This paper describes 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 QPSK signal with a rolloff factor of 0.5 is applied. This amplifier module (Fujitsu FMC080802-04) uses three GaAs FETs. The amplifier's saturated power is 31 dBm, and its maximum power-added efficiency is 50 X .Excellent operation of the NLDC was confirmed within a backoff range of more than 4 dB. Within this range, the out-ofband 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.
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