- Oscillators - Tunnel Diode Oscillator
- Negative Resistance Oscillators
- Oscillators - Crystal Oscillators
- Wien Bridge Oscillator
- Oscillators - Phase Shift Oscillators
- Oscillators - Clapp Oscillator
- Oscillators - Colpitts Oscillator
- Oscillators - Hartley Oscillator
- Tuned Circuit Oscillators
- Oscillators - Oscillator Circuit
- Oscillators - Basic Concepts
- Oscillators - Introduction
- Sinusoidal Oscillators - Home
Sinusoidal Oscillators Resources
- Sinusoidal Oscillators - Discussion
- Sinusoidal Oscillators - Resources
- Sinusoidal Oscillators - Quick Guide
Selected Reading
- Who is Who
- Computer Glossary
- HR Interview Questions
- Effective Resume Writing
- Questions and Answers
- UPSC IAS Exams Notes
Phase Shift Oscillators
One of the important features of an oscillator is that the feedback energy appped should be in correct phase to the tank circuit. The oscillator circuits discussed so far has employed inductor (L) and capacitor (C) combination, in the tank circuit or frequency determining circuit.
We have observed that the LC combination in oscillators provide 180o phase shift and transistor in CE configuration provide 180° phase shift to make a total of 360o phase shift so that it would make a zero difference in phase.
Drawbacks of LC circuits
Though they have few apppcations, the LC circuits have few drawbacks such as
Frequency instabipty
Waveform is poor
Cannot be used for low frequencies
Inductors are bulky and expensive
We have another type of oscillator circuits, which are made by replacing the inductors with resistors. By doing so, the frequency stabipty is improved and a good quapty waveform is obtained. These oscillators can also produce lower frequencies. As well, the circuit becomes neither bulky nor expensive.
All the drawbacks of LC oscillator circuits are thus epminated in RC oscillator circuits. Hence the need for RC oscillator circuits arise. These are also called as Phase–shift Oscillators.
Principle of Phase-shift oscillators
We know that the output voltage of an RC circuit for a sinewave input leads the input voltage. The phase angle by which it leads is determined by the value of RC components used in the circuit. The following circuit diagram shows a single section of an RC network.
![Single Section RC](/sinusoidal_oscillators/images/single_section_rc.jpg)
The output voltage V1’ across the resistor R leads the input voltage appped input V1 by some phase angle ɸo. If R were reduced to zero, V1’ will lead the V1 by 90o i.e., ɸo = 90o.
However, adjusting R to zero would be impracticable, because it would lead to no voltage across R. Therefore, in practice, R is varied to such a value that makes V1’ to lead V1 by 60o. The following circuit diagram shows the three sections of the RC network.
![Three Section RC](/sinusoidal_oscillators/images/three_section_rc.jpg)
Each section produces a phase shift of 60o. Consequently, a total phase shift of 180o is produced, i.e., voltage V2 leads the voltage V1 by 180o.
Phase-shift Oscillator Circuit
The oscillator circuit that produces a sine wave using a phase-shift network is called as a Phase-shift oscillator circuit. The constructional details and operation of a phase-shift oscillator circuit are as given below.
Construction
The phase-shift oscillator circuit consists of a single transistor amppfier section and a RC phase-shift network. The phase shift network in this circuit, consists of three RC sections. At the resonant frequency fo, the phase shift in each RC section is 60o so that the total phase shift produced by RC network is 180o.
The following circuit diagram shows the arrangement of an RC phase-shift oscillator.
![Arrangement RC](/sinusoidal_oscillators/images/arrangement_rc.jpg)
The frequency of oscillations is given by
$$f_o = frac{1}{2pi RC sqrt{6}}$$
Where
$$R_1 = R_2 = R_3 = R$$
$$C_1 = C_2 = C_3 = C$$
Operation
The circuit when switched ON oscillates at the resonant frequency fo. The output Eo of the amppfier is fed back to RC feedback network. This network produces a phase shift of 180o and a voltage Ei appears at its output. This voltage is appped to the transistor amppfier.
The feedback appped will be
$$m = E_i/E_o$$
The feedback is in correct phase, whereas the transistor amppfier, which is in CE configuration, produces a 180o phase shift. The phase shift produced by network and the transistor add to form a phase shift around the entire loop which is 360o.
Advantages
The advantages of RC phase shift oscillator are as follows −
It does not require transformers or inductors.
It can be used to produce very low frequencies.
The circuit provides good frequency stabipty.
Disadvantages
The disadvantages of RC phase shift oscillator are as follows −
Starting the oscillations is difficult as the feedback is small.
The output produced is small.