and drain are connected via capacitors 48 and 49 to the outputs 42 and 43 respectively of noise reduction circuit 40 the voltage of the voltage reference source 44 determines the threshold voltage which is indicated by d in graph d if the pulse shaped signals d 39 exceed this threshold voltage d a pulse e 39 appears at the output of the voltage comparison circuit 14 as shown in graph e the pulse e 39 activates the monostable multivibrator 17 which results in a pulse f 39 shown in graph f which has a pulse duration determined by the time constant of the adjustable rc elements 18 19 the pulse duration corresponds to the previously mentioned first period of time the voltage across the capacitor 20 arranged between a first voltage source and the output of the monostable multivibrator 17 follows owing to a rapid charge time constant rather accurately the variation of the pulse f 39 however only to its falling edge the voltage of capacitor 20 can only be reduced to its original value i e the value prior to the ocurrings of the pulse f 39 by a charge transfer via the controllable current source 21 to 25 inclusive to this end the controllable current source 21 to 25 inclusive comprises a transistor 21 the collector of which is coupled to the connection between the capacitor 20 and the output of the monostable multivibrator 17 and whose emitter is connected via an emitter resistor 22 to the first voltage source the base of the transistor 21 is connected on the one hand via a supply resistor 25 to a second voltage source and on the other hand via a series arrangement.