Adaptively preserving power amplifier linearity under antenna mismatch

Abstract

An adaptive method is proposed to preserve power amplifier linearity under severe antenna mismatch condition, as an alternative to an isolator. At high output power and mismatch conditions voltage clipping, due to collector voltage saturation of the bipolar power transistor, is demonstrated to be the main cause of distortion. Linearity degradation can be properly monitored by minimum voltage detection, which is used adaptively to control the gain of a pre-amplifier in order to prevent collector voltage clipping. Measurement results demonstrate a significant linearity improvement under severe mismatch conditions. For an EDGE power amplifier an EVM improvement of 5% and an ACPR improvement of 10dB have been obtained at 28.5 dBm output power and a VSWR of 4:1.