Abstract: A fraction of current passing through the P-emitter region and N-base region of a thyristor is by-passed to the base-emitter junction of a PNP transistor. The amount of the base current is dependent on the thyristor current. Thus, as the anode current of the thyristor increases, the base current and hence the collector current of the PNP transistor increases. The collector current by-passed to the PNP transistor is fed, via a switch which is closed during the off-time of the thyistor, to the base-collector path of an NPN transistor whose collector and emitter are respectively connected to the gate and cathode of the thyristor. The turn-on voltage across the collector and emitter of the NPN transistor accordingly becomes lower than the gate-cathode voltage of the thyristor. The base-emitter current of the NPN transistor equals the collector current of the PNP transistor, the collector current being a fraction of the anode current by-passed to the PNP transistor.