Harmonic tuning of power FETs at X-band

A study is presented of high-efficiency, harmonic-tuned, class-B operation of power MESFETs at X-band. Hybrid, single-stage 1200- mu m power FET (field-effect transistor) amplifiers were fabricated with the output circuit designed to provide optimum load impedance at the fundamental frequency (10 GHz) and short at the second harmonic. Power-added efficiency of 61% at an output power level of 450 mW and 7-dB power gain were obtained at 10 GHz. The corresponding drain efficiency was 75%. The second harmonic level in the output was suppressed to less than -40 dBc over a 4% frequency bandwidth. The efficiency was improved at the expense of a lower operating voltage and power density (0.4 W/mm) when compared with class-A or class-AB amplifiers made from similar devices. Theoretical harmonic-balance analyses of these tuned class-B amplifiers were also performed, and the results agree fairly well with the measured data.<>