Download EXPERIMENT NO.(5) RC OSCILLATORS

Electronics II Laboratory
EXPERIMENT NO.(5)
RC OSCILLATORS
Object
To study the characteristics of RC oscillators namely:
1) Phase shift oscillators.
2) Wein bridge oscillators.
Equipments Components
1- Dual Power supply
2- Oscilloscope
3- Operational amplifier
4- Transistors
5- Components as shown in figures
6- Function generator
Theory
The oscillator is an amplifier with positive feedback that generates
a number of waveforms usually used in instrumentation and test
equipments. An oscillator that generates a sinusoidal output is called a
harmonic oscillator; the transistor is usually acts in the active region. The
output of the relaxation oscillator is not sinusoidal depending on the
transient rise and decay of voltage in RC or RL circuits.
There are two types of RC oscillators:
1. Phase shift oscillators in which the output of an amplifier must be
180o out of phase with input. A general circuit diagram of a phase shift
oscillator is shown in Fig.(l), where the amplifier is an ideal one. A
phase shift network (usually a resistor-capacitor network) is used to
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produce an additional phase shift of 180 at one particular frequency to
develop the required positive feedback.
From the mesh network equations of the feedback network, we find the
feedback factor β as,

Vf
1

2
Vo 1  5  j 6   3


Where= 1 / RC

The phase shift of the feedback network must be 180 then:
6   3  0    6
so,
f
1
2 6RC
At this frequency = 1/29 and it is required that (A) must be at
least 29 to satisfy oscillation condition as shown in Fig.(2).
The phase shift oscillator is used to the range of frequencies for
several hertz to several kilohertz and so includes the range of audio
frequencies. The frequency depends on the impedance elements in the
phase shift network.
The phase shift oscillator circuit is not very suitable for generating
variable frequency because the resistors and capacitors must be
simultaneously changed to obtain the required frequency control over a
wide range therefore it is used mostly in fixed frequency applications.
2. The Wien bridge oscillator is used to obtain variable frequency
signal. The frequency of oscillation can be changed by using two gang
variable capacitors or two gang variable resistors. The circuit diagram
is shown in Fig.(3). In this circuit, there are two types of feedback:a. positive feedback through Z1 and Z2 whose components determine the
frequency of oscillation.
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Electronics II Laboratory
b. negative feedback through Rl and R2 whose elements affect the
amplitude of oscillation.
B
V f
Vo
A1

 Z2
Z1  Z2
R1
R2
Moreover, the loop gain BA is given by
BA 

R1 

.
1



R2 
3  j( 1   2 ) 
; Where RC
Now to satisfy the oscillation condition with
BA = 1 requires
= 1
Then;
1
R1
1
3 f 
R2
2RC
Continuous variation of frequency is accomplished by varying
simultaneously two capacitors. Change in frequency range accomplished
by switching in different values for the two identical resistors R.
Procedure
1. Connect the circuit as shown in Fig.(2), insert potentiometer of l k in
the feedback arm.
2. Measure the frequency of oscillation (f o) and the amplitude of the
output voltage.
3. Measure and draw the waveforms of points A, B, C and D.
4. Observe the effect of variation of the potentiometer on the frequency of
oscillation.
5. Observe the effect of the variation of RE and RB on fo.
6. Break the feedback network and measure the gain by connecting the
signal generator to point d.
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7. Connect the circuit as shown in Fig.(3).
8. Measure the frequency and amplitude of oscillation.
9. Change the value of R and C their effect on the frequency and
amplitude of oscillation.
C
C
Amplifier
C
with 180
phase shift
R
R
Vo
R
Fig.(1) : Schematic of phase shift (RC) Oscillator
Vcc
8.2k
-12V
390
10uF
0.01uF
A
0.01uF
B
560
0.01uF
D
C
560
680
Vo
Q1
BCY71
10
47uF
Fig.(2): RC phase shift oscillator
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Electronics II Laboratory
Rf
2.2k
+15V
+
741
eo
-15V
R1
1k
220
0.1uF
0.1uF
220
Fig.(3): Wien bridge oscillator
Discussion
l. What are the general oscillation conditions in feedback amplifiers?
2. Drive an expression for the frequency of oscillation in both phase shift
and wein bridge oscillators.
3. Compare between phase shift and wein bridge oscillators.
4. Comment on the wave forms at points A, B, C and D in figure.
5. Discuss the effect of changing RB and RE on fo.
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