The use of Operational amplifiers can improve the performance of a wide variety of signal processing circuits. In rectifier circuits, the voltage drop that occurs with an ordinary semiconductor rectifier can be eliminated to give precision rectification.
The below shown circuit is the precision full wave rectifier. It consists of following sections:
The input voltage Vin is applied to one terminal of the summing amplifier along with resistor R3 and to the input of the precision rectifier. The output of precision rectifier is applied to another terminal of summing amplifier. The precision half-wave rectifier circuit uses an inverting amplifier configuration.
When the input signal Vin is positive, Op-Amp output terminal is negative, Diode D1 is reverse biased and D2 Diode is forward biased, the circuit is
Vb = -(R2/R1) * Vin
In the circuit, R1 and R2 have been chosen such that R2 = 2R1.
So the voltage at Vb = -2Vin.
Thus during the positive half cycle of the rectified voltage Vb is applied to terminal B of the inverting summing amplifier is -2Vin.
The voltage at terminal A is Va = +Vin. The output from the summing circuit with R3=R4=R5 is Vo = – (Va+Vb). Hence Vo = -(Vin – 2Vin) = +Vin. So during the negative half cycle of the input, the Op-Amp output terminal goes positive, causing D2 to be reverse biased. Without D1 in the circuit, the Op-Amp output would be saturated in the positive direction. However, the positive voltage at the Op-Amp output forward biases the D1. This tends to pull the Op-Amp inverting terminal in a positive direction. But, such a move would cause the Op-Amp output to go negative. So, the output settles at the voltage close to ground level. So to be clear, the negative half-cycle is clipped off. That is Vb=0 and Va = -Vin. Totally the Vo will be Vo = -(-Vin+0) = +Vin.
It is seen that the output is a full wave rectified version of the input voltage. A Precision full-wave rectifier is also known as absolute value circuit. This means the circuit output is the absolute value of the input voltage regardless of polarity.
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