Fig: RC differentiating circuit
Figure shows a RC differential circuit. It is also known as high pass filter. The reactance of a capacitor decreases with increasing frequency. The higher frequency components in the input signal appear at the output i.e. the capacitor acts as a short circuit for very high frequencies and virtually all the input appears at the output. Therefore, it is also called as high pass RC circuit. In above circuit the voltage drop across R will be very small in comparison with the drop across C. Hence, the current is completely determined by the capacitance C. The value of current I will be,
I = C x dvi / dt
The output voltage Vo is as given,
. Vo = iR
Vo = RC x dvi / dt (where R and C are constants)
Vo α dvi / dt
i.e. the output signal is directly proportional to the derivative of the input signal.