An amplifier includes a first FET having a first back-gate end, a second FET having a second back-gate end, a third FET having a third back-gate end, a first power supply terminal configured to apply a voltage to the first back-gate end, a second power supply terminal configured to apply a voltage to the second back-gate end, and a third power supply terminal configured to apply a voltage to the third back-gate end. In the stated amplifier, the first through third power supply terminals are configured such that different voltages can be set to the first through third power supply terminals.