MrAl,
Yes, and you can move the steering wheel by twisting the tires when the car is on a hoist. But who does that except a mechanic?
The transistor is not a car. The transistor is an electronic component. Strange though, you seem to say that no one would want to move the steering wheel that way yet you mention at least one person that would anyway. Which is it?
There are many ways to model a transistor, but forgetting something is not my style.
You dont really have to forget, you simply have to take a fresh look at it, as you do with any black box.
It is a matter of perspective and the region of operation. Sure, there is a one to one correspondence between, say Ib and Ic in the active region. You cannot change one without the other. But it is usually assumed that in the active region, Ib is the functional control.
That's just the opposite. You can change one without the other. That's why i mentioned the black box and taking a fresh look at this. The key word there too is "usually". That's right, usually you do assume Ib has the control, but in this case it doesnt. Ic has control, and if you look at it as a black box you can possibly get control in other ways too.
Look at it like this:
I present you with a *new* device, *not* a transistor, but it too has three leads. That means we can measure three voltages 'across' and three currents 'through' the device. If we call the voltages v1, v2, and v3, and the currents i1, i2, and i3, we can relate one parameter to the other two algebraically as either of:
v1=v2+v3
v2=v1+v3
v3=v1+v2
and
i1=i2+i3
i2=i1+i3
i3=i1+i2
which means we can control at least one parameter with the other two, and and there are other relationships too if we can establish some inter-terminal resistances, which means some of the currents might depend on some of the voltages, or vice versa.
There could also be internal amplification, which would mean that perhaps v3 for example depended on v2 times a constant like A. Thus, v3=v2*A, and that means other things are possible too.
What this all means is that a three terminal device is a much more complex device that needs to be looked at very carefully, and what is more is that we can not assume that one terminal is the only terminal that can control something else about the device unless the device, under test, proves this to be true.
Let me describe another type of device, which i wont say what it is yet, but we will mark the terminals the same as a transistor, b, c, and e, but those are only letters this time. Thus lead 1 is marked 'b', lead 2 marked 'c', and lead 3 marked 'e'. Now i wont specify what kind of device this is yet, just a few measurements.
Ok, so now we have our new device which we'll call the bce device, and to make it a little simpler we'll connect the 'e' terminal to ground, 0v. Next we measure 1ma flowing into the 'b' terminal from a current generator we have set up to pump 1ma into the 'b' terminal. Next we measure 10ma flowing into the 'c' terminal from another different current generator. Next we measure across the 'c' and 'e' terminals and we measure 0.15 volts. Next we increase the current generator connected to the 'c' terminal to 200ma, and we note that the only thing that changed was the voltage between 'c' and 'e', as it went up to 9.6 volts. Thus, when we increased the current into the 'c' terminal we saw the 'c' to 'e' voltage rise by 9.45 volts, from 0.15 to 9.6 volts.
Any problem with this so far?
Here's yet another 3 terminal device:
Ground the 'e' terminal, pump 1ma into the 'b' terminal, pump 10ma into the 'c' terminal,
measure 150mv between the 'c' terminal and the 'e' terminal. Increase the current into the
'c' terminal to 200ma, measure 2.86 volts from 'c' to 'e' terminals.
Any problem with that?
Any problem with either of these two devices?