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, 10K)                                              -           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                    
9. Bread board

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 V_C2 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, V_C1 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 V_CC exponentially with a time constant T₁ = 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. V_C2 increases and makes Q₁ fully saturated. During this time C₂ has been charging through V_CC, 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 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:

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   C = 0.1F

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 .

2. Define quasi stable state?

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

3. 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.

4. 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.

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

 Ans: when Q₂ is ON capacitor C₂ charges through resistor RC ₁ from V_cc 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.    

6. 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

7.  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.

8. 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.

9. 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.

10. Which multivibrator has two quasi-stable states?

Ans: Astable multivibrator

11. 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)  

12. 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% h_{fe min)}=40, V_CC=12V, I_C(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.