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This paper describes the design, implementation, and characterization of a high-efficiency 10-GHz amplifier with dynamic drain bias control that maintains high efficiency over a range of output power levels. The power amplifier (PA) operates in class-E switched mode with 67% drain efficiency at an output power of 20.3 dBm, 0.7 dB less than the specified maximum power for the device. A coupler and detector at the output of the PA provide a feedback signal to the drain-bias controller based on a 96% efficient Buck dc-dc converter. When compared with a PA with constant drain bias (4 V), the average efficiency of the PA with dynamic biasing is improved by a factor of 1.4. Over an output power between 15-20 dBm, the drain bias varies from 2 to 4 V, and the efficiency improves from 22% to 65% at the lower power level. The efficiency includes the losses in the dc-dc converter.