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Problem 7—What are precision rectifiers?

Answer

Precision rectifiers (also called small signal rectifiers) are used to rectify small signals on the order of a few millivolts. A normal silicon diode has a forward voltage drop of about 0.7V and cannot be used directly to rectify small signals. Precision rectifiers make use of operational amplifiers to overcome the limitations of diodes. A typical precision rectifier using an opamp is shown below:

When Vin goes positive, the opamp output is driven negative, which switches D1 off and D2 on. This turns the circuit into a voltage follower that clamps the inverting output to ground (the same potential as the noninverting input). This means that Vout is essentially connected to ground through the feedback resistor, holding it at 0V.

The circuit acts as a half-wave rectifier, and it can rectify signals down to approximately the input offset voltage of the opamp (millivolts or even microvolts).

Such a circuit can work well throughout the audio range, but there is some distortion at zero crossing created by the slew-rate limitations of the opamp (the output must jump by two diode drops), making the circuit fairly useless for RF.

Contributor: Naveen P N

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