Abstract: A spark ignition system includes a variable reluctance pick-up providing the input signal to a saturating amplifier which includes a feedback path constituted by a capacitor and a resistor in series. This amplifier has a transfer function while it is being driven towards saturation, such that its output is the sum of an integral part and a proportional part and does not saturate as a result of oscillatory high frequency spurious signals since these have a low integral. A resistor in the input of the amplifier is shorted out by a diode connected transistor when the polarity of the signal from the pick-up reverses, to increase the gain of the amplifier while it is being driven towards de-saturation, such change of polarity signalling production of a spark.