Principle of Phase Shift Oscillators
Principle of Phase Shift Oscillators-One desirable point of an oscillator is that it should feed back energy of correct phase to the tank
circuit to overcome the losses being in it. In the oscillator circuits bandied so far, the tank
. circuit employed inductive (L) and capacitive (C) rudiments.
In similar circuits, a phase shift of 180º was
. attained due to inductive or capacitive coupling and a farther phase shift of 180º was attained due to
In this way, energy supplied to the tank circuit was in phase with the generated
. oscillations. The oscillator circuits employing L-C rudiments have two general downsides.
.they suffer from frequence insecurity and poor waveform. Secondly, they can not be used for veritably
. low frequentness because they come too important big and precious.
Good frequence stability and waveform can be gain from oscillators employing resistive and capacitive rudiments.
Similar amplifiers are call R-C or phase shift oscillators and have the
fresh advantage that they can be used for veritably low frequentness.
In a phase shift oscillator, a
phase shift of 180º is gain with a phase shift circuit rather of inductive or capacitive coupling.
A farther phase shift of 180º is introduce due to the transistor parcels. Therefore, energy supplied back
. to the tank circuit is assure of correct phase.
Phase shift circuit
A phase- shift circuit basically consists of an R-C network. Fig-1 (i)
shows a single section of RC network. From the abecedarian proposition of electrical engineering, it can be shown that interspersing voltage V ′
1 across R leads the applied voltage V1 by φº.
The value of φ depends
upon the values of R andC.However, the value of φ also changes, If resistance R is varied. If R were reduce
to zero, V ′1 will lead V1 by 90º i.e. φ = 90º. Still, conforming R to zero would be inoperable
. because it would lead to no voltage acrossR. Thus, in practice, R is varied to such a value that makes V ′
1 to lead V1 by 60º.
Fig-1 (ii) shows the three sections of RC network.
Each section produces a phase shift of
60º. Consequently, a total phase shift of 180º is produce i.e. voltage V2 leads the voltage V1 by 180º.
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