Feed-forward amplifier

Abstract

In a feed-forward amplifier which has an error detection circuit for detecting a nonlinear distortion component of a main amplifier and an error rejection circuit for amplifying the detected distortion component by an auxiliary amplifier and injecting it into the main amplifier to cancel an error component, a first pilot signal is injected into a signal input path and a second pilot signal is injected into a signal amplification path of the error detection circuit. A first variable attenuator and a first variable phase shifter of the error detection circuit are adjusted by a control circuit so that the level of the first pilot signal component on an error injection path of the error rejection circuit decreases to minimum. A second variable attenuator and a second variable phase shifter of the error rejection circuit are adjusted by the control circuit so that the level of the second pilot signal component on a signal output path decreases to a minimum. The first pilot signal component which appears on the signal output path is rejected by a filter or cancelled by supplying the error injection path or signal output path with the first pilot signal after adjusting its amplitude and phase. Alternatively, the frequency of the first pilot signal to be injected into the signal input path is spread to lower the level of the first pilot signal per unit spectrum which appears on the signal output path.