Peak Detector - Working and Applications

Peak Detector is a circuit which is used to detect the peaks of the applied input signal. It basically follows the input voltage and stores the peak voltage. It employs a diode and capacitor to implement this function.

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Types of Peak Detector

Active peak detector
Passive peak detector

Active peak detector

Active peak detectors use active components like op-amps, transistors, etc. It is more accurate as the losses incurred due to resistive elements are taken care of by the active components.

Passive peak detector

Passive peak detectors use passive components like diodes, capacitors, etc. It is less accurate because of the losses occurring in different elements mainly resistors.

Active Peak Detector Circuit

Figure 1 shows an active peak detector circuit. It consists of an op-amp, diode (D) and capacitor (C). Let us assume that all are ideal. Input (V_i) is applied to the non-inverting terminal of the op-amp. V_o is the output voltage.

Working

Figure 2 shows the working of the active peak detector circuit. The working principle of this circuit is that the peak of the input waveform is followed and stored in terms of voltage (V_c) across the capacitor.

Whenever the input voltage is greater than the capacitor voltage V_c, the diode D will be ON and there will be the charging of the capacitor C. The circuit will be working as a non-inverting amplifier and capacitor voltage V_c follows input voltage V_i. The circuit for this case is shown in figure 3.

Whenever the input voltage V_i is lesser than the capacitor voltage V_c, the diode D will be OFF and the op-amp is working in an open-loop configuration. In this case, capacitor voltage V_c holds its previous maximum charged voltage until the input voltage V_i becomes more than this maximum voltage. The circuit for this case is shown in figure 4.

Let us understand this with an example. Consider the figure 5. Figure 5(a) shows the input voltage waveform and Figure 5(b) shows the capacitor or output voltage waveform. From figure 2 we can see that capacitor voltage V_c is equal to the output voltage V_o.

Fig. 5 (a) input voltage waveform (b) output or capacitor voltage waveform

Assume that capacitor voltage V_c (V_c = V_o) is 0 initially. As the input voltage is applied to the non-inverting terminal, the diode D is turned ON and the capacitor voltage will follow the input waveform till time t₂. At this time, V_i = V₂. Therefore, V_o = V_c = V₂. After time t₂, the input voltage starts decreasing. As the cathode voltage is greater than the anode voltage, the diode is turned OFF. The capacitor will retain this maximum voltage till V_i is greater than the V_c (i.e. till time t₃). After time t₃, the V_c again follow the V_i till time t₄. At this time, V_i = V₃. Therefore, V_o = V_c = V₃. After time t₄, the input voltage starts decreasing. As the cathode voltage is greater than the anode voltage, the diode is turned OFF. The capacitor will retain this maximum voltage till V_i is greater than the V_c (i.e. till time t₅). After time t₅, the V_c again follow the V_i till time t₆. At this time, V_i = V₄. Similarly, the capacitor will charge up to the maximum voltage V₅ and retain it until the next peak or maximum voltage comes. This how the active peak detector works.