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Precision rectifier:Part-1

The  precision rectifier, which is also known as a super diode, is a configuration obtained with an operational amplifier in order to have a circuit behaving like an ideal diode or rectifier. It can be useful for high-precision signal processing

 Basic Circuit

 untitled-1

 

 This circuit has the benefit that the op-amp never goes in saturation, so the only thing affecting its frequency response is the amplification and the gain-bandwidth product.

Similar circuitry can be used to create a precision full-wave rectifier circuit.

 The basic circuit implementing such a feature is shown on the right, where RL can be any load. When the input voltage is negative, there is a negative voltage on the diode, too, so it works like an open circuit, there is no current in the load and the output voltage is zero.

           

When the input is positive, it is amplified by the operational amplifier and it turns the diode on. There is current in the load and, because of the feedback, the output voltage is equal to the input.

 

In fact the threshold of the super diode is not actually zero, as it should be for an ideal one, but it equals the threshold of the normal diode divided by the gain of the operational amplifier, that is almost zero.

 

 This basic configuration has a problem so it is not commonly used: when the input becomes (even slightly) negative, the output of the operational amplifier can easily become greater than the voltage supplied to the op-amp, thus causing saturation. Then, if the input becomes positive again, the op-amp has to get out of the saturation to amplify again. This change takes some time, and this greatly reduces the frequency response of the circuit. 

    Improved Circuit

 untitled-2

In this case, when the input is greater than zero, D1 is OFF and D2 is ON, so the output is zero, because one side R2 is connected to the virtual ground, and there is no current through it. When the input is less than zero, D1 is ON and D2 is OFF, and the output is like the input with an amplification of ? R2 / R1. Its transfer characteristic is the following:

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This circuit has the benefit that the op-amp never goes in saturation, so the only thing affecting its frequency response is the amplification and the gain-bandwidth product.

Similar circuitry can be used to create a precision full-wave rectifier circuit.

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