PURPOSE: To expand a usable frequency band by providing a feedback circuit formed with the use of a drain grounding field effect transistor(FET) between an input and an output with 180°C phase difference separately from a negative feedback circuit, and suppressing the input/output VSWR of the broad band amplifier in an allowable range.

CONSTITUTION: An FET 20 controls a feedback quantity from an output terminal 9 to an input terminal 8. The feedback quantity is determined by the gains of an amplifying part 30 and the FET 20. Since the gain of the amplifying part 30 and the FET 20 are both decreased as the frequencies are increased, the feedback quantity by the FET 20 is decreased as the frequencies are increased. Accordingly, the gain of the amplifier 30 is approximately flat with respect to the frequencies. Consequently without largely deteriorating the input/ output VSWR of the amplifier, the frequency characteristic of the gain can be drastically improved. The feedback circuit by the FET 20 controlling the feedback quantity can be used between the terminals having the 180°C phase difference of a microwave voltage.