# Precision Rectifiers  Written by

A precision rectifier is a circuit that behaves like a perfect diode. It conducts perfectly when forward-biased, without any forward voltage drop, and it blocks perfectly when reverse biased. They are frequently used in precision circuits for peak detection or clamping.

The simplest version of a precision rectifier is shown in Figure 1. It looks a bit like a non-inverting op amp buffer, and indeed that is how it operates if the input voltage vi is positive, since diode D1 will be forward biased. The feedback loop makes sure the output follows the input, negating the diode’s forward drop. If the input voltage vi is negative, D1 is reverse biased (you could imagine it is open circuit) and the output voltage vo, will be zero since it is effectively floating. Figure 1This is the simplest precision rectifier circuit. The output voltage follows the input for positive input voltages. For negative input voltages D1 is reverse biased, and the op amp is effectively open loop with the output saturated. The transition between closed loop and open loop states can cause unwanted artifacts on the output in some circumstances.

In this latter scenario the op amp is operating open loop and its output will be saturated to the negative rail. Here lies the problem with this circuit. Since it takes the op amp a finite time to come out of saturation when the input voltage returns to positive, unwanted artefacts can be produced on the output.

The circuit of Figure 2 is an improvement that addresses the saturation problem. This time we have a circuit that looks a bit like an inverting amplifier with a few added diodes. When the input is positive, D1 is reverse biased (so can be considered open circuit).  The output vo will be zero since it is connected to the virtual ground via R2. Figure 2This circuit overcomes the shortcomings of the rectifier in Figure 1. For positive input voltages D1 is reverse biased, and the output voltage is zero but D2 maintains the op amp in closed loop mode. For negative inputs D2 is reverse biased and the circuit acts as an inverting amplifier. The transfer function is shown on the right.

Unlike the circuit of Figure 1, D2 will be forward biased providing a negative feedback path around the op amp allowing it to maintain closed-loop operation.

When vi is negative, the D1 is forward biased and D2 is reverse biased and the circuit behaves as an inverting amplifier. Assuming R1 and R2 are equal, the output vo will be equal to –vi. The graph in Figure 1 shows the output characteristic for the circuit. Note that the op amp always operates in closed loop mode, so it’s output does not saturate and the output remains clean when the input voltage transitions from one sign to another.

These circuits are great to emulate a single precision diode or half-wave rectifier, but what if we need a full-wave rectifier? Figure 3 shows how we can adapt the circuit of Figure 2 to achieve this. The first op amp, U1, forms a precision rectifier as before, although the diodes are reversed. This means that the output at va will be zero for negative inputs and –vi for positive inputs. Figure 3This fill-wave rectifier circuit includes a half wave rectifier similar to that in Figure 2 (although with the diodes reversed) and a summing amplifier. Together these create a full wave rectifier with the characteristic shown in the chart at right.

The second op amp, U2 forms an inverting summing amplifier with an output voltage given by the equations below.

The output, shown in graphical form on the right of Figure 3 will therefore always be positive with a magnitude equal to the input voltage – exactly what we’d expect from a precision full-wave rectifier.

There are of course plenty of variations on these circuits. The most important thing to be aware of is that some circuits allow the op amp to operate open loop in some circumstances. There is nothing inherently wrong with this, but op amp behaviour can be unpredictable as it transitions in an out of open loop, so you need to be sure this won’t introduce any unwanted behaviour in your application.

References

Horowitz, Paul, and Winfield Hill. The Art of Electronics. Third edition, 11th printing, with Corrections. Cambridge New York, NY: Cambridge University Press, 2017.

Engineering LibreTexts. “7.2: Precision Rectifiers,” May 2, 2018. https://eng.libretexts.org/Bookshelves/Electrical_Engineering/Electronics/Operational_Amplifiers_and_Linear_Integrated_Circuits_Theory_and_Application_(Fiore)/07%3A_Nonlinear_Circuits/7.02%3A_Precision_Rectifiers.

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