INSTRUMENTATION & CONTROL

Objective

To design phase shift oscillator using operational amplifier IC-741C and verify its functionality.

Components

Operational amplifier 741 IC
Resistors (1kΩ , ½ W ; 3.3 kΩ , ¼ W )
Capacitor (0.1 µF , 63 volts)
CRO Probes
Bread board and Hook up wires
Potentiometer (100 kΩ)
CRO
Power Supply DC (± 15 volts)

Theory

Figure shows a phase shift oscillator, which consists of an operational-amplifier as the amplifying stage and three RC cascaded networks as the feedback circuit. The feedback circuit provides feedback voltage from the output back to the input of the amplifier.
The operation amplifier is used in the inverting mode therefore, any signal that appears at the inverting terminal is shifted by 180 at the output. An additional 180 phase shift around the loop is 360 (or 0). At some specific frequency when the phase shift of the cascade RC networks is exactly 180 and gain of the amplifier is sufficiently large, the circuit will oscillate at that frequency. This frequency is called the frequency of oscillation f0 and is given by,
f0 = 1/(2П√6 RC ) = 0.065/RC ………………(1)
At this frequency the gain AV must be equal to at least 29. That is,
RF/R1 = 29 ……………………………………(2)
Thus the circuit will produce a sinusoidal waveform of frequency f0 if the gain is 29. and the total phase shift around the circuit is exactly 360. For a desired frequency of oscillation, choose a capacitor C, and then calculate the value of R from equation (1).

Procedure

1. Connect the circuit as shown in figure.
2. Adjust the value of the potentiometer such that the ratio of RF and R1, must be greater than 29.
3. Adjust different knobs of CRO to get stable waveform.
4. Calculate the time period and thus frequency of output waveform.
5. Compare practical and theoretical value of f0.