An oscillator is a circuit that creates a waveform output from a direct current input. The two main types of oscillator are harmonic and relaxation. The harmonic oscillators have smooth curved waveforms, while relaxation oscillators have waveforms with sharp changes.
An oscillator is a circuit which works on the principle of positive feedback and is used to generate signals. oscillator does not have input but it gives output. In oscillator phase shift is 0 or 360. [AB]=1.