# Astable Multi Vibrator

Prior to the Lab session:

1. Study the operation and working principle Astable Multivibrator.
2. Study the procedure for conducting the experiment in the lab.

Objectives:

1. To study the operation and observe the wave forms of Astable Multivibrator.
2. To Design an Astable Multivibrator to generate a square wave of 1KHz frequency using Transistor.

Apparatus:

1. CRO 0 to 20 MHz (Dual Channel)                               -           1 No.
2. Function Generator 1Hz to 1 MHz                              -           1 No.
3. Bread board                                                             -           1 No.
4. Resistor (1K$\Omega$, 10K$\Omega$)                                              -           2 Nos. each
5. Capacitors (0.1µF)                                                    -           2 No.’s
6. Transistor (BC 107)                                                   -           2 No.’s
7. Regulated D.C Power Supply 0 to 30V    (dual)            -          1 No.
8. Connecting wires

Circuit diagram:

 Fig. 1.1 Astable Multivibrator

Theory:

The Astable circuit has two quasi-stable states. Without external triggering signal the Astable configuration will make successive transitions from one quasi-stable 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 Q2 conducts more than Q1 i.e. current flowing through transistor Q2 is more than the current flowing in transistor Q1. The voltage VC2 drops. This drop is coupled by the capacitor C1 to the base by Q1 there by reducing its forward base-emitter voltage and causing Q1 to conduct less. As the current through Q1 decreases, VC1 rises. This rise is coupled by the capacitor   C2 to the base of Q2. There by increasing its base- emitter forward bias. This Q2 conducts more and more and Q1 conducts less and less, each action reinforcing the other. Ultimately Q2 gets saturated and becomes fully ON and Q1 becomes OFF. During this time C1 has been charging towards VCC exponentially with a time constant T= R1C1. The polarity of C1 should be such that it should supply voltage to the base of Q1. When C1 gains sufficient voltage, it drives Q1 ON. Then VC1 decreases and makes Q2 OFF. VC2 increases and makes Q1 fully saturated. During this time C2 has been charging through VCC, R2, C2 and Q2 with a time constant T2 = R2C2. The polarity of C2 should be such that it should supply voltage to the base of Q2. When C2 gains sufficient voltage, it drives Q2 On, and the process repeats.

Design Procedure:

The period T is given by

T = T1  + T2 = 0.69 (R1C1  + R2C2)

For symmetrical circuit, with R1 = R2 = R & C1 = C2 = C

T = 1.38 RC

Let VCC  = 12V; hfe = 51 (for BC107), VBESat  = 0.7V; VCESat  = 0.3V Let C = 0.1$\mu$F & T = 1mSec.

10-3 = 1.38 x R X 0.1 X 10-6

R = 7.24K$\Omega$ (Practically choose 10K$\Omega$) i.e., R1 and R2 resistors.

Let ICmax=10mA

R$\frac{V_{cc} - V_{cesat}}{I _{cmax}} = \frac{12 - 0.3}{0.01}$ = 1.17K$\Omega$ ( 1K$\Omega$ is selected for Rc1 and Rc2)

Procedure:

1. Make then connections as per the circuit diagram.
2. Observe the Base Voltage and Collector Voltages of Q1 & Q2 on CRO  in DC mode and  measure the frequency (f = 1/T).
3. Trace the waveforms at collector and base as each transistor with the help of dual trace CRO and plot the waveforms.
4. 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$\Omega$   C = 0.1$\mu$F

Result:

An Astable Multivibrator is designed; the waveforms are observed and verified the results theoretically.

Viva Questions:

1. What are the other names of Astable multivibrator?

Ans: Free running oscillator,Square wave generator  and Relaxation oscillator .

1. Define quasi stable state?

Ans:The state which is unstable or semi stable is called quasi stable state.

1. Is it possible to change time period of the waveform with out changing R & C?

Ans:Yes it is possible,Astable multivibrator is called voltage controlled oscillator and volatage to frequency converter by changing input voltage we can change output frequency and time period.

1. Why Astable multivibrator is called free running oscillator?

Ans: Astable multivibrator is called as Free-running Multivibrator as they do not require any additional inputs or external assistance to oscillate. Astable oscillators produce a continuous square wave from its output or outputs, which can then be used to flash lights.

1. Explain charging and discharging of capacitors in an Astable Multivibrator?

Ans: when Q2 is ON capacitor C2 charges through resistor RC 1 from Vcc and the capacitor c1  discharges through resistor R 1, when Q1 is ON and Q2  is OFF the capacitor C1 charges through

resistor RC2 and capacitor  C2  discharges  through resistor  R2.

1. How  can an Astable multivibrator be used as VCO?

Ans: it converted into VCO by changing its   1. Voltage comparision levels. 2. Charging/discharging current by a controlling voltage

1.  What are symmetrical triggering and unsymmetrical triggering?

Ans: symmetrical triggering is one in which each successive triggering signal induces a transition regardless of the state in which binary happens to be,, I,e  symmetrical triggering requires only one  source unsymmetrical triggering is one which the  triggering signal is effective in inducing a transition in only one direction.

1. What are the applications of Astable Multivibrator?

Ans: Astable and analog circuits multivibrators are used in amateur radio equipment to receive and transmit radio  signals .These   are also used in morse code generators, timers, and systems that require a square wave, including television broadcasts.

1. How  can Astable multivibrator be used as a voltage to frequency converter?

Ans:A voltage-controlled oscillator (VCO) is one in which the frequency of oscillations varies as a function of voltage. The same circuit is also called a voltage-to-frequency converter (VFC) because a given voltage gives rise to a specific frequency.

1. Which multivibrator has two quasi-stable states?

Ans: Astable multivibrator

1. What is the formula for frequency of oscillations? Design R and C for a frequency of 2KHz of a symmetric Astable oscillator.

Ans:  F= 1/ T        F=  (1/1.38RC)

1. What is duty cycle?

Ans:  It is defined as the ratio of conducting time period to total time period of the cycle.

Design problems

1. Design a collector coupled astable multivibrator using 2-BC107 transistors to operate at 1.5KHz and with a duty cycle of 45% hfe min)=40, VCC=12V, IC(sat)=10mA.
2. Design voltage to frequency converter using astable multivibrator.

Outcomes: After finishing this experiment students are able to design Astable Multivibrator and explain the operation of the same.

• Updated
Aug 21, 2019
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12,683
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