A consideration of phase distortion in linear power amplification of QPSK and two tone sinusoidal stimuli

The effects of phase distortion, resulting from AM-PM conversion in an RF power amplifier, and the amplifier's linearity, power, and efficiency performance is examined for two tone sinusoidal and quadrature phase shift key (QPSK) digitally modulated RF stimuli. These effects are examined using the quadrature modeling technique based on single tone CW derived I/Q gain transfer functions. Amplifier performance characteristics are evaluated for varying amounts of phase distortion as well as for several stimuli. The results indicate that even moderate amounts of phase distortion significantly degrade the available efficiency performance of the amplifier at a given level of spectral distortion. Consequently, compensating the phase distortion via pre-distortion or by other means, potentially allows the amplifier to be driven further into gain compression thereby achieving higher efficiency at peak power.