Single-Ended Branch PA Characterisation for Outphasing Amplifiers

Back-off efficiency is a key requirement for load modulated power amplifier (PA) architectures such as Chireix outphasing. This paper studies the impact of second and third harmonic impedance tuning (Z(2f₀),Z(3f₀)) on the performance of a 10 Watt GaN HEMT branch PA subject to outphasing-like load modulation, operating at 900 MHz. Performance variation of up to 3 dB output power (P_OUT) and 50% points drain efficiency (DE) with phase manipulation alone of Z(2f₀) is recorded. Optimal Z(2f₀) is revealed to vary - throughout a fixed and for different - outphasing trajectories. A simple technique is presented, using continuous class F^-1 closed form equations, which predicts the intrinsic optimal Z(2f₀) for multiple points from five outphasing load trajectories, de-embedded to the current generator (CG) plane of the device.