An oscillator consists basically of an amplifier and a phase shifting network. The amp receives the o/p of phase shifting network. The amp then amplifies it, phase shifts it through 180 degrees and applies it to the i/p of phase shifting network.

The phase shifting network shifts the amp o/p through another 180 degrees and alternates it before applying it back to amp i/p.

Due to the total phase shift of 360 degree, the feedback becomes a + ve feedback, which gives rise to oscillations, if the Barkhausen criterion is satisfied.

There is no external ac i/p voltage applied to an oscillator still it produces a sinusoidal o/p voltage An oscillator is an amp with +ve feedback.