The ideal rectifier provide an output proportional to the average value of a repetitive waveform.There are several measurement applications where the peak value of a single must be detected and then held as a dc level for later analysis.The peak of a pulse wave train is often of more interest than its average value.In performing destructive testing,tracking and holding of peak signal is necessary.Spectral and mass spectrometer analysis also needs the use of peak detectors.
The peak detector circuit computers the peak value of the input signal.The circuit has a property that it can follow the input signal.The circuit has a property that it can follow the maximum value(peak) of a signal and the highest value is stored on a capacitor.If a higher peak value comes its new value gets stored in capacitor.
The highest peak value remains stored until the capacitor is discharged.
If a capacitor and a MOSFET reset switch are added at the output of the diode rectifier circuit as shown in figure,a peak detector results.
Assume initially that the capacitor is charged up to Vc(say).Now,when Vin exceeds vc,the diode is forward biased and the circuit behaves as a voltage follower circuit(unity gain amplifier).Because of this,the output voltage vout follows vin as long as Vin remains more than Vc. When vn falls below Vc,the diode gets reverse baised and the capacitor holds the ccharge till input voltage vin again attains a value greater than Vc,the voltage waveshape for the positive detector.
The circuit can be modified to hold the lowest or most negative voltage of a signal by reversing the diode.
If the output is loaded a buffer voltage follower should be used at the output of capacitor to prevent the load from discharging capacitor C.
The circuit finds extensive application in measurement instrumentation especially in communication circuits like AM (amplitude modulation).
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