text: As the amplitude of the output nears the power supply voltage rails, the amplifier begins to saturate on the peaks (top and bottom) of the sine wave, flattening or "clipping" the peaks.  Since the output of the amplifier can no longer increase with increasing input, further increases in amplitude cause the equivalent gain of the amplifier and thus the loop gain to decrease.  The amplitude of the sine wave, and the resulting clipping, continues to grow until the loop gain is reduced to unity, , satisfying the Barkhausen criterion, at which point the amplitude levels off and steady state operation is achieved, with the output a slightly distorted sine wave with peak amplitude determined by the supply voltage.  This is a stable equilibrium; if the amplitude of the sine wave increases for some reason, increased clipping of the output causes the loop gain  to drop below one temporarily, reducing the sine wave's amplitude back to its unity-gain value.  Similarly if the amplitude of the wave decreases, the decreased clipping will cause the loop gain to increase above one, increasing the amplitude.
  The amount of harmonic distortion in the output is dependent on how much excess loop gain the circuit has:
  If the small signal loop gain is made close to one, just slightly greater, the output waveform will have minimum distortion, and the frequency will be most stable and independent of supply voltage and load impedance.  However, the oscillator may be slow starting up, and a small decrease in gain due to a variation in component values may prevent it from oscillating.