Electronic Circuit Analysis & Design - II RC phase shift oscillator

Department of Electronics Engg., P.V.P.I.T., Budhgaon

Circuit Diagram:

RC Phase Shift Oscillator Circuit Diagram

Electronic Circuit Analysis & Design - II RC phase shift oscillator

Department of Electronics Engg., P.V.P.I.T., Budhgaon

Title: RC phase shift oscillator.

Aim: To Study and design RC phase shift oscillator.

Equipments: CRO, Power supply.

Components: Resistors, Capacitors, Transistors.

Theory:

Any circuit which is used to generate a periodic voltage ac input signal is called

oscillator. To generate the periodic voltage the circuit is supplied with energy from a dc

oscillator is called as sinusoidal or harmonic oscillator.

In some systems, output of amplifier is feedback to the input in such a way, so

as to reduce input. It results in reduction of overall gain, stability increases, distortion

reduces, bandwidth increases, input resistance increases & output resistance

decreases. This is termed as negative feedback. The result of positive feedback is

oscillations. Oscillator provides a source of ac voltage at a desired frequency, thus

oscillator provides sinusoidal output at frequencies varying from audio band up to

several Giga Hz (GHz) i.e. (20 KHz – 3 GHz).

An example of an oscillator circuit that follows the basic development of a

feedback circuit is the phase-shift oscillator. This circuit is as shown in Figure. Recall

that the requirements for oscillation are that the loop gain, Av, is greater than unity and

that the phase shift around the feedback network is 180° (providing positive feedback).

In the present idealization, we are considering the feedback network to be driven by a

perfect source (zero source impedance) and the output of the feedback network to be

connected into a perfect load (infinite load impedance).

Barkhausen Criteria:

In practice it is not sufficient to just make Av=1 in equation.

βAv is made larger then 1, As β usually cannot be greater than 1 (as it is attenuation

network) then βAv≥1 means Av≥ .The output is relatively free from distortion βAv

should be only slightly >1. Consider diagram in which amplifier causes phase shift of

180⁰ between Vi & Vo (Av is negative in single stage common emitter amplifier).

Electronic Circuit Analysis & Design - II RC phase shift oscillator

Department of Electronics Engg., P.V.P.I.T., Budhgaon

Observations:

Frequency of Sine wave: _______________

Amplitude of Sine wave: _______________ V p-p

Electronic Circuit Analysis & Design - II RC phase shift oscillator

Department of Electronics Engg., P.V.P.I.T., Budhgaon

In order to provide positive feedback the feedback network

1. Must provide positive phase shift of 180⁰ so as to provide signal that can be

replace Vi.

2. Not more attenuation than since KAv must be ≥1.

This condition of unity look gain Av=1 is called Barkhausen criteria with this criteria is

satisfied system can be closed loop and output will continue.

Av=1 at an angle 0⁰.

Working:

In these oscillators required phase shift of 180⁰ in different loop from output to

input is obtained by using R & C components. In this circuit common emitter amplifier is

followed by 3 sections of RC phase shift network. The output of last RC section is being

returned to input. In order to make 3 RC sections identical R is chosen as R’=R - Ri.

Where, Ri=Input impendence circuit.

If values of R & C are selected such that, for given frequency it will give phase

shift of is 60⁰ for each RC section. Thus such ladder network produces total phase shift

of 180⁰ between input & output voltage for given frequency. Therefore at special

frequency fo, total phase shift from base of transistor around the circuit & back to base

in exactly 360⁰ or 0⁰, thereby satisfying barkhausen condition for oscillations.

The frequency of oscillation is,

fo =

The mean gain required to sustain oscillation is,

hfe(min)=

The mean gain required should be at least 29 if we select R=Rc the minimum hfe

is 56. If the ratio is changed, required gain of hfe(min) should produce distorted

output.

Electronic Circuit Analysis & Design - II RC phase shift oscillator

Department of Electronics Engg., P.V.P.I.T., Budhgaon

Procedure:

1. Design the given problem carefully.

2. List out the component values and take the components.

3. Mount the components on Bread Board (BB), do the connection as per the circuit

diagram.

4. Check the connections before applying power supply.

5. Show the connections to Lab Instructor and verify the connections.

6. Apply the power supply connection and switch ON the power supply.

7. Observe the waveform on C.R.O./D.S.O.

8. Note down the readings (frequency and amplitude) carefully.

9. Plot the graph on graph paper.

10. Disconnect the power supply and components.

Electronic Circuit Analysis & Design - II RC phase shift oscillator

Department of Electronics Engg., P.V.P.I.T., Budhgaon

Problem:______________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

Solution:

Electronic Circuit Analysis & Design - II RC phase shift oscillator

Department of Electronics Engg., P.V.P.I.T., Budhgaon

Electronic Circuit Analysis & Design - II RC phase shift oscillator

Department of Electronics Engg., P.V.P.I.T., Budhgaon

Electronic Circuit Analysis & Design - II RC phase shift oscillator

Department of Electronics Engg., P.V.P.I.T., Budhgaon

Conclusion:

The RC phase shift oscillator is used to generate a sine wave of required

frequency and amplitude. The amplitude is ________ and frequency is _________.