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Phase Shift Oscillators
One of the important features of an oscillator is that the feedback energy applied 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 applications, the LC circuits have few drawbacks such as
Frequency instability
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 stability is improved and a good quality
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 eliminated in RC oscillator circuits.
Hence the need for RC oscillator circuits arise. These are also called as Phaseshift
Oscillators.
Principle of Phase-shift oscillators
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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.
The output voltage V1 across the resistor R leads the input voltage applied 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.
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
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oscillator circuit are as given below.
Construction
The phase-shift oscillator circuit consists of a single transistor amplifier 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.
The frequency of oscillations is given by
1
fo =
–
2πRC √6
Where
R1 = R2 = R3 = R
C1 = C2 = C3 = C
Chapters Categories
Operation
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The circuit when switched ON oscillates at the resonant frequency fo. The output Eo of
the amplifier 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 applied to the transistor
amplifier.
The feedback applied will be
m = E i /E o
The feedback is in correct phase, whereas the transistor amplifier, 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 stability.
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.
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