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What is a Precision Rectifier?

by Sonali

The op-amp circuits used as rectifiers are called precision rectifiers. Usually the AC voltages below the forward voltage drop of 0.7V of a diode cannot be rectified. To overcome such problems we use precision rectifiers. The precision rectifier has the capability of rectifying voltages which are of smaller magnitude than the forward voltage drop. The application of these diodes is in AM detectors where only information is required in the signal with negligible power. 

The above figure shows a non-inverting precision rectifier. The diode present here is a precision diode. When the voltage V­I is positive the voltage VOA is also positive. When Vi<0the voltage VOA becomes negative and hence reverse biasing the diode and making Vo =0.

When the value of input voltage Vi<  (cut-in voltage). The diode again becomes reverse biased as VOA becomes negative. The op-amp then comes to negative saturation.  There is no current through RL and V0= 0.

When the value of input voltage Vi>  (cut-in voltage) the circuit acts as voltage follower and the output voltage follows the input voltage during this positive half cycle of input.

This rectifier works on the same principle i.e conversion of AC to DC but with an op-amp. This op-amp provides an additional advantage of rectifying the smaller values of the input voltage which are below 0.7V.  

As we take the feedback from the output of the diode and the op-amp compensates for any voltage drop across the diode the input and output waveforms of the precision rectifier circuits are equal to the input. The diode behaves as an ideal diode.

Full wave precision rectifier circuit

We add a summing amplifier at the output of a full wave rectifier circuit . Since the points P1 to P2 act as basic precision rectifier circuit and the output is negative voltage.

The output from the precision rectifier is fed to the summing amplifier through R3. There is a summer amplifier between the points P2 and P3.

Through resistor R4 the input from the point P1 is fed to the summing amplifier. The gain of the op-amp is 1X dependent upon the resistors R4 and R5 respectively. 

We get the final output from P3 and the output at P2 is provided as input to the summing amplifier which has 2X gain. Hence, the output obtained is twice the value of input.

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