For example, an analog feedback amplifier disclosed in a Patent Document 1 comprises a variable phase shifter that adjusts the phase of a baseband input signal, a variable gain amplifier that amplifies the input signal, and power amplifier that amplifies the input signal, and further comprises a control system that controls the phase shift of the variable phase shifter and the gain of the variable gain amplifier.
The control system comprises the following components.
(1) An attenuator that attenuates the signal amplified by the power amplifier, and outputs the signal after the attenuation as a feedback signal.
(2) A detector that detects the input signal.
(3) An amplitude comparator that detects the difference between the amplitude of the input signal detected by the detector and the feedback signal output from the attenuator.
(4) A phase comparator that detects the phase difference between the input signal and the feedback signal output from the attenuator.
(5) Again controller that controls the gain of the variable gain amplifier in accordance with the difference detected by the amplitude comparator.
(6) A loop filter that controls the phase shift of the variable phase shifter in accordance with the phase difference detected by the phase comparator.