text: Amplitude-stabilized oscillators In applications where a 'pure' very low distortion sine wave is needed, such as precision signal generators,  a nonlinear component is often used in the feedback loop that provides a 'slow' gain reduction with amplitude.  This stabilizes the loop gain at an amplitude below the saturation level of the amplifier, so it does not saturate and "clip" the sine wave.  Resistor-diode networks and FETs are often used for the nonlinear element.  An older design uses a thermistor or an ordinary incandescent light bulb; both provide a resistance that increases with temperature as the current through them increases.
  As the amplitude of the signal current through them increases during oscillator startup, the increasing resistance of these devices reduces the loop gain.  The essential characteristic of all these circuits is that the nonlinear gain-control circuit must have a long time constant, much longer than a single period of the oscillation.  Therefore over a single cycle they act as virtually linear elements, and so introduce very little distortion.  The operation of these circuits is somewhat analogous to an automatic gain control (AGC) circuit in a radio receiver.  The Wein bridge oscillator is a widely used circuit in which this type of gain stabilization is used.