Patent ID: 7262668

Claim:
A semiconductor amplifier circuit comprising: a field effect transistor equipped with a gate electrode, a drain electrode and a source electrode; a first power source for supplying a bias voltage to the gate electrode of the field effect transistor; a second power source for supplying a bias voltage to the drain electrode of the field effect transistor; a protective resistance connected between the gate electrode of the field effect transistor and the first power source; an NPN transistor connected between the drain electrode of the field effect transistor and the second power source; a first differential amplifier to detect a voltage generated between both ends of the protective resistance; and a second differential amplifyer for applying a control voltage generated based on an output signal of the first differential amplifyer to a base electrode of the NPN transistor; wherein the second differential amplifier has a plus input terminal and a minus input terminal, the output of the first differential amplifier is input to the plus input terminal of the second differential amplifier and a negative voltage is input to the minus input terminal of the second differential amplifier.