An amplifier includes a first transformer configured to output first differential signals, a first differential amplifier coupled to the first transformer, a second transformer coupled to the first differential amplifier, a second differential amplifier coupled to the second transformer, a third transformer configured to transform second differential signals output from the second differential amplifier to a single-ended output signal, and a first bias circuit configured to supply a first bias voltage to a first secondary coil of the first transformer, wherein the first bias circuit sets the first bias voltage to a voltage greater than or equal to a first voltage based on the input signal in a first operating area where power of the output signal is greater than or equal to a first power so that power-gain characteristics of the output signal become closer to characteristics where gain of the output signal becomes constant.