Abstract: A constant duration and level of spark is obtained over a wide range of engine speeds in an ignition circuit, including a gate-turn-off device having a main current path connected in series with the primary winding of an ignition coil. An operating voltage is selectively applied, in timed relation to the operation of the internal combustion engine, to an LC resonant circuit for shocking the resonant circuit into oscillation. These oscillations are applied to the gate electrode of the gate-turn-off device, turning the gate-turn-off device off and on each successive half cycle, for inducing a relatively high level spark voltage in the secondary winding of the ignition coil.