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Precision Rectifier

The major limitation of conventional rectifiers is that it cannot rectify AC voltages below forward voltage drop VD (0.7V) of a diode. The precision rectifier will make it possible to rectify input voltage of a very small magnitude even less than forward voltage drop of diode. The diode can be used in AM detector where power is negligible and we want information in the signal. Rectifier circuits used for circuit detection with op-amps are called precision rectifiers.
Necessity of Op-Amp:
When forward biased voltage is less than 0.7V, then diode is not conducting. In case of normal power rectifier input applied is much larger than 0.7V. So diode is not operated. Therefore Op-amp is used to help diode to conduct.
The precision rectifiers are classified in two categories.
1. Precision HWR
2. Precision FWR

Precision Half wave rectifier(HWR) :
In HWR, the diode conducts in one of the half cycles of applied ac input signal. Because of this again we can classify HWR as positive PHWR (output is positive) and negative PHWR (output is negative).

In positive half cycle of applied ac input signal output of op-amp is negative, so diode D1 is forward biased and D2 is reversed biased. The output of op-amp is virtually shorted to ground and output voltage is zero.
In negative half cycle of applied ac input signal output of op-amp is positive, so diode D2 is forward biased and D1 is reversed biased. The circuit works as an inverting amplifier with gain of (-Rf/R1 ) Therefore Vo=Vin×A.  But in negative half cycle input magnitude is negative therefore we get, Vo=(-Vin )[-Rf/R1]
∴Vo= Rf/R1 (Vin )
Thus in negative half cycle output is positive with a gain of(Rf/R1 ).

Precision Full wave Rectifier:
In PFWR, for both the half cycles output is produced & in one direction only.  In positive half cycle of applied ac input signal, output of first op-amp (A1) is Negative. Therefore diode D2 is forward biased & diode D1 is reverse biased.