Adjustable time constant operating circuit

Claim 1

In a differentiating circuit including the combination of an operational amplifier exhibiting relatively high input impedance and gain characteristics, an input capacitance means providing an input channel to an input terminal of said operational amplifier and feedback resistance means providing an electrical channel between an output terminal of said operational amplifier and said input terminal thereof, apparatus for adjusting the time constant of said differentiating circuit, comprising: switch means coupled to said feedback resistance means and adapted to selectively interrupt said electrical channel upon the activation of said switch means, said feedback resistance means including first and second resistance means connected in series relationship and said switch means including a switching device connected to the junction formed by said first and second series connected resistance means, said switching device being operative to interrupt said electrical channel for a portion P of each periodic interval of time T such that said effective time constant of said differentiating circuit is adjusted by the factor T/T- P; and energizing means coupled to said switch means for providing an activating signal to activate said switch means at periodic intervals of time, whereby the effective time constant of said differentiating circuit is dependent upon said periodic intervals, said energizing means generating a periodic signal having a duration P and a period equal to T.