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.
- 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.
Frequency of oscillation (F):
Gain of the Op Amp inverting amplifier (G):
Attenuation offered by the feedback RC network is 1/29, so the gain of inverting amplifier should be 29
Use Ri=1.2 KΩ
Use 50KΩ potentiometer and adjust its value to obtain output on CRO