Barkhausen stability criterion

Barkhausen Stability Criterion was put forth by German physicist Heinrich Georg Barkhausen (1881-1956) for sustained oscillations and it states that:

1. The loop gain of the oscillator circuit must be greater than or equal to unity, that is, βA>=1, where β is the gain of the feedback network and A is the voltage gain of the oscillator circuit.
2. The phase shift around the circuit must be zero or an integral multiple of 2π.