Electronic Circuit Design Lab Experiments

Analyse the working of RC Phase shift Oscillator using BJT.

Sep 24 2023

Aim

Apparatus Required:

Breadboard ,Workstation, Function Generator, Trainer Kit, Patch cords, DC Power supply

The instrument comprises of the following built in parts:-

Fixed DC Regulated power Supply of +12VDC
A Phase Shift Network consist of three sections R1C1, R2C2,&R3C3. The total Phase Shift produced by The RC network is 180 (60-60-60)
One NPN Transistor for further phases shift of 180.

Theory:

Oscillators using RC network are Phase Shift Oscillators. Lc oscillators are used for generating high frequency. These are not suitable for low frequency ranges. Such as audio Frequency. The alternative is RC network.

The above block diagram shows the essential components of a transistorized Phase Shift Oscillator

Tank Circuit:The tank circuit consists of three resistance connected in parallel with three capacitors. It is known as frequency determining network. The frequency of oscillations in the circuit depends upon the values of resistance & capacitors.

Transistor Amplifier: The function of the amplifier is to amplify the oscillations produced by rc circuit. The amplifier receives DC power from battery & converts it into AC power supplying to the tank circuit. The oscillations produced in the tank circuit are applied to the input of the transistor. The transistor increases the output of these oscillations.

Feedback Circuit:The function of feedback circuit is to transfer a part the output energy to R. C. network in proper phase. When the feedback is positive, the overall gain of the amplifier is written as

Af=A/1-AB

Where AB is feedback factor or loop gain.

If AB =1, Af=c . thus the gain becomes infinity i.e., there is output without any input. In other words, the amplifier works as oscillator. The condition AB=1 known as Barkhausen criterion of oscillation

A transistor Phase Shift Oscillator must introduce in-phase feedback from the output to the input to sustain oscillation. If a common emitter amplifier is used with a resistive collector load, there is a 180 phase shift between the base & collector. hence the phase shift feedback network between collectors & base must introduce an additional 180 phase shift, at some frequency if oscillation is to take plase. A transistor connected as a Phase Shift Oscillator is shown on the collector on the engraved front panel of the instrument. In this common emitter amplifier, feedback is from the collector to the base, i.e. from the output to the input. The 3 section phase shift network consists of C1R1,C2R2,C3R3. So that each section may introduce a 60 phase shift (approx) at the resonant frequency.

The frequency of the oscillations may be expressed as

F = 1/[2π (6RC)1/2]

Where C = 0.01µF

Calculated frequency = 649.7Hz

Circuit diagram:

Procedure

Connect the CRO probe at output terminals.
Switch ON the instrument with an oscilloscope.
Observe the output frequency. It will be around 650Hz.
Now connect other CRO probe at lst yellow socket and adjust CRO’s Time/Division knob at ‘XY’ mode to observe Phase Shift of 0 as shown below.
Compare the other Phase Shifts by connecting the other yellow sockets one by one.
Lissajous pattern for different phase shift are shown below.

Note: Adjust the Volt/division knobs of CRO if required to the desired phase shift pattern

Results:Result has been verified by observing Lissajous Patterns with Different Phase Shift

Viva Questions:

Q.-1. Explain Low Frequency Oscillator

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Q.-2. Derive the expression of frequency for RC Phase Shift Oscillator

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Q.-3. How RC Phse Shift Oscillator different from Weinbridge Oscillator

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