astable multivibrator circuit!! ( about frequency, and Vcc)

Discussion in 'Homework Help' started by lemoneyes, May 20, 2009.

  1. lemoneyes

    Thread Starter New Member

    Apr 20, 2009
    6
    0
    I had a experiment about astable multivibrator circuit
    ( especially, interest in frequency )
    i attached a picture(involved circuit and equation).
    in that picture,, equation about frequency is shown....
    after experiment,, i found frequency is diffrent with Vcc value !!!
    but, Vcc is not related with equation!!!

    why is this happen??
    thanks, guys
     
    Last edited: May 20, 2009
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    Actually you have it slightly off. The discharge is from the base bias resistor (say Rb2) through the cap (C1) to ground. The drawing is a bit off from how I learned it, but I'm having trouble placing it.

    But then, my memory is old on the subject...

    http://en.wikipedia.org/wiki/Astable_multivibrator
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The frequency changes and the transistors might be damaged if the supply voltage is higher than about 6V which is the max allowed reverse emitter-base voltage of silicon transistors.

    The capacitors cause the base of the transistors to go to a negative voltage. If the voltage is more than about 6V then the emitter-base junction experiences avalanche breakdown that clamps and discharges the capacitor.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    If we make a few assumptions:

    1. Vcc < Vbe(breakdown), which Audioguru pointed out
    2. Rc >≈ 5*RL
    3. Vce(sat)≈0

    Then T (half cycle) =-RbC*ln[(Vbe-Vcc)/Vbe-2Vcc)].
    If Vbe were zero, then T=-RbC*ln(1/2).
    But Vbe≠0, which is the point made by sirkituk.
     
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