Answer:  Sinusoidal Oscillators:-Sinusoidal oscillators produce a sine wave output where as non-sinusoidal oscillators produce square or pulsed output. A multivibrator circuit is a type of non-sinusoidal oscillator. Sinusoidal Oscillators are essentially non-linear which complicates the technical design and analysis of such circuits. The quality of the sine wave is expressed through the total harmonic distortion coefficient or THD, defined as:

$\implies \mathrm{THD=\sqrt{D_2^2+D_3^2+D_4^2+.....}}$

Where $\mathrm{D_k}$ represents the relationship between the amplitude of the harmonic $\mathrm{k}$and the fundamental harmonic described in Fourier series.

Barkhausen Criterion:- Consider a basic inverting amplifier with an open loop gain A. The feedback network attenuation factor $\beta$ is less than unity. As basic amplifier is inverting, it produces a phase shift of $ 180^0$ between input and output as shown in the below figure.