John Sorensen said:
This is fundamentally wrong. BJTs are current control devices.
okay lets talk about fundamentals.
why is a transistor called a transistor???
well, that because a transistor "transfers resistance"!!!!!
okay so what is resistance??
from Ohm's Law we have R = V/I which means that when you change the voltage across a resistor the current changes proportionally. so a change in voltage results in a change in current.
okay now what is transfer of resistance??
resistance is transfered when a change in voltage between two points results in a change in the current at some third point. so its basically just like resistance but the change in voltage and current is happening at different places.
thats exactly what happens in a transistor. a voltage across two terminals controls the current through the third terminal.
so does that mean that the equation Ic = BETA * Ib is wrong??? no, it isnt. it isnt wrong but it is just an approximation. it works in explaining alot of applications of transistors and thats why it is used so much. but that doesnt mean that a transistor is a current amplifier. to understand differential amplifiers, logarithmic converters, temperature compensation and other important applications you must think of a transistor as a transconductance device - collector current is determined by the base emitter voltage. and according to the Eber Molls model of the transistor
Ic = Is [ exp( Vbe/VT ) - 1 ]
where Is is the saturation current and VT is the thermal voltage.
the quantity Ib is just a by-product of the whole transistor-action. its something like who came first? the chicken or the egg? :lol:
and if you remember the voltage divider bias amplifier (VDB) you would remember that Ib and BETA are eliminated from the calculations. when designing a good amplifier Ib must be zero. so now i ask you that lets say you have a control panel consisting of Ib, Is, BETA, VT etc and lets name that control panel a "transistor" and the manual of this control panel says that keep Ib = zero for an optimum performance then how would you say that Ib is a controlling factor. either it is the controlling factor or it is an un-necessary factor in the whole process. thats up to you to decide.
now i know that it Ib can never be zero in a BJT. but thats the defficiency of the transistor. if it had been ideal there werent be any Ib. so if you see that Ib is always present then that doesnt mean that Ib is the controlling factor.
one thing more, do you know why Ib should be kept zero. because if Ib is zero then all the electrons entering from the emitter will go to the collector and become the collector current (in an NPN transistor) but if some of the electrons combine with the holes of the base they wont become the collector current. thats why Ib should be zero.
for further reference read the book "The Art of Electronics" from page 79 and onward.
and read the article
http://amasci.com/amateur/transis.html
i hope that helps