Page 62 - Semester-IV-Electronics

R-C Phase Shift Oscillator

 Works as a phase shift oscillator in which the transistor is in

common emitter mode and produces a change of 180° in the

input signal.

 For positive feedback, part of the output voltage at the

collector is fed back at the base with a phase change of 180°.

 This phase-change is obtained by three R-C circuits in cascade

in which each R-C circuits changes the phase by 60°.

 In the cascaded R-c circuit, the capacitors are connected in

series and these are shunted by resistances.

 The last resistance is R` = R – h because this resistance R` is
ie
connected to the base of the transistor so that by addition of

internal resistance h ie of the transistor, the total resistance

becomes R.

 Hence, all the thee R-C circuits are identical.
62