Thursday, August 02, 2012

Astable Multivibrator

Transistorised 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.

Astable Multivibrator working





























  • 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.

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