US Patent 7940122 Amplifier circuit and communication device

There is provided an amplifier circuit having a common-source amplifier, an output load connected to an output terminal of the common-source amplifier, a buffer circuit connected to the output terminal of the common-source amplifier, a feedback circuit connected between an output terminal of the buffer circuit and an input terminal of the common-source amplifier, and a control circuit for controlling an impedance of the feedback circuit in accordance with a gain of the common-source amplifier.