An improved Doherty amplifier using cascaded digital predistortion and digital gate voltage enhancement

This paper describes algorithms used to improve linearity, efficiency, and peak power of a Doherty amplifier whose auxiliary transistor gate voltage is adjusted digitally as a function of the signal envelope. Two predistortion stages are used to compensate for high-order memoryless nonlinearities and low-order memory effects. The digital gate voltage waveform is a sigmoid function of the predistorted signal´s envelope with two voltage limits corresponding to class C and AB bias levels. The instantaneous gate voltage is controlled by two adjustable parameters: a breakpoint and a slope. The parameters are adjusted to reduce the variance of the AM-AM curve of the power amplifier. The enhancement of the gate voltage at higher signal envelope levels increases the peak power and improves the efficiency of the Doherty amplifier structure.