Peak detector
The precision rectifier, also known as a super diode, is a configuration obtained with an operational amplifier in order to have a circuit behave like an ideal diode and rectifier. It is useful for high-precision signal processing.
The op-amp based precision rectifier should not be confused with the power MOSFET-based active rectification ideal diode.
The basic circuit implementing such a feature is shown on the right, where can be any load. When the input voltage is negative, there is a negative voltage on the diode, so it works like an open circuit, no current flows through the load, and the output voltage is zero.
When the input is positive, it is amplified by the operational amplifier which switches the diode on. Current flows through the load and, because of the feedback, the output voltage is equal to the input voltage.
The actual threshold of the super diode is very close to zero, but is not zero. It equals the actual threshold of the diode, divided by the gain of the operational amplifier.
This basic configuration has a problem so it is not commonly used. When the input becomes (even slightly) negative, the operational amplifier runs open loop, as there is no feedback signal through the diode. With a typical high open loop gain operational amplifier, 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.
An alternative version is given on the right.
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