RC Phase Shift Oscillator Lab Manual

Experiment 4
RC Phase Shift Oscillator

Aim: To design and test RC Phase Shift Oscillator circuit to generate sinusoids of
given frequency
Circuit Diagram:
Figure 1: BJT RC Phase Shift Oscillator
Theory:
An ideal RC phase shift oscillator circuit is shown below.
The amplifier in the circuit amplifies the input signal by a factor of ‘A’ and also
introduces 1800 phase shift. The output of the amplifier is fed to three cascaded
identical RC sections present in the feedback network. The feedback network
introduces 1800 (each section introduces 600) phase shift between its input and
output signal. The output of feedback network is provided as input to the amplifier
creating a positive feedback to the amplifier.
The feedback section attenuates the input signal by a factor of βF = 1/29. To satisfy
Barkhausen criteria for sustained oscillations, the amplifier must have a gain A > 29.
The major advantage of RC phase shift oscillator is that it can produce pure
sinusoidal oscillations. However, it cannot be used to generate sinusoidal signals
beyond 20 KHz.
Figure 1 shows a version of RC phase shift oscillator that uses BJT as active
element of the amplifier stage. The output of the feedback network is loaded by
relatively low input resistance hie of the transistor. To minimize the loading effect, the
feedback signal is coupled through the feedback resistor Rf in series with the
amplifier stage input resistance Ri
Frequency can be adjusted by varying either the capacitors or resistors of RC

section. The frequency of oscillations is given by
1 ...(1)
𝑓=
2𝜋𝑅𝐶√6 + 4𝑘
Where
𝑅𝑐
𝑘=
𝑅
BJT must have
29
hfe > 4k + 23 +
k
The optimum values of k and hfe are
𝑘 < 2.7 𝑎𝑛𝑑 ℎ𝑓𝑒 > 44.5
Design Steps:
a. Amplifier Design
Sl.No. Steps Working
Beta of transistor
1. 𝛽=
Choose transistor whose 𝛽 is greater than 44.5
2. Voltage gain of Amplifier (AV > 29) AV = 50
VCC =
3. Choose VCC and IC
IC =
Calculate VC, VE and VB VC =
𝑉𝐶𝐶
4. 𝑉𝐶 = VE =
2
𝑉𝐶𝐶
𝑉𝐸 =
10 VB =
𝑉𝐵 = 𝑉𝐸 + 𝑉𝐵𝐸
Calculate IB and IE
𝐼𝐶
5. 𝐼𝐵 = IB =
𝛽
𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
Calculate RC and RE
𝑉𝐶𝐶 − 𝑉𝐶
𝑅𝐶 =
𝐼𝐶 RC =
6.
𝑉𝐸 RE =
𝑅𝐸 =
𝐼𝐸
Calculate re
7. 26𝑚𝑉 re =
𝑟𝑒 =
𝐼𝐸
Calculate RE1 and RE2
𝑅𝐶 RE1 =
|𝐴𝑉 | =
8. 𝑟𝑒 + 𝑅𝐸1
𝑅𝐶
𝑅𝐸1 = − 𝑟𝑒 RE2 =
𝐴𝑉
𝑅𝐸2 = 𝑅𝐸 − 𝑅𝐸1
Calculate R1 and R2
Choose
𝛽𝑅𝐸 R2 =
𝑅2 ≤
9. 10
Calculate
(𝑉𝐶𝐶 − 𝑉𝐵 )𝑅2 R1 =
𝑅1 =
𝑉𝐵
10. Choose lower cut off frequency fLS fLS = 100Hz

Let XCE = RE2/10 XCE =
11. 1
𝐶𝐸 =
2𝜋𝑓𝐿𝑆 𝑋𝐶𝐸 CE =
b. Feedback Network Design

Sl.No. Steps Working
12. Frequency of oscillations f=
Choose R and calculate Rf such that
𝑅𝐶
< 2.7 and Rf > 0
𝑅
R=
13. 𝑅𝑓 = 𝑅 − 𝛽(𝑟𝑒 + 𝑅𝐸1 )
Rf =
Calculate C
𝑅𝐶
𝑘=
14. 𝑅 C=
1
𝐶=
2𝜋𝑓𝑅√6 + 4𝑘
Procedure:
1. Design the circuit to match the given specifications.
2. Test the components.
3. Build only the amplifier part and measure and make sure that the DC voltages
VC, VB and VE are as per the design.
4. Apply a small input voltage to the amplifier circuit using signal generator.
Make sure that the amplitude of input signal is small enough and does not
cause distortion of the output signal having frequency equal to desired
frequency of oscillations.
5. Check the gain of the amplifier.
6. If gain of the amplifier is as desired, disconnect the input signal.
7. Build the feedback network and connect to the amplifier to complete the
oscillator circuit.
8. Observe the output voltage waveform on the CRO and measure the
frequency of oscillations.
Observation:
Peak to Peak output voltage Vo(p-p) = _________________
Desired frequency of oscillations f = __________________
Measured frequency of oscillations fmeasured = _______________
|𝑓−𝑓𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 |
% Error = ⋅ 100 = ___________________
𝑓
Sample Viva Questions

1. What is an oscillator?
2. Explain working of an oscillator.
3. What are the different types of oscillators?
4. What are tuned oscillators?
5. Explain Barkhausen’s criteria of sustained oscillations
6. Mention the applications of oscillators.
Concepts to explore (Beyond syllabus)
Crystal Oscillator
The circuit of crystal oscillator is shown in Figure 2. A Common Emitter amplifier

provides adequate amplification. Crystal exhibits a property known as piezoelectric
effect. When mechanical stress is applied across one set of crystal’s faces, a
potential difference develops across the opposite the other set of faces. This is
called piezoelectric effect. Similarly, a voltage applied across one set of faces of the
crystal causes mechanical distortion of the crystal. Every crystal has its own natural
resonant frequency and is very precise. Crystal can be electrically represented as
below
Crystal exhibits series resonant frequency at which its impedance is minimum and
parallel resonant frequency at which its impedance is maximum. The circuit in Figure
2 uses parallel resonant mode. The tank circuit formed by crystal is connected in the
feedback path of an amplifier to satisfy Barkhausen criteria.
The advantage of crystal oscillator is its precise resonant frequency and hence the
oscillations are very stable. Crystals do not wear out very easily and as a result the
oscillator incorporating crystals can operate for a long time. However, any change in
frequency requires replacement of crystal and provides very little flexibility for tuning
ie., can be tuned only between crystal’s series and parallel resonant frequencies
which are usually very close.
Figure 2: Crystal oscillator circuit

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Experiment 4

RC Phase Shift Oscillator

Figure 1: BJT RC Phase Shift Oscillator

An ideal RC phase shift oscillator circuit is shown below.

Frequency can be adjusted by varying either the capacitors or resistors of RC

𝑘 < 2.7 𝑎𝑛𝑑 ℎ𝑓𝑒 > 44.5

10. Choose lower cut off frequency fLS fLS = 100Hz

b. Feedback Network Design

Desired frequency of oscillations f = __________________

Measured frequency of oscillations fmeasured = _______________

Sample Viva Questions

The circuit of crystal oscillator is shown in Figure 2. A Common Emitter amplifier

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