PURPOSE: To make a bias circuit unnecessary, and also, to widen a flat gain band in respect of a flat gain amplifier by a CMOS circuit.

CONSTITUTION: First p- and n-channel field effect transistors 3,4 are cascade- connected between power supply 1 and the ground 2, and the gates of these transistors 3,4 are connected in common so as to be an input terminal 5. Besides, second p- and n-channel field effect transistors 6,7 are cascade-connected between the power supply 1 and the ground 2, and their cascade-connection point, the gates of these transistors 6,7, and the cascade-connection point of the first p- and n-channel field effect transistors 3,4 are connected in common so as to be an output terminal 8.