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 oscillators for synchronous receivers, musical instruments, study purposes, etc.
The main part of an RC phase shift oscillator using 741 is the op-amp itself. It’s used as an 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 capacitance of the capacitors, the frequency of oscillations can be varied. Here, the feedback network has a phase shift from zero to 270° at low and high frequencies respectively.
Circuit Diagram of RC Phase Shift Oscillator Using 741
Components Required for RC Phase Shift Oscillator
- Resistors (1.2KΩ, 1KΩx2)
- Potentiometer (50KΩ)
- Capacitors (0.01µFx3)
- 741 Op Amp
Transient Output Waveform
Sustained/Steady-State Output Waveform
Working Procedure of RC Phase Shift Oscillator Using 741
- 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 his 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.
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
An op-amp can be in inverting and non-inverting modes. Here, in this RC phase shift oscillator using 741 op-amp is built keeping it in inverting mode. Hence the pin connections are truly vital while implementation.