Astable Multivibrator
Prior to the Lab session:
 Study the operation and working principle Astable Multivibrator.
 Study the procedure for conducting the experiment in the lab.
Objectives:
 To study the operation and observe the wave forms of Astable Multivibrator.
 To Design an Astable Multivibrator to generate a square wave of 1KHz frequency using Transistor.
Apparatus:
 CRO 0 to 20 MHz (Dual Channel)  1 No.
 Function Generator 1Hz to 1 MHz  1 No.
 Bread board  1 No.
 Resistor (1K, 10K)  2 Nos. each
 Capacitors (0.1µF)  2 No.’s
 Transistor (BC 107)  2 No.’s
 Regulated D.C Power Supply 0 to 30V (dual)  1 No.
 Connecting wires
 Bread board
Circuit diagram:
Fig. 1.1 Astable Multivibrator 
Theory:
The Astable circuit has two quasistable states. Without external triggering signal the Astable configuration will make successive transitions from one quasistable state to the other. The Astable circuit is an oscillator. It is also called as free running multivibrator and is used to generate “Square Wave”. Since it does not require triggering signal, fast switching is possible.
Operation:
When the power is applied, due to some imbalance in the circuit, the transistor Q_{2} conducts more than Q_{1} i.e. current flowing through transistor Q_{2} is more than the current flowing in transistor Q_{1}. The voltage V_{C2} drops. This drop is coupled by the capacitor C_{1} to the base by Q_{1} there by reducing its forward baseemitter voltage and causing Q_{1} to conduct less. As the current through Q_{1} decreases, V_{C1} rises. This rise is coupled by the capacitor C_{2} to the base of Q_{2}. There by increasing its base emitter forward bias. This Q_{2} conducts more and more and Q_{1} conducts less and less, each action reinforcing the other. Ultimately Q_{2} gets saturated and becomes fully ON and Q_{1} becomes OFF. During this time C_{1} has been charging towards V_{CC} exponentially with a time constant T_{1 }= R_{1}C_{1}. The polarity of C_{1} should be such that it should supply voltage to the base of Q_{1}. When C_{1} gains sufficient voltage, it drives Q_{1} ON. Then VC1 decreases and makes Q_{2} OFF. V_{C2} increases and makes Q_{1} fully saturated. During this time C_{2} has been charging through V_{CC}, R_{2}, C_{2} and Q_{2} with a time constant T2 = R_{2}C_{2}. The polarity of C_{2} should be such that it should supply voltage to the base of Q_{2}. When C_{2} gains sufficient voltage, it drives Q_{2} On, and the process repeats.
Design Procedure:
The period T is given by
T = T_{1} + T_{2} = 0.69 (R_{1}C_{1} + R_{2}C_{2})
For symmetrical circuit, with R_{1} = R_{2} = R & C_{1} = C_{2} = C
T = 1.38 RC
Let V_{CC} = 12V; h_{fe} = 51 (for BC107), V_{BESat} = 0.7V; V_{CESat} = 0.3V Let C = 0.1F & T = 1mSec.
10^{3} = 1.38 x R X 0.1 X 10^{6}
R = 7.24K (Practically choose 10K) i.e., R1 and R2 resistors.
Let I_{Cmax}=10mA
R_{C }= = 1.17K ( 1K is selected for Rc1 and Rc2)
Procedure:
 Make then connections as per the circuit diagram.
 Observe the Base Voltage and Collector Voltages of Q1 & Q2 on CRO in DC mode and measure the frequency (f = 1/T).
 Trace the waveforms at collector and base as each transistor with the help of dual trace CRO and plot the waveforms.
 Verify the practical output frequency with theoretical values f = 1/T, where T = 1.38 RC
Expected Waveforms:
Theoretical calculations: F = 1/ T = (1/1.38RC)
R = 10K C = 0.1F
Result:
An Astable Multivibrator is designed; the waveforms are observed and verified the results theoretically.
Viva questions:
 What are the other names of Astable multivibrator?
 Define quasi stable state?
 Is it possible to change time period of the waveform with out changing R & C?
 Explain charging and discharging of capacitors in an Astable Multivibrator?
 How can an Astable multivibrator be used as VCO?
 Why do you get overshoots in the Base waveforms?
 What are the applications of Astable Multivibrator?
 How can Astable multivibrator be used as a voltage to frequency converter?
 What is the formula for frequency of oscillations?
Outcomes:
After finishing this experiment students are able to design Astable Multivibrator and explain the operation of the same.

UpdatedJan 07, 2014

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