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how much is the current in a simple bjt circuit?

Discussion in 'General Electronics Discussion' started by cyjfox, Jul 23, 2010.

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  1. cyjfox


    Jul 23, 2010
    consider a very simple circurt.
    if Vbc of a npn transistor is zero and Vbe is 5V,how much is each current of the bjt?
    I used the following formula.
    Icc = IS[exp(qVbe/kT)-1]
    Iec = IS[exp(qVbc/kT)-1]
    Ict = Icc - Iec
    IE = -Ict-Icc/BF
    IC = Ict-Iec/BR
    IB = Icc/Bf+Iec/BR
    IS is the saturation current and value is 2e-16
    BF is the forward beta and value is 200
    BR is the reverse beta and value is 0.1
    so i calculated:
    let q/kT = 25mv then qVbe/kT is 200
    exp(200) = 7.22597377e86(sucn a huge number)
    exp(200)-1 is almost the same value = 7.23e86
    Icc = 1.446e81
    since Vbc = 0,Iec = 0
    Ict = Icc-Iec = Icc
    so i concluded:
    IE and IC is almost 1.446e81 which seemed impossible(too huge).
    so,what am i doing wrong?
    thanke you.
  2. Resqueline


    Jul 31, 2009
    It might be correct for all I know, but look at a real-world Vbe versus Ib (or Vf versus If of a diode) graph instead of those theoretical formulae and see if it clears up a little.
    All I know is that applying 5V (with unlimited current resources) to the b-e of a bjt will destroy it instantly..
  3. cyjfox


    Jul 23, 2010
    Really thank you,Resqueline
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