The figure shows a Wien Bridge circuit with non-linear feedback. The resistance of the RL lamp is selected to have a nominal value equivalent to half the feedback resistance Rf. Thus, the lamp acts as an AGC, given its non-linear operating characteristic, maintaining the output voltage with small variations. Some circuits use a diode instead of the lamp, as a limiter. The diodes reduce distortion by providing a smooth limitation to the output voltage. An AGC should be used when none of these techniques allows for automatic gain control.
