Conditions which are required to be satisfied to operate the circuit as an oscillator are called as “Barkhausen criterion” for sustained oscillations.

The Barkhausen criteria should be satisfied by an amplifier with positive feedback to ensure the sustained oscillations.

For an oscillation circuit, there is no input signal “Vs”, hence the feedback signal Vf itself should be sufficient to maintain the oscillations.

The Barkhausen criterion states that:

• The loop gain is equal to unity in absolute magnitude, that is, | β A | = 1 and

• The phase shift around the loop is zero or an integer multiple of 2π: ∠ β A = 2 π n, n ∈ 0, 1, 2,….

The product β A is called as the “loop gain”.