Astable Multivibrator

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.


  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.


  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


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.


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 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. 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 = 0.1F & 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 ICmax=10mA

R=  = 1.17K ( 1K is selected for Rc1 and Rc2)


  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


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?
  2. Define quasi stable state?
  3. Is it possible to change time period of the waveform with out changing R & C?
  4. Explain charging and discharging of capacitors in an Astable Multivibrator?
  5. How  can an Astable multivibrator be used as VCO?
  6. Why do you get overshoots in the Base waveforms?
  7. What are the applications of Astable Multivibrator?
  8. How  can Astable multivibrator be used as a voltage to frequency converter?
  9. What is the formula for frequency of oscillations?


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