Fast second harmonic injection characterization for efficiency enhancement of RF power amplifiers

A fast characterization method to determine the optimal second harmonic injection required at the input for enhancing the efficiency of RF power amplifiers is demonstrated. The second harmonic is injected with a small frequency offset to automatically sweep the phase during one modulation cycle. The power amplifier performance figures of merit are characterized over the second-harmonic amplitude and phase space. Time varying voltage and current waveforms are measured using a large signal network analyzer for a 15 W peak power packaged GaN device. The constructive second-harmonic injection exhibited improvements in efficiency of 11.7% and 30.5% compared, respectively, to the cases of (1) no injection with 50Ω termination and (2) destructive second-harmonic injection.