Circuits involving capacitive decay:

Consider the circuit shown below. Depending on the time constant of the circuit, relative to the period of the square wave applied to it, the response of the circuit can vary widely. Assuming T is half the period of the square wave.

If CR is slightly less than T, the waveform in the top diagram is produced at the output (across C).

If Cr<

If CR>>T, the circuit is an integrating circuit, since the output waveform is that of the integral of the square wave, that is the area underneath it. This is shown in the lower diagram.

If the positions of the resistor and capacitor are reversed and the voltage across the resistor measured, then the waveform produced will be that of the current, since V=IR.

If CR is short enough then a stream of pulses is produced when a square wave is applied to the input. Shown in the top diagram.

If CR<

When CR>>T the circuit is called a coupling circuit. A coupling circuit allows the input waveform to pass to the output whilst blocking the passage of any d.c.