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Chapter: Linear Integrated Circuits : Applications of Operational Amplifier

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Peak Detector using Operational Amplifier

Square, Triangular, Saw tooth and pulse waves are typical examples of non-sinusoidal waveforms.

Peak Detector

 

Square, Triangular, Saw tooth and pulse waves are typical examples of non-sinusoidal waveforms. A conventional AC voltmeter cannot be used to measure these sinusoidal waveforms because it is designed to measure the RMS value of the pure sine wave. One possible solution to this problem is to measure the peak values of the non-sinusoidal waveforms. Peak detector measures the +ve peak value of the square wave input.



i) During the positive half cycle of Vin:

the o/p of the op-amp drives D1 on. (Forward biased)

Charging capacitor C to the positive peak value Vp of the input volt Vin.

 

ii) During the negative half cycle of Vin:

D1 is reverse biased and voltage across C is retained.

The only discharge path for C is through RL since the input bias IB is negligible.

For proper operation of the circuit, the charging time constant (CRd ) and discharging time constant (CRL) must satisfy the following condition.

CRd <= T/10        

Where Rd = Resistance of the forward-biased diode.

T = time period of the input waveform.

CRL >= 10T         (2)

Where RL = load resistor.

If RL is very small so that eqn. (2) cannot be satisfied.

·           Use a (buffer) voltage follower circuit between capacitor C and RL load resistor.

·           R is used to protect the op-amp against the excessive discharge currents.

·           Rcomp = minimizes the offset problems caused by input current

·           D2 conducts during the –ve half cycle of Vin and prevents the op-amp from going into negative saturation.

Note: -ve peak of the input signal can be detected simply by reversing diode D1 and D2

 

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