We had already discussed about Op amp RC phase shift oscillator in earlier posts. Different types of oscillators are available which are based on their performance and components used, like Wein bridge oscillator, Hartly Colpitts oscillator etc. Here we are going to study how we can obtain RC Phase shift Oscillator using a silicon transistor. In an oscillator the frequency generating circuit is decided by the circuit elements used.A basic oscillator circuit contains the following parts
- 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˚.
- Resistors(47kΩ; 10kΩ; 2.2kΩ; 4.7kΩ x 2; 680Ω)
- Capacitors(22µF; 1µF; .01µFx3)
- Transistor BC107
Sustained Output Waveform
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
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.