Astable multivibrator

Discussion in 'General Electronics Chat' started by chersam1176, Jan 21, 2014.

  1. chersam1176

    Thread Starter New Member

    Jan 15, 2014
    Hi Experts,
    I recently joined this forum.
    I was going through a circuit for astable multivibrator, but wonder how it works since there is no charging path for capcitor when either of transistor is ON. Can any body explain how it woks?

    Last edited by a moderator: Jan 22, 2014
  2. BillB3857

    Senior Member

    Feb 28, 2009
    If you notice, both transistors have bias resistors tied to their base leads. Since no two devices are identical, one or the other will have slightly higher gain than the other. The one with the highest gain will start to conduct slightly more than the other. That slight difference results in the collector of that transistor starting to drop voltage slightly. That slight drop is coupled via the capacitor to the base of the opposite transistor tending to lower its emitter/collector conduction. That drop in conduction causes that transistors collector voltage to rise. That rise is coupled to the first transistor causing it to conduct even harder. Once it reaches saturation, there is no longer a change of voltage to be coupled through the capacitors, and as they start to equalize, the situation reverses.
    Another explanation would be thermal noise within the transistors is amplified and the one with the most noise will be the first to start the conduction process.

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    chersam1176 likes this.
  3. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    hi cher,
    In addition to Bill's description.
    I would not recommend the Bistable configuration you have used, it will work, but as you can see in the plots compared for your circuit, type 'B' and the 'usual type 'A' configuration the waveforms are cleaner and swing to 0V for the 'A' circuit.
    chersam1176 likes this.
  4. chersam1176

    Thread Starter New Member

    Jan 15, 2014
    Hi ,
    Thanks for quick reply. That was quite useful.