are shown in fig2 which constitutes a schematic diagram of an illustrative embodiment of the present invention with a spike filter incorporated in the ic 10 in fig2 the fet 49 is shown along with the current feedback resistor 51 the voltage feedback from the current feedback resistor 51 is connected to a pin 53p of the ic 10 and the pin 53p is connected through a resistor 54 to the base of a pnp transistor 55 the collector of the transistor 55 is grounded and its emitter is connected to the output of a current source 56 to ground through a capacitor 57 and to one input of the comparator 14 the other input of the comparator is connected to the voltage reference input unit 13 fig1 to the emitter of a pnp transistor 59 which functions with a current source 58 and which has its collector and base connected to ground to establish a fixed threshold voltage level in operation spikes which may appear in the voltage feedback applied to the pin 53p and interfere with proper operation of the circuit a waveform of a typical input signal applied to pin 53p is illustrated in fig3 a showing the occurrence.