An RC phase shift oscillator is a type of electronic oscillator circuit that produces a sinusoidal output waveform. It uses a common-emitter (CE) BJT (Bipolar Junction Transistor) amplifier along with an RC (Resistor-Capacitor) network that acts as a feedback circuit to provide the necessary phase shift for oscillations.

RC Phase Shift Oscillator Fundamentals

For any oscillator to sustain oscillations, it must satisfy the Barkhausen criteria. These two conditions are crucial for stable, continuous oscillations:

• Magnitude Condition: The loop gain, which is the product of the amplifier gain (A) and the feedback network attenuation ($\beta$), must be equal to or greater than unity, i.e., $|A\beta| \ge 1$.
• Phase Condition: The total phase shift around the closed loop must be 0 degrees or 360 degrees (or multiples of 360 degrees). This means the sum of the phase shift introduced by the amplifier and the phase shift introduced by the feedback network must be 0° or 360°.

Role of BJT Amplifier in RC Phase Shift Oscillator

In an RC phase shift oscillator, a common-emitter (CE) BJT configuration is typically used as the amplifier. A CE amplifier inherently provides a 180-degree phase shift between its input and output signals. This means if the input signal has a certain phase, the output signal will be inverted, or shifted by 180 degrees, relative to the input.

Feedback Network Phase Shift

Given that the BJT amplifier (CE configuration) provides a 180-degree phase shift, for the total phase shift around the loop to be 360 degrees (or 0 degrees for oscillation), the feedback network must provide the remaining phase shift. The calculation is as follows:

Total Phase Shift = Phase Shift by Amplifier + Phase Shift by Feedback Network

To satisfy the Barkhausen criteria, Total Phase Shift = $360^\circ$ (or $0^\circ$).

So, $360^\circ = 180^\circ$ (from BJT amplifier) + Phase Shift by Feedback Network

Therefore, Phase Shift by Feedback Network = $360^\circ - 180^\circ = 180^\circ$.

The RC phase shift oscillator typically uses three identical RC stages in its feedback network. Each RC stage can provide a maximum phase shift of 90 degrees, but for stable oscillations, each stage is designed to provide 60 degrees phase shift, summing up to a total of 180 degrees (3 stages $\times$ 60 degrees/stage = 180 degrees) from the feedback network at the oscillation frequency.

Therefore, in an RC phase shift oscillator using BJT, oscillations happen at the frequency where the feedback network shifts the phase of the amplifier output by 180 degrees.