RC phase shift oscillator is a sinusoidal oscillator used to produce sustained well shaped sine wave oscillations. It is used for different applications such as local oscillator for synchronous receivers, musical instruments, study purposes etc. The main part of an RC phase shift oscillator is an op amp inverting amplifier with its output fed back into its input using a regenerative feedback RC filter network, hence the name RC phase shift oscillator.

By varying the capacitor,the frequency of oscillations can be varied. The feedback RC network has a phase shift of 60 degrees each, hence total phase shift provided by the three RC network is 180 degrees. The op amp is connected as inverting amplifier hence the total phase shift around the loop will be 360 degrees. This condition is essential for sustained oscillations. We have already discussed about RC phase shift oscillator using transistor.

## Circuit Diagram

## Components Required

- Resistors (1.2KΩ, 1KΩx2)
- Potentiometer (50KΩ)
- Capacitors (0.01µFx3)
- 741 Op Amp

## Output Waveform beginning

## Sustained Output Waveform

## Working of RC Phase shift oscillator

- The feedback network offers 180 degrees phase shift at the oscillation frequency and the op amp is configured as an Inverting amplifier, it also provide 180 degrees phase shift. Hence to total phase shift around the loop is 360=0degrees, it is essential for sustained oscillations.

- At the oscillation frequency each of the resistor capacitor filter produces a phase shift of 60° so the whole filter circuit produces a phase shift of 180°.

- The energy storage capacity of capacitor in this circuit produces a noise voltage which is similar to a small sine wave, it is then amplified using op amp inverting amplifier.

- By taking feedback, the output sine wave also attenuates 1/29 times while passing through the RC network, so the gain of inverting amplifier should be 29 in order to keep loop gain as unity.

- The unity loop gain and 360 degree phase shift are essential for the sustained oscillation.

- RC Oscillators are stable and provide a well shaped sine wave output with the frequency being proportional to 1/RC and therefore, a wider frequency range is possible when using a variable capacitor.

- However, RC Oscillators are restricted to frequency applications because at high frequency the reactance offered by the capacitor is very low so it acts as a short circuit.

## Why it uses 3 RC stages?

- Number of RC stages help improve the frequency stability. The total phase shift introduced by the feedback network is 180 degrees, if we are using N RC stages each RC section provide 180/N degree phase shift.

- When 2 RC sections are cascaded, the frequency stability is low. For 3 sections cascaded the phase change rate is high so there is improved frequency stability. However for 4 RC sections there is an good phase change rate resulting in the most stable oscillator configuration. But 4 RC sections increases cost and makes circuit complexity.

- Hence phase shift oscillators make use of 3 RC sections in which each section provides a phase shift of 60 degree. The latter is generally used in high precision applications where cost is not much regarded and only accuracy plays a major role.

## Design

Attenuation offered by the feedback RC network is 1/29, so the gain of inverting amplifier should be 29

Use R

So, R

Use 50KΩ potentiometer and adjust its value to obtain output on CRO

_{i}=1.2 KΩSo, R

_{f}=35KΩUse 50KΩ potentiometer and adjust its value to obtain output on CRO

Thank You for this Post

THanks it helped me a lot for my exams

You are welcome and keep visiting. Wish all the best for your exams.

really thanks for providing the whole datas..

hi i have a question

the oscillator will not work like that we must applied signal on the enter of a.o.p to beggin oscillation after that we remove signal and oscillation will continu .

is that correct?

Hi Nasreddine,

We can not say like this, because a noise (or switching transients) will trigger the oscillator. It is a theoretical concept we can’t simulate or demonstrate practically.

this note is very helpful to me

Hi irappadesai,

Thank you. By the way checkout other circuits which can be used to your academic projects..

hi all,

i’m requiring a suitable circuit for high frequency RF oscillator, if somone having idea about it , pls. post it.

thnk u.

can u plz explain exactly how to simulate this on multisim(i mean noise)

Hi Danish,

It is theoretical concept, unable to simulate with Multisim.

Thank you…

Hi Jaseem,

Thank you for this post! can you please attach the MULTISIM file (*.ms12) of the above?

I tried to simulate this circuits myself, but it didnt work..

thanks,

Eyal

Thank you very much, I simulated it in Proteus ISIS 7.10 and worked, but I used a LM324 and changed the pin 3 input from 0 to 5V DC. When the input DC voltage is variable, the amplitude (AC) of the sin wave is variable too; it means that you get a variable amplitude sin wave.