Astable Multivibrator using transistors – Transistorised Circuit wave form and operation

Transistorized Astable Multivibrator is a cross coupled transistor network capable of producing sharp continuous square wave. It is free running oscillator or simply a regenerative switching circuit using positive feedback. Astable Multivibrator switches continuously between its two unstable states without the need for any external triggering.
Time period of Astable multivibrator can be controlled by changing the values of feedback components such as coupling capacitors and resistors. NE 555 IC based astable multivibrator circuit is already explained in the previous article. Here we are dealing with Transistorised Astable Multivibrator. Animation of working circuit is also provided with this article it makes easy to understand the basic working principle behind Astable multivibrator.
Also read: How transistor acts as a switch 
555 Timer Astable calculator for Astable Multivibrator Designing

Circuit diagram of Transistorised Astable Multivibrator

Components required

  • Resistors (1KΩx2, 1MΩx2)
  • Capacitors (1µFx2)
  • Transistors (BC548x2)
  • LEDs
  • DC power supply

Working principle

  • Assume anyone of the transistors Q1 or Q2 turns ON due to parameter variation or due to some switching transients, let it be Q1.
  • Then the collector voltage of Q1=Vce(sat)=0.2V, it is cross coupled to base terminal of Q2 through C1, then Q2 remain in OFF state.
  • During Q1 ON, the current path through R1 charges the capacitor C1, the capacitor C1 voltage is coupled to base of transistor Q2.
  • While charging of C1, when the capacitor voltage exceeds 0.7V, Q2 become turns ON.
  • As soon as Q2 ON, its collector voltage falls to Vce(sat)=0.2V, it is coupled to base terminal of Q1 then Q1 become OFF.
  • At the same time capacitor C2 starts charging through R2, when the C2 voltage exceeds 0.7 V, Q1 turns ON due to cross coupling.
  • This process continues.
  • We can also take output from collector terminals of the transistors, such circuit is show below. To know the working of below circuit you must read How transistor acts as a switch?

Output waveform

This is the output taken from the collector terminal of Q1

Design of Transistorised Astable Multivibrator

If the reference transistor is Q1, Time period of positive cycle of transistorised astable multivibrator is given by,

Ton=0.69 R1xC1 sec 

Toff=0.69R2xC2 sec 

We can also take output from Q2, if so 

Ton=0.69 R2xC2 sec 

Toff=0.69R1xC1 sec 

In my article, the circuit is designed for 0.6 sec ON and 0.6 sec OFF 

Assume C1=C2=1µF 

Then R1=R2=1MΩ 


Ton=0.69x1MΩx1µF=0.69 sec 

Toff=0.69x1MΩx1µF=0.69 sec

Time Period (T)

T=Ton + Toff 

  = 0.69 (R1C1+R2C2) 

Frequency (F)

Duty cycle (D)

i-St@r Lab

37 thoughts on “Astable Multivibrator using transistors – Transistorised Circuit wave form and operation

  1. can u plz give there working when we use complementry Q1 is npn and Q2 is pnp then how it work and diagram alsoo….

  2. Hi,
    Very gud explanation!!! but i have some doubts,
    1.Can You tell me why do we consider only 0.69 constant in delay i.e.,
    Ton=0.69*R1C1 or Toff=0.69*R2C2
    2.Which transistor will conduct first and how, since we are using the same transistors and RC value.(Based on what parameters).
    Thanks in advance

  3. I think you’ll find that the capacitor polarity is back-to-front…… in an NPN based multivib, the collector voltage is always higher than the base voltage of the other device. Also a diode in series with each transistor base will prevent reverse breakdown damage to the base-emitter junctions, if using a high enough supply voltage to induce this problem.

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