Precision rectifier

The precision rectifier, sometimes called a super diode, is an operational amplifier (opamp) circuit configuration that behaves like an ideal diode and rectifier. The op-amp-based precision rectifier should not be confused with the power MOSFET-based active rectification ideal diode.

The precision rectifier, sometimes called a super diode, is an operational amplifier (opamp) circuit configuration that behaves like an ideal diode and rectifier. The op-amp-based precision rectifier should not be confused with the power MOSFET-based active rectification ideal diode. ==Basic circuit== The basic circuit implementing such a feature is shown on the right, where R_\text{L} can be any load. When the input voltage is negative, the opamp puts its most negative voltage on the diode's anode, so the diode is reverse biased and works like an open circuit. Since almost no current will flow through the diode, the output voltage V_\text{out} will be pulled down to ground through R_\text{L} . When the input becomes positive, it is amplified by the opamp, which switches the diode on. Because of the negative feedback, just enough current will flow through R_\text{L} so that V_\text{out} equals the input voltage. The actual threshold is very close to zero, but is not zero. It equals the actual threshold of the diode, divided by the gain of the opamp. This basic configuration has a problem, so it is not commonly used. When the input becomes (even slightly) negative, the opamp runs open-loop, as there is no feedback signal through the diode. For a typical opamp with high open-loop gain, the output saturates. If the input then becomes positive again, the op-amp has to get out of the saturated state before positive amplification can take place again. This change generates some ringing and takes some time, greatly reducing the frequency response of the circuit. ==Improved circuit== An alternative version is given on the right. In this case, when the input is greater than zero, D1 is off, and D2 is on, so the output is zero because the other end of R_2 is connected to the virtual ground and there is no current through R_2 . When the input is less than zero, D1 is on and D2 is off, so the output is like the input with an amplification of -R_2 / R_1 . Its input–output relationship is the following: This circuit has the benefit that the op-amp never goes into saturation, but its output must change by two diode voltage drops (about 1.2 V) each time the input signal crosses zero. Hence, the slew rate of the opamp and its frequency response (gain–bandwidth product) will limit high-frequency performance, especially for low signal levels, although an error of less than 1% at 100 kHz is possible. Similar circuitry can be used to create a precision full-wave rectifier circuit.

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