Patent ID: 7423491

Claim:
A semiconductor amplifier comprising: a semiconductor amplifying element, having a first electrode, a second electrode, and a third electrode, configured to amplify a high-frequency signal that is input to the first electrode and to output an amplified signal to the second electrode; a first power source configured to supply a first bias voltage to the first electrode of the semiconductor amplifying element; a second power source configured to supply a second bias voltage to the second electrode of the semiconductor amplifying element; a third power source configured to supply a negative voltage; a protective resistance connected between the first electrode of the semiconductor amplifying element and the first power source; a voltage detector for detecting a voltage generated between two terminals of the protective resistance; a differential amplifier having a first input terminal connected to the voltage detector and a second input terminal connected to the third power supply, the differential amplifier being configured to output a control voltage based on the negative voltage supplied from the third power supply and an output signal of the voltage detector; and a bias voltage controller, connected between the second electrode of the semiconductor amplifying element and the second power source, configured to control the second power source bias voltage to be supplied to the second electrode of the semiconductor amplifying element based on the control voltage output from the differential amplifier.