PA Design: Steve Cripps on Outphasing Pas

Most existing analyses of the Chireix outphasing circuit assume that the active devices behave as voltage sources. Once this rusty creaking door is forced open, the analysis poses few problems and shows how the combined output power can be controlled over a useful range and with enhanced effi ciency by varying the differential phase of the two input signals. But in almost all other applications and PA analyses, transistors are not usually considered to behave as voltage sources. As such, it is surprising that after 80 years the outphasing configuration still sits on such shaky foundations. This presentation analyzes the Chireix outphasing circuit using a novel analytical model for the transistor I-V knee characteristics, rather than the approximation of a simple voltage source, and shares various new design pointers. It also incorporates input drive level variation, usually a critical independent variable in any other PA analysis, but curiously underrated by the RF power amplifier (RFPA) outphasing community. The result is a more comprehensive understanding of how the outphasing circuit works.