Title- To design a RC phase shift oscillator.

Objective- To design a RC phase shift oscillator and measure the frequency and amplitude of oscillator.

Theory- Phase shift oscillator consists of a negative gain amplifier with a three section RC ladder network in the feedback. Consider the RC network shown in figure-1. For the shown three methods one can readily write out the mesh equations and can obtain:

Where α=1/ωCR. In order that the phase difference between the input voltage Vi and the output voltage Vo be 180°, we must have

and,

These conditions give and . It follows that if Vo is

fed to the input of an amplifier providing a gain of 29 and a phase shift of 180° and id the output of the amplifier is feed back to the input of RC network, an

oscillation of frequency f=1/(2π ) Hz will be sustained by the system. This

type of oscillation is called RC or phase shift oscillation. Since the RC network is required to give a phase shift of 180° a minimum number of three identical RC stages must be used. It may be mentioned that to generate audio frequency signals the use LC tank circuit becomes inconvenient as impractically large values of L and C are required. However, such frequencies are conveniently generated with the phase oscillator.

Circuit Diagram:

Procedure- 1) Construct the circuit on the bread board using the values of the component as obtained from design.

2) Connect the output of the op-amp to the CRO.

3) Switch on the power supply.

4) Note the required waveform with frequency and amplitude from CRO.

Repeat the steps 1-4 for different frequency waveform.

Apparatus and accessories:

1) Operational Amplifier (IC 741C)

2) Resistors (12KΩ, 470KΩ, 4.7KΩ)

3) Potentiometer (10KΩ)

4) Capacitors (0.047µF)

5) Bread Board

6) Connecting Wires

7) Cathode Ray Oscilloscope

8) DC Power supply

9) Multimeter

Calculation and results : Theoretical value of frequency

Table 1: Measurement of frequency of the phase shift oscillator.

Set no. R KΩ C µF Theoretical frequency Frequency, f observed from CRO (Hz)

Discussion:

1) What do you mean by an electronic oscillator?

2) What classes of oscillators are commonly used in AF and RF ranges and why?

3) What is the basic principle of oscillators?

4) Is an external input signal necessary for the output of an oscillator? If not how are oscillations initiated?