PURPOSE: To amplify high frequency signals over a wide band with low waveform distortion and low power consumption by connecting a specified impedance matching circuit for the distortion of a field-effect transistor to the transistor.

CONSTITUTION: The source of the field-effect transistor (FET) Q as an active element is connected to a ground terminal, the impedance matching circuit Ain composed of a coil L1 and a capacitor C1 is connected to the gate and the impedance matching circuit Aout composed of the coil L2, and the capacitor C2 is connected to the drain. Then, DC bias is set to let the FET Q perform the amplification operation of an AD class. In this case, the respective output impedances Z11 and Z12 of the impedance matching circuits Ain and Aout are decided so as to match with the output impedance when power addition efficiency becomes maximum to the fixed third distortion of the FET Q. Thus, the distortion and adjacent channel leakage power are reduced and the power consumption is reduced.