1.In a BJT based RC phase shift oscillator,BJT is used as an active element of the amplifier.

2.Here,the output of the feedback network is loaded appreciably by the relatively low resistance of the transistor.

3.Thus,the resistance R of the feedback network is in parallel with the low input resistance hie  of the transistor,which reduces the effective value of  R in the last section of the feedback network.

4.In this oscillator, the BJT amplifier provides a phase shift of  $180 ^{\circ} $ and the feedback RC network provides the remaining $180 ^{\circ} $ phase shift to obtain a total phase shift of $360^ {\circ}$

DERIVATION OF FREQUENCY OF OSCILLATOR:

The small signal ac equivalent model is shown below.We have simplified the circuit by replacing the current source by voltage source.