Peak detector circuit can be configured using a diode and a parallel combination of resistance and capacitance. If input voltage is highly fluctuating then we get a DC output from peak detector. Let input signal (peak value Vp) applied to peak detector circuit. For ideal diode, in forward bias diode conducts and capacitor voltage Vc is increased with input voltage and reach at Vp. After that diode in reverse bias and capacitor starts to discharge through resistor R1. This discharge time can be expressed as R1ŚC1. If we want to hold the peak value of signal for circuit activation, we have to increase the RC value. For practical diode we have to consider the voltage drop across the diode.