A regular diode rectifier is useless for small signals. A silicon diode needs about 0.6 V to conduct, so anything below that vanishes entirely, and signals just above it suffer huge nonlinearity. If you're trying to measure a 50 mV audio envelope, a 100 mV thermocouple AC component, or the output of a low-level sensor, a plain bridge rectifier will lie to you. The fix is a precision rectifier (sometimes called a "superdiode") — an op-amp wrapped around the diode so that feedback forces the output to behave as if the diode had zero forward drop.

The half-wave superdiode: Connect a diode between the op-amp output and the inverting input, with the input signal driving the non-inverting pin. When V_in goes positive, the op-amp drives its output high enough to overcome V_F, and the inverting input tracks V_in exactly. When V_in goes negative, the diode blocks and the op-amp saturates at its negative rail. That saturation is the catch — recovery from saturation takes microseconds, so this topology struggles above a few kHz.

The improved half-wave rectifier solves this by using two diodes and an inverting configuration. D1 conducts on negative input swings, providing the rectified output through R_f; D2 conducts on positive swings, clamping the op-amp output one diode drop below ground so it never saturates. With R_in = R_f, gain is −1 for negative inputs, 0 for positive. Cascade it with a summing inverter to get full-wave (absolute value) behavior.

Practical example: True-RMS-to-DC converter front end. You want to measure the AC ripple on a 3.3 V rail — maybe 20 mV_pp. Feed it through a coupling cap into a full-wave precision rectifier built from an OPA2188 (zero-drift, low offset), then RC-filter the output. With a 100 nF coupling cap and 100 kΩ input resistor, your high-pass corner is 16 Hz — fine for switching ripple measurement. The op-amp's 25 μV offset is your error floor, not the diode's 600 mV.

Rule of thumb: Pick an op-amp with slew rate ≥ 4× ω × V_peak. For a 1 V peak signal at 10 kHz, that's at least 0.25 V/μs — easy. But push to 100 kHz with the same amplitude and you need 2.5 V/μs, which rules out cheap parts like the LM358. Also use fast switching diodes (1N4148, BAT54) rather than 1N4001 power rectifiers — the reverse recovery time dominates high-frequency error.