Consider a basic inverting amplifier with an open loop gain ‘A’.
As basic amplifier is inverting it produces a phase shift of 1800 between input and output
The amplifier to work as an oscillator two conditions are required and called as barkhausen criteria for the oscillator.
The barkhausens criterion states that:
The total phase shift around a closed loop as the single proceed from input through amplifier output, feedback network back to input completing a closed loop is 00 or 3600.
The magnitude of the product of the open loop gain of the amplifier (A) and feedback factor βis unity i.e.
Two conditions are diagrammatically illustrated