| 摘要 |
A power amplifier circuit for efficiently generating an output signal into a load at more than one alternative power level has at least two amplifiers, each having optimum efficiency at different power output levels. The power amplifier circuit may be operated in several modes, including activating one amplifier during an entire cycle of an input signal to be amplified, or alternatively activating one amplifier based on a control signal that is independent of the amplified output signal. In either case, remaining amplifiers are deactivated. Coupling between the amplifiers prevents deactivated amplifiers from impeding the flow of power from the activated amplifier to the load. In another aspect of the invention, optimum efficiency at different power output levels is achieved by means of coupling each amplifier to a common power source and to the load via an impedance matching circuit, the impedance transformation being different for different amplifiers. The impedance matching circuit may comprise a push-pull transformer having taps to provide different turns ratios between respective amplifiers and the load. Alternatively, the matching circuit may comprise a number of quarter-wavelength lines each coupled at one end to the load and at the other end to one of the amplifiers via a balun.
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