In an amplifier including first and second power supply terminals, first and second output terminals, a first load connected between the first power supply terminal and the first output terminal, a second load connected between the first power supply terminal and the second output terminal, a constant current source connected to the second power supply terminal, a first transistor connected between the first output terminal and the constant current source, a control terminal of the first transistor being adapted to receive an input voltage, and a second transistor connected between the second output terminal and the constant current source, a control terminal of the second transistor being adapted to receive a reference voltage, an amplification and output impedance switching circuit is connected between the first and second output terminals, so that the amplifier and output impedance switching circuit controls an amplification and output impedance of the amplifier in accordance with a control signal.