SV: Re: [sdiy] stupid ebers-moll question

[René Schmitz]

Hi Karl and all,

Karl Ekdahl wrote:
> Well, i'm looking at your expo-tutorial actually,
> trying to figure out how the expo-converter in the
> 4069 works but i can't really figure out stuff without
> the Is parameter. Since info about the Is parameter
> seems extremely hard to come by, does it get's
> cancelled out somehow by allways using two transistors
> thus getting the Is parameter twice (assuming they're
> the same for both transistors)? I'm really confused...

Yes, it disappears. Ic=Is*exp(Vbe*q/(kT)). If you set up two of these 
equations, one for each transistors, and solve for delta Vbe, you need 
to divide Is1/Is2. If you assume that they are the same, then it turns 
into unity (or if the thermal behaviour of Is is equal, then some other 
constant).

Cheers,
  René

P.S.: Now I see why its really Icb0 and not Ice0 which is equal to Is.
With all three transistor layers there is some additional transistor 
action by carriers injected to the base, which raise Ice0 above Icb0.

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159