Pierce Crystal Oscillator

The Barkhausen criteria states that for a positive feedback system, oscillation will occur when the loop gain (product of forward gain and feedback gain) has a phase shift of zero (180 degrees for negative feedback systems) and a magnitude of unity. In practical oscillator circuits, the analysis should indicate a loop gain magnitude greater than one, which will cause the amplitude of oscillation to grow exponentially until clipping or other nonlinear phenomena cause the gain magnitude to decrease to unity.