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astable multivibrator

Discussion in 'General Electronics Discussion' started by Ricardo7890, Aug 14, 2012.

  1. Ricardo7890

    Ricardo7890

    6
    0
    Jul 17, 2012
    In this circuit i don't understand why the the base of both transistors aren't fed by the r2 and r3 resistor, it doesn't make any sense.
     

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  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,689
    Jan 21, 2010
    Google it. There are many good explanations on the internet.

    The basic thing is that turning one transistor on turns the other one off for a while, after that the other one can turn on, which turns the other one off.

    Yes, in theory, if both transistors wee identical and all parts were exactly matched, you could turn both transistors on simultaneously, in practice it doesn't happen (and you stay away from component values the would make it easier)
     
  3. Ricardo7890

    Ricardo7890

    6
    0
    Jul 17, 2012
    i haven't been able to find any explanation why the r2 and r3 resistors don't supply the base directly, they're in series with those terminals, and i can' figure out how the capacitors are relevant at all
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    The capacitors either provide base current or rob current from the base depending on the state of the other transistor.

    Understanding the capacitors is at the heart of understanding how this circuit works.

    I recommended you google this device. It appears you did not take that advice.

    The first hit on Google is this. Read it.
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    9,134
    1,843
    Nov 17, 2011
    But these resistors do supply the bases. They are directly connected. *steve*'s link gives a thorough explanation.
     
  6. john monks

    john monks

    693
    1
    Mar 9, 2012
    I believe I see your confusion. What happens when both transistors are in a state of "saturation" or "fully on" at the same time? Let's look at this junction by junction.
    Q1 base is sitting at .7 volts and Q8 base is sitting at .7 volts.
    Now the collectors are sitting at about 0.1 volts.
    They are said to be in "saturation". But are they really? We need to look more carefully at the characteristic curves of the transistors.
    Suppose you increase the voltage on the base of Q1 by a slight amount. Doesn't the collector change by even more than that? You find that it generally does. This voltage changes by a slightly more amount and in the opposite direction. This gets fed into the base of Q2, gets increased and inverted and fed back to the base of Q1 and the circuit takes off. That is what is called a "positive feedback".
    This characteristic of transistors is rarely highlighted in datasheets but never the less is very real. And for the circuit to take advantage of this characteristic the base resistor (R2 and R3) is usually about ten times the value of the collector resistor (R1 and R4). For if you made these resistors to close to each other in value you would see a situation in which both transistors would go into saturation and the circuit would "lock up".
     
    Last edited: Aug 15, 2012
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