Feed-forward circuit

Claim 1

A feed-forward amplifier, comprising: a signal cancellation loop including a main amplifier to which an input signal is applied, to obtain an amplified but distorted output signal, a first coupler for coupling a portion of the output signal from the main amplifier, a first delay line to which the input signal is also applied, to delay the signal by substantially the same amount as the main amplifier, equalization means, to make amplitude and phase adjustments in the main amplifier output signal, to match it with the output signal from the first delay line, and a second coupler, to subtract the delay line output signal from the equalizer means output signal, to effectively cancel the input signal component and leave only error signal components; an error signal loop including an error signal amplifier connected to receive an error signal from the second coupler, a second delay line connected to delay the distorted output signal from the main amplifier by substantially the same amount as the error amplifier, and a third coupler, connected to subtract the output of the error amplifier from the output of the second delay line, to cancel error signal components in the distorted output signal; wherein the first and second delay lines are microstrip lines, and the main and error amplifiers have a high gain and a relatively linear phase-frequency characteristic; and wherein the main amplifier and the error amplifier each include a single-ended first section having two common-emitter stages, and an output stage of cascode push-pull configuration.