R-C Phase shift Oscillator

• RC phase shift oscillator basically consists of an amplifier and a feedback network consisting of resistors and capacitors arranged in ladder fashion. Hence such an oscillator is also called ladder type RC phase shift oscillator

The basic RC circuit which is used in the feedback network of oscillator


• The total impedence of the circuit is,
Z = R - j Xc = R - j[1/2πfc] Ω = |Z| ∠-φ Ω
• In such a RC circuit, current I leads input voltage Vi by angle φ where the angle φ is decided by the components R and C which is given by

• The drop Vr is in phase with current I while the drop Vc lags current I by 90* i.e. I leads Vc by 90*.

Phasor Diagram


• Such 3 R-C sections are used, each contributing 60* phase shift hence the total phase shift contributed by the feedback network(3*60*)=180*
• All the resistance values and all the capacitors values are same, so that for a particular frequency, each section of R and C produces a phase shift of 60*

feedback network Diagram in ladder fashion