Subject: Liner Integrated Circuits

Topic: Non-Linear Applications of Operational Amplifier

Difficulty: Low

1.Rectifier circuit gives average value of input signal; but in practice we need peak value of input signal. This is achieved by peak detector circuit. The following figure shows a simple peak detector circuit using diode and capacitor.

2.In the positive half cycle, diode D is forward biased and capacitor C starts charging. When input reaches its peak value capacitor gets charged to positive peak value.

3.In negative half cycle, as input decreases, diode D is reversed biased and capacitor is isolated and holds the peak value of previous cycle. Hence called as peak detector.

4.But in practice, output is taken across some load RL, so when input voltage decreases capacitor discharges through load RL. To avoid this select RL of very large value so that capacitor discharges very slowly hence almost holds the charge. Whatever charge it lost through RL is gets back in next half cycle.

Limitation:

The diode D is acting as an instant switch, so supply gets loaded. To avoid the loading while charging capacitor, we use op-amp as follows. Op-amp is placed between input and diode D so loading is avoided