An inverter power supply has a field-effect transistor (16) as its switching device. Control pulses from a pulse transformer (T1) are applied to the gate electrode through a transistor circuit comprising a transistor (Q2P) with its base connected to the secondary of the pulse transformer and its emitter-collector circuit forming a discharge path for the gate capacitance. On the termination of a control pulse, the transistor (Q2P) becomes conducting and rapidly discharges the gate capacitance of the field-effect transistor, causing it to switch off very rapidly.; A second transistor (Q2N) may have its base connected to the output of the pulse transformer and its emitter-collector circuit connected between the gate electrode of the field-effect transistor and a source of potential, so that on arrival of a pulse it becomes conducting and rapidly charges the gate capacitance of the field-effect transistor (16), causing it to switch on very rapidly. The potential source may be a capacitor (C1) whose potential is maintained in operation by base-collector current flowing in the second transistor (Q2N). The circuit is constructed throughout of elements capable of operating at very high frequencies.