF output, the second load having a higher impedance than the first load, and a switch means for alternately connecting the first signal amplification unit and the second signal amplification unit to the RF output.
REFERENCES:
patent: 3449685 (1969-06-01), Holmes
patent: 5307512 (1994-04-01), Mitzlaff
patent: 5565823 (1996-10-01), Fratti
Holt “Electronic Circuits Digital and Analog” 1978, by John Wiley & Sons, Inc. pp 384 and 545.*
McMorrow, Robert J. et al., “The Microwave Doherty Amplifier,” 1994 IEEE MTT-S Digest, pp. 1653-1656, 1994.
Upton, David M. et al., “A New Circuit Topology to Realize High Efficiency, High Linearity, and High Power Microwave Amplifiers,” RAWCON'98 Proceedings, pp. 317-320, 1998.
Youngoo Yang et al., “Experimental Investigation on Efficiency and Linearity of Microwave Doherty Amplifier,” 2001 IEEE MTT-S Digest Phoenix, AZ, 4 pp, 2001.
Iwamoto, Masaya et al., “An Extended Doherty Amplifier with High Efficiency Over a Wide Power Range,” 2001 IEEE MTT-S Digest Phoenix, AZ, 4 pp., 2001.
Hsiao Shuo-Yuan
Wang Nanlei Y Larry
Zhou Wei-Shu
EiC Corporation
Shingleton Michael B
Townsend and Townsend / and Crew LLP
Woodward Henry K.
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