Department of Electronics Engg., P.V.P.I.T., Budhgaon Circuit Diagram:
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).
Department of Electronics Engg., P.V.P.I.T., Budhgaon Observations:
Frequency of Sine wave: _______________ Amplitude of Sine wave: _______________ V p-p
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.
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.
Department of Electronics Engg., P.V.P.I.T., Budhgaon Problem:______________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ Solution:
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 _________.
