Constant-gain envelope tracking in a UHF outphasing transmitter based on continuous-mode class-E GaN HEMT PAs

A UHF outphasing transmitter, based on continuous-mode class-E power amplifiers (PAs) and implementing a constant-gain envelope tracking (ET) strategy, is presented in this paper. Drain terminating and biasing networks are designed to provide near optima impedance values at the fundamental and higher order harmonics to the selected GaN HEMT device. A high-efficiency wideband performance, 80% for a 630–890 MHz range, is obtained, besides being amenable for load-modulation through a compact outphasing scheme, incorporating a series Chireix combiner and an impedance transformer. Once characterized in a pure output power phase-coding regime, the observed limitation in dynamic range is overcome by operating the amplifiers in a sort of continuous class-J mode, while forcing the load impedance to follow a constant-gain trajectory. A 1c-WCDMA signal, with peak-to-average power ratio (PAPR) of 8.4 dB is shown to be reproduced, satisfying the linearity requirements, with an average efficiency of 58%.