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SUBJECT:
ANALOG ELECTRONICS
(2130902)
EXPERIMENT NO. 01
TITLE:
TO BUILD TRANSISTOR BASED RC PHASE SHIFT
OSCILLATOR CIRCUIT, AND MEASURE AND VERIFY ITS
FREQUENCY OF OPERATION.
DATE :
DOC. CODE : DIET/EE/3rd SEM
REV. NO. : 1.00/JUNE-2015
AIM:
To build transistor based RC phase shift oscillator circuit, and measure and verify its frequency of
operation.
APPARATUS:
Analog board, AB70.
Oscilloscope and probes
2mm patch cords.
THEORY:
In this experiment we would like to introduce you a transistor oscillator circuit, called as RC Phase shift
Oscillator. First of all we need to know what an oscillator is. An oscillator is an electronic circuit which
acts as a sine wave generator.
The PHASE-SHIFT OSCILLATOR is a sine-wave generator that uses a resistive-capacitive (RC)
network as its frequency-determining device. In the common-emitter amplifier configuration there is a
180-degree phase difference between the base and the collector signal. To obtain the regenerative
feedback in the phase-shift oscillator, circuit need a phase shift of 180 degrees between the output and
the input signal.
In an RC Oscillator circuit the input is shifted 180o through the amplifier stage and 180o again through a
second inverting stage giving us “180o + 180o = 360o” of phase shift which is effectively the same as
0o thereby giving us the required positive feedback. In other words, the phase shift of the feedback loop
should be “0”.
An RC network consisting of three RC sections provides the proper feedback and phase inversion to
provide this regenerative feedback. Each section shifts the feedback signal 60 degrees in phase. Since
the impedance of an RC network is capacitive, the current flowing through it leads the applied voltage
by a specific phase angle.
The phase angle is determined by the amount of resistance and capacitance of the RC section. If the
capacitance is a fixed value, a change in the resistance value will change the phase angle. If the
resistance could be changed to zero, we could get a maximum phase angle of 90 degrees. But since a
voltage cannot be developed across zero resistance, a 90-degree phase shift is not possible. With a small
value of resistance the achieved phase shift is less than 90 degrees, but in the phase-shift oscillator it
required 180-degree phase shift for regenerative feedback, therefore, at least three RC sections are
required.
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Number of RC stages will improve the frequency durability. The total phase shift established by the
feedback network must be 180 degrees for constant oscillations. If we are using ‘N’ RC stages, each RC
section provides 180/N degree phase shift. When 2 RC sections are cascaded, the frequency stability is
poor. However for 4 RC sections there is a good phase change rate resulting in the most stable oscillator
configuration. But 4 RC segments enhance cost and makes circuit complexity.
Fig.1: Transistor Phase Shift Oscillator
The values of resistance and capacitance are generally chosen so that each section provides about a 60degree phase shift. Resistors R1, R2, and RC provide base and collector bias. Capacitor CE bypasses ac
variations around the emitter resistor RE. Capacitors C1, C2, and C3 and resistors RB1, RB3, and RB2
form the feedback and phase-shifting network. Resistor R2 is variable for fine tuning to compensate for
any small changes in value of the other components of the phase-shifting network. A change in the flow
of base current results in an amplified change in collector current which is phase-shifted the 180
degrees. When the signal is returned to the base, it has been shifted 180 degrees by the action of the RC
network, making the circuit regenerative.
If all the resistors, R and the capacitors, C in the phase shift network are equal in value, then the
frequency of oscillations produced by the 3 stage RC oscillator is given as:
=
1
2
√6 + 4
Where: ƒo is the Output Frequency in Hertz
R is the Resistance in Ohms
C is the Capacitance in Farads
6=2N where N is the number of RC stages. (N = 3)
Where K = RC/R.
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The phase shift oscillator is suited for the range of frequency forms several Hertz to several hundred
Kilohertz, and so includes the range of audio frequencies. At Megahertz frequency it has no marked
advantage over LC circuit. By changing the value of RC combination we can change the output
frequency of the Oscillator
Fig.2: Phase Shift Oscillator kit
PROCEDURE:
1. Connect +12V DC power supplies and ground at their indicated position from external source or
ST2612 Analog Lab.
2. Calculate the frequency of phase shift oscillator by using eq.1
3. Rotate the Potentiometer ‘R2’ fully clock wise to set its value at 20KΩ
4. Rotate the Potentiometer ‘RL’ clock wise also to have maximum amplitude of the output.
5. Connect Oscilloscope CHI at socket ‘Vout’ and ground
6. Switch ‘On’ the power supply.
7. Observe the output if the distortion is appearing rotate the Potentiometer ‘R2’ anti clock wise till
the out is a perfect sine wave.
8. Still if the output is not as desired, decrease the value of RL.
9. Measure the frequency of output.
10. Compare measured frequency with the theoretically calculated value.
11. Now connect Oscilloscope CHII at TP1, TP2 and TP3 respectively
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12. Measure phase difference between test points TP1, TP2 and TP3 and output with the help of dual
channel CRO.
13. Observe the output at TP4 also to see the effect of coupling capacitor.
14. The phase shift should be at the margin of 60º
OBSERVATION TABLE:
Sr.
No.
Parameter
1
Theoretical value of output frequency
2
Practical value of output frequency
3
Phase shift between test point TP1 & output
4
Phase shift between test point TP2 & output
5
Phase shift between test point TP3 & output
Value
CALCULATION:
CONCLUSION:
LAB-INCHARGE
Darshan Institute of Engineering And Technologies, Rajkot
H.O.D
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