Abstract: Power amplifier operating at a fixed frequency or a narrow bandwidth and based upon a non-linear gain element. The power amplifier includes a resonant circuit operably connected to the non-linear gain element, the circuit being resonant at a harmonic of substantially the band center frequency of the non-linear gain element and being arranged to dissipate energy resulting from harmonic generation by the non-linear gain element during signal amplification. In a particular aspect of the power amplifier, resonant circuits comprising inductor-capacitor harmonic trap circuits tuned to different harmonics are placed at both the input and the output of a gallium arsenide heterojunction bipolar transistor HBT providing the non-linear gain element.