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how much is the current in a simple bjt circuit?

cyjfox

Jul 23, 2010
2
Joined
Jul 23, 2010
Messages
2
hello,everyone.
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.
let
Icc = IS[exp(qVbe/kT)-1]
Iec = IS[exp(qVbc/kT)-1]
Ict = Icc - Iec
then
IE = -Ict-Icc/BF
IC = Ict-Iec/BR
IB = Icc/Bf+Iec/BR
where
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.
 

Resqueline

Jul 31, 2009
2,848
Joined
Jul 31, 2009
Messages
2,848
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..
 
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