the precision rectifier
A plain diode cannot rectify a signal smaller than its forward drop of roughly 0.6 V, so millivolt-level signals just disappear. The precision rectifier solves this by putting a diode inside an op-amp's feedback loop. The op-amp simply pushes its output 0.6 V harder to overcome the diode, so the rectified output is accurate even for tiny signals. This op-amp-plus-diode combination is nicknamed the superdiode.
Here is the mechanism. Negative feedback forces the minus input to follow the input voltage. To do that with a diode in the way, the op-amp drives its own output one diode-drop higher, so the load sees exactly the input voltage when the input is positive, while the diode blocks and the output goes to 0 when the input is negative. The leftover 0.6 V error is divided down by the op-amp's huge loop gain, effectively to zero. A 50 mV AC signal, which a bare diode would entirely ignore, is rectified faithfully.
Precision rectifiers are the front end of AC voltmeters, peak detectors, and RMS converters that must handle small signals. The honest caveat: when the diode turns off, the feedback loop opens and the op-amp can saturate; recovering from that takes time, limited by the op-amp's slew rate. So at higher frequencies a two-diode full-wave precision rectifier, which keeps the loop closed, is preferred, and the op-amp's bandwidth and slew rate set the accuracy.
Send a 20 mV audio sine wave into a precision rectifier. A silicon diode alone would output nothing because 20 mV never reaches its 0.6 V threshold, but the superdiode delivers a clean rectified 20 mV.
Feedback hides the diode drop, letting millivolt signals be rectified accurately.
It removes the diode drop only while the loop is closed. Near the zero crossing the loop briefly opens and the op-amp's recovery (slew rate) limits accuracy at high frequency.