I. Introduction
To meet the increasing demand of data throughput, many modern wireless standards employ spectrum efficient modulation schemes, which often present a large peak-to-average power ratio (PAPR) for the transmitted signal. Consequently, the power amplifier (PA) needs to frequently operate at a large power back-off (PBO) with degraded efficiency. Among the PBO efficiency improving techniques, Doherty PA has recently gained much attention due to its potentially large modulation bandwidth and moderate implementation overhead [1]–[6].