A signal separating unit separates an input signal into a first branch signal and a second branch signal in such a manner that, as the amplitude of the input signal decreases, the amplitude of the first branch signal and the second branch signal decreases and the difference between a phase of the first branch signal and a phase of the second branch signal increases and, as the amplitude of the input signal increases, the amplitude of the first branch signal and the second branch signal increases and the difference between the phase of the first branch signal and the phase of the second branch signal decreases. An amplifier amplifies the first branch signal. Another amplifier amplifies the second branch signal. A combining unit combines signals that are output from the amplifiers together, thereby generating an output signal.