There is provided an amplifier apparatus and a signal processing apparatus that reduce a delay or distortion of a power supply voltage used for drain modulation. An amplifier apparatus is an amplifier apparatus that performs drain modulation, and includes a printed circuit board having a first surface and a second surface; an amplifier circuit disposed on the first surface; and a modulation power supply circuit that supplies a variable power supply voltage for performing drain modulation, to the amplifier circuit The modulation power supply circuit has an output portion that outputs the power supply voltage. The amplifier circuit has an input portion to which the power supply voltage is supplied. The output portion is located on the side of the second principal surface of the printed circuit board and is connected to the input portion through a conductor penetrating through the printed circuit board.