Question

Draw the circuit of phase shift oscillator using n-p-n transistor. Explain how
positive feedback is achieved in this circuit.

Answer 1

Here is the phase-shift oscillator circuit with NPN transistor:

The positive feedback is provided by the RC phase-shift network. In the example above each $C_nR_n$network shifts the phase angle between each successive RC section by an angle of approx. 60°, thus making total phase shift 3x60°=180°.

60° is a result of choosing appropriate capacitor and resistor to achieve

$$\phi=\text{tan}^{-1}\frac{1}{2\pi fC_nR_n}=60^\circ.$$