Le Chaud Lapin
- Jan 1, 1970
Kevin Aylward said:Well, I wouldn't go that far;-) I am really just trying to correct this
incredible widespread misconception that surrounds basic transistor
operation. Its potential differances that make charge move.
Base current is obviously something that needs to be accounted for.
Right off the bat, if your doing a power amp, you have to work out the
drive current from Iload/Hfemin.
Kevin is right. There is a subtlety here, and if you're not
quasi-anal about how you let knowledge to get into your head, you
might miss it.
When I first learned semiconductor physics, I remember being almost
offended at one professor's proclamation that the base current
"controls" the collector current. This simply isn't true. First, the
base current is smaller than the collector current, and second, if you
remove the application of potential from collector to emitter, you can
still get current flow, but the original model breaks down. What
Is there a relationship between IB and IC?? Yes, of course. But the
purist/fundamentalist quickly sees there the base current doesn't
really "control" anything, and the charge distribution model is for
more appropriate. Here, the differnce in potential causes a certain
distrubution of chage in a 3-D space on all sides of the P-N
junctions, and there is drift, diffusion, and recombination of this
charge leading to current flow based upon the application of
Gauss/Poissons/Maxwell/Einstein/Fick's Laws, and it is this special
arrangement from which the flow of current is determined.
So the second intepretation is intuitive and permits much greater
insight than the other, and allows the engineer to remain useful when
someone starts taking the model and twisting it around, as did one
astute professor who understood the mechanics so well that he would
take circuits and rearrange the transistors in ways few people would
dare try, then still managed to predict with suprising accuracy the
behavior of the circuit without help from a computer. My guess is
that he was relying on the voltage-control (charge distribution)
model, not a neatly packaged concept of Bf.