Different types of oscillators are available all around with their own pros and cons. Their classification is based on their performance and the components used. Among them, the Wein bridge oscillator and Hartly Colpitts oscillator are quite prominent.

In an oscillator, the frequency generating circuit is decided by the circuit elements used. Here we are going to study how we can obtain RC phase shift oscillator using BJT.

## What Is An Oscillator

Before we jump into a transistor-based oscillator circuit or RC phase shift oscillator using a transistor, let’s define the oscillator itself. An oscillator is an electronic circuit that acts as a sine wave generator.

The only requirement of an oscillator is a DC power supply source. It is widely used in frequency variable signal generators. So it is a common basic electronic circuit.

A basic oscillator circuit contains the following parts –

- Amplifier
- Frequency selective network
- Positive feedback from output to input

## Condition for Sustained Oscillations

The basics of oscillator circuit must obey Barkhusen’s criteria to provide sustained oscillation, it states that

- Total loop gain of the circuit must be equal to unity. ie; Aß = 1. (Where A is the gain of the amplifier and ß is the loop gain or feedback factor.)
- Net phase shift (or total phase shift) around the circuit must be 0˚or 360˚.

## Circuit Diagram of RC Phase Shift Oscillator Using BJT

## Components Required for Phase Shift Oscillator

- Resistors(47kΩ; 10kΩ; 2.2kΩ; 4.7kΩ x 2; 680Ω)
- Capacitors(22µF; 1µF; .01µFx3)
- Transistor BC107
- Potentiometer

## Sustained Output Waveform of RC Phase Shift Oscillator

## Working of Transistorized RC phase shift oscillator

- Here we are using a BC107 transistor for implementing RC phase shift oscillator. BC107 is an audio frequency transistor which is made up of silicon.
- If we use a common emitter amplifier with a resistive collector load, there will be a 180˚ phase shift between the voltages at base and collector. It will also amplify the signal.
- Feedback circuit section must produce another 180˚ shift to meet the Barkheusan criterion.
- Three sections of phase shift networks are used which is constituted by resistive-capacitor combination. In that each section introduces 60˚ phase shift at resonant frequency.
- The positive feedback from output to input will lead the circuit to operate as an oscillator.
- Phase shift oscillator is a particular type of audio frequency oscillator. Output signal is obtained across 1µF capacitor and ground terminal as shown in circuit schematic.

## Design of RC Phase Shift Oscillator Using BJT

If all the resistors, R and the capacitors, C in the phase shift network are equal in value, then the frequency of oscillations produced by the RC oscillator is given as:

Where, ƒr is the Output Frequency in Hertz

R is the Resistance in Ohms

C is the Capacitance in Farads

N is the number of RC stages. (N = 3)

## Why RC phase shift oscillator uses 3 RC stages

- Number of RC stages will improve the frequency permanence. The total phase shift established by the feedback network must be 180 degrees for sustained oscillations. If we are using ‘N’ RC stages, each RC section provides 180/N degree phase shift.
- When 2 RC sections are cascaded, the frequency stability is poor. For 3 sections cascaded, the phase change rate is high so we get improved frequency stability. However for 4 RC sections there is a good phase change rate resulting in the most stable oscillator configuration. But 4 RC segments enhance cost and makes circuit complexity.
- Hence phase shift oscillators make use of 3 RC sections and in that each section gives a phase shift of 60 degree. The latter 3 RC networks are generally used in high precision applications where cost is not much considered and only accuracy plays a major role.

## Conclusion

Apart from designing an RC phase shift oscillator using transistor you can implement it using op amp 741. Note that we have kept a 4.7k ohm potentiometer instead of a regular resistor.

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