An Automatic Gain Control loop adjusts an amplifier's gain in real time so the output amplitude stays roughly constant even when the input swings across 40+ dB. It's the reason a distant AM station and a local blowtorch both come out of your radio at similar volume, and why your webcam's microphone doesn't clip when someone shouts.

An AGC has four pieces stitched into a feedback loop:

• Variable-gain element — a VCA, JFET operating in its triode region, MOSFET, PIN diode attenuator, or a multiplying DAC. Gain is controlled by a voltage or current.
• Envelope detector — a precision rectifier feeding an RC network that extracts the output signal's amplitude.
• Error amplifier — compares detected level against a reference (the desired output amplitude).
• Loop filter — sets attack and release time constants. This determines how fast gain responds to level changes.

The critical design tension is the attack/release ratio. Fast attack catches sudden loud transients before they clip; slow release avoids "pumping" — the audible gain riding up between words or musical notes. A typical audio AGC uses attack ≈ 1–10 ms and release ≈ 100 ms – 2 s. Asymmetric time constants require separate charge and discharge paths on the detector cap (small series resistor for charging through a diode, large resistor to ground for discharge).

Real-world example: A shortwave receiver AGC. The IF stage uses a dual-gate MOSFET (like the BF998). Gate 2 receives a DC voltage from the AGC line — 0 V gives maximum gain (~30 dB), 4 V collapses gain to near zero. A diode envelope detector at the demodulator output charges a 4.7 µF cap through 1 kΩ (attack ≈ 5 ms) and discharges through 220 kΩ (release ≈ 1 s). An op-amp compares the DC envelope to a 2 V reference and drives the gate. Result: a 60 dB range of signal strengths produces less than 6 dB of audio output variation.

Rule of thumb for loop bandwidth: the loop's crossover frequency must sit well below the lowest signal frequency of interest, or the AGC will try to flatten the signal itself. For 300 Hz–3 kHz voice audio, keep loop bandwidth under ~30 Hz — about 10× below the lowest audio component.

Watch out for hang AGC designs in SSB radios: after a strong signal ends, a comparator holds the gain suppressed for 200–500 ms before releasing, preventing static crashes between words. And in RF, always place AGC control early in the chain (front-end and IF) to prevent later stages from saturating on strong signals before the loop can respond.