Efficiency of Outphasing RF Power-Amplifier Systems

TLDR

The outphasing technique (LINC) merges two nonlinear RF power amplifiers into a linear PA system, but the resulting time‑varying impedances alter the amplifiers’ DC power consumption and efficiency. The system drives two class‑B PAs with phase‑shifted signals, controlling the phases so their outputs add to the desired amplitude; power and efficiency characteristics are derived for simple transformer‑coupler and Chireix transmission‑line‑coupler configurations, and average efficiency versus shunt reactance is evaluated for amplitude‑modulated signals. Simple outphasing systems exhibit the efficiency of a linear class‑B PA, while a Chireix outphasing system can reach peak efficiency at a particular output amplitude, and tailoring the shunt reactance to the signal can double the efficiency.

Abstract

The outphasing technique (LINC) combines two nonlinear RF power amplifiers into a linear RF power-amplifier (PA) system. The two PA's are driven with signals of different phases, and the phases are controlled so that the addition of the PA outputs produces a system output of the desired amplitude. However, the resultant time-varying impedances presented to the PA's alter their dc power consumption and efficiency. Power and efficiency characteristics are derived for both simple (transformer-coupler) and Chireix (transmission-line-coupler with shunt reactance) outphasing systems using saturated class-B PA's. Simple outphasing systems have the efficiency characteristic of a linear class-B PA. Through proper selection of the shunt reactance, the efficiency of a Chireix outphasing system can be maximized at a specific output amplitude. The average efficiency with various amplitude-modulated signals is determined as a function of shunt reactance. Selecting the shunt reactance to fit the signal can improve efficiency by as much as a factor of 2.