Astable Multivibrator


  To design and simulate an Astable Multivibrator circuit.




EDWin Components Used


Number of components required

BC107 BC107A Transistor 2
RES RC05 Resistor 4
CAP CASE-A600 Capacitor 2
VDC SMB_VDC Dc voltage source 1
GND SMB_SPL0 Ground 1



  Astable Multivibrator is a two stage switching circuit in which the output of the first stage is fed to the input of the second stage and vice versa.

  The outputs of both the stages are complementary. This free running multivibrator generates square wave without any externaltriggering pulse.

  The circuit has two stable states and switches back and forth from one state to another, remaining in each state for a time depending upon  

  thedischarging of the capacitive circuit.

  The multivibrator is one form of relaxation oscillator, the frequency of which may be controlled by external synchronizing pulses.

  In our experiment we are using transistor, as the amplifying device and also it is a collector coupled multivibrator.

  Figure shows the basic symmetrical astable multivibrator in which components in one half of a cycle of the circuit are identical to their counterpart

  in the other half. Square wave output can be obtained from the collector point of Q1 or Q2.


  When supply voltage, VCC is applied, one transistor will conduct more than the other due to some circuit imbalance. Initially let us assume that Q1

  is conducting and Q2 is cut-off. Then VC1, the output of Q1 is equal to VCESAT which is approximately zero and VC2 is equal to VCC. At this instant

  C1 charges exponentially with the time constant R1C1 towards the supply voltage through R1 and correspondingly VB2 also increases

  exponentially towards VCC. When VB2 crosses the coupling voltage Q2 starts conducting and VC2 falls to VCESAT. Also VB1 falls due to capacitive

  coupling between collector of Q2 and base of Q1, thereby driving Q1 into OFF state. The rise in voltage VC1 is coupled through C1 to the base

  of Q2 causing a small overshoot in voltage VB2. Thus Q1 is OFF and Q2 is ON. At this instant the voltage levels are:

  VB1 is negative, VC1=VCC, VB2=VBESAT and VC2=VCESAT.

  When Q1 is OFF and Q2 is ON the voltage VB1 increases exponentially with a time constant R2C2 towards VCC . Therefore Q1 is driven to 

  saturation and Q2 to cut-off. Now the voltage levels are:

  VB1=VBESAT, VC1=VCESAT, VB2 is negative and VC2=VCC.

  From the above it is clear that when Q2 is ON the falling voltage VC2 permits the discharging of capacitor C2 which inturn drives Q1 into

   cut-off. The rising voltage of VC1 is fed back to the base of Q2 tending to turn it ON. This process is regenerative.

 Derivation of time period

  The charging equation for a capacitor is given by

  Capacitor voltage,


where VC - the capacitor voltage,

VINIT – the initial capacitor voltage,

VFIN – the final capacitor voltage

t – the time period of charging.

R and C – the resistor and capacitor through which charging occurs.

The capacitor discharges from –VCC to VCC.

Therefore VIN=(-VCC), VFIN=VCC, VC@ 0V.

Substituting this in equation (2)

  Taking natural logarithm

  For a symmetrical astable multivibrator

  Charging and discharging time periods are given by

From equation (5)

where T is the total time period.

  Since the multivibrator is symmetrical


Design Specifications

Manufacturer’s specifications

  Applying KVL for the collector side of Q2.

since it is a symmetrical astable multivibrator

  Applying KVL for the base loop of the circuit

since it is a symmetrical astable multivibrator.

  Design of C

The total time period T is given by

since it is a symmetrical astable multivibrator.

From equations (1) and (2)

The free running frequency is given by

Assume the frequency as 100Hz.




   EDWin 2000 -> Schematic Editor: The circuit diagram is drawn by loading components from the library. Wiring and proper net

   assignment  has been made. The values are assigned for relevant components.

   EDWin 2000 -> Mixed Mode Simulator: The circuit is preprocessed. The desired test points and waveform markers are placed. The Transient 

   Analysis parameters have been set. The Transient Analysis is executed and output observed in Waveform Viewer.


   The output waveform may be observed in the waveform viewer.