12-RC Phase shift Oscillator.pdf - Experiment No. 12 RC...

Electronic Circuits Lab, Department of Electrical Engineering, College of Engineering Trivandrum
1
Experiment No. 12
RC PHASE SHIFT OSCILLATOR USING BJT
AIM
To design RC phase shift oscillator.
THEORY
An oscillator is essentially a source of emf. Its output may be a sine wave, a square
wave, a triangular wave or a ramp. It is possible to vary the frequency and amplitude of the
output. In this experiment we will be concerned only with sine wave oscillators.
Feedback is said to exist
in an amplifier if a part of its
output is brought back into the
input circuit. Consider the
situation in figure 1. A fraction β
( β < 1) of the output V
o
is brought
back into the input circuit.
The net input V
i
to the
amplifier can then be written in
two different ways.
i s o
V V V    (1)
where V
s
is an external input
In equation (1) the feedback is said to be positive. Both V
s
and βV
o
drive the input
terminal. So, this is positive feedback.
If A is the voltage gain of the amplifier.
o i
V AV  (2)
From equation (1) and (2)
o
s o
V
V V
A
  
Hence gain Ar of the whole circuit including the feedback network is given by
(1 )
o
s
V A
Ar
V A 
 

Thus, positive feedback increases the gain from A to Ar . If V s is reduced to 0 and Vo
made equal to V
i
, we get an amplifier that supplies its own input. An output voltage is
obtained even in the absence of an external input. This happens when 1 A   .
An amplifier that supplies its own input is an oscillator. Then relation Aβ = 1 is called
Barkhausen criterion for oscillators. It implies that βV
o
must have the same magnitude and
the same phase as the input. In general, A is complex. This means that the output of an
Fig 1. Basic oscillator block diagram