An oscillator is a circuit, which generates ac output signal without giving any input ac signal. This circuit is usually applied for audio Frequencies only. The basic requirement for an oscillator is positive feedback. The operation of the RC Phase Shift Oscillator can be explained as follows. The RC phase shift starting voltage is provided by noise, which is produced due to random motion of electrons in resistors used in the circuit. The noise voltage in the RC phase shift contains almost all the sinusoidal frequencies. This low amplitude noise voltage gets amplified and appears at the output terminals. The amplified noise drives the feedback network which is the phase shift network. Because of this the feedback voltage is at maximum in a particular frequency, which in turn represents the frequency of oscillation. Furthermore, the RC phase shift required for positive feedback is correct at this frequency only. The voltage gain of the amplifier with positive feedback is given by:

From the above equation we can see that if:
 the gain becomes infinity means that there is output without any input. 

• I.e. the amplifier becomes an oscillator. This condition:

         is known as the Barkhausen criterion of oscillation. Thus the output contains only a single sinusoidal frequency. In the beginning, as the oscillator is switched on, the loop gain is greater than unity. The oscillations build up. Once a suitable level is reached the gain of the amplifier decreases, and the value of the loop gain decreases to unity. So the constant level oscillations are maintained. Satisfying the above conditions of oscillation the value of R and C for the phase shift network is selected such that each RC combination produces a phase shift of 60°. Thus the total phase shift produced by the three RC networks is 180°. Therefore at the specific frequency fo the total phase shift from the base of the transistor around the circuit and back to the base is 360° thereby satisfying Barkhausen criterion. We select R1=R2=R3 =R and C1=C2=C3=C 

The frequency of oscillation of RC Phase Shift Oscillator is given by

In order that 

it is required that the amplifier gain
for oscillator operation.

That's it. Next tutorial is about Tuned-LC Oscillator.