The paper presents a model of the radio transmitter output stage for generating signals with high peak-to-average ratio waveform (AM, single-sideband modulation, QAM), based on a series outphasing modulation circuit with Chireix compensation and class EF2 power amplifiers (PAs). The property of the class EF2 PA to maintain high efficiency when complex load resistance varies over a wide range is shown. It allows to preserve high efficiency of amplitude modulation at 15 times power back-off. Due to a higher power output capability than in a Chireix outphasing circuit with class E PAs, the proposed circuit can be widely used in broadcasting (short wave AM and DRM) and communication (WCDMA and LTE) transmitters with different output power levels and frequency ranges.