Abstract: Current in the drain-source path of a microwave high-power GaAs MESFET is controlled by a regulating circuit including an integrating differential amplifier whose output controls the gate of the MESFET. One input of the differential amplifier is supplied with a fraction of the supply voltage, and the other is supplied with a voltage dependent upon the current to be controlled. During switch-on and switch-off of the supply voltage, the risk of Gunn effect oscillation of the MESFET is avoided by a switching circuit which controls the differential amplifier to maintain substantially zero drain-source current when the supply voltage is below a predetermined level less than its nominal level.