Backoff improvement of an 800-MHz GaAs FET amplifier for a QPSK transmitter using an adaptive nonlinear distortion canceller

The use of a nonlinear distortion canceller (NLDC) with a Cartesian feedback loop to improve the output backoff of an 800 MHz GaAs FET power amplifier when a quadrature phase shift keying (QPSK) signal with a rolloff factor of 0.5 is applied is described. This amplifier module uses three GaAs FETs. The amplifier´s saturated power is 31 dBm, and its maximum power-added efficiency is 50%. Excellent operation of the NLDC was confirmed within a backoff range of more than 4 dB. Within this range, the out-of-band emission was kept 55 dB below the center frequency level. Since the peak output power coincides with the saturation power at a 4 dB backoff, the NLDC improves the backoff to the theoretical limit