Transistor as Current Source. Forward Active?

[fallen186]

Homework Statement

Why does a transistor with a constant $V_{B}$ (e.g. $V_{B}=1V$) only need $V_{C} \approx 0.3V$ to act as a current source;

If $V_{C} < V_{B}$ doesn't that mean the transistor is not in forward active mode and thus not a mode we want to work in.

Homework Equations

The Attempt at a Solution

PSpice Code:

Understanding Transistors as Current Sources

.model my_npn NPN(BF=150 NF=1.1 VAF=80V CJC=6P CJE=10P)

*Voltage Sources
VC 1 0 dc 0;
VB 2 0 dc 1;

*Transistor C B E
Q1 1 2 0 my_npn;

.Probe
.OP
.DC VC 0.1 1 0.001;
.end

 

[NascentOxygen]

So you were allowing base current to reduce (in whatever way it chose to) as Ic increased? You have no idea what the base current is doing here, do you? Try plotting it, too. What you have is an unusual way of making a constant current source (and I'm not sure that we could have foreseen how well it would perform, on paper, anyway). I'd say we don't usually make a controlled current source this way because of the extreme difficulty of holding VBE perfectly constant. Try varying VBE by a tiny amount, and your "constant" current source will probably reveal its extreme vulnerability.

It is easy to hold IB relatively constant by a resistor from a voltage V where V>>VBE.

http://www.du.edu/secs/departments/ece/media/images/TransistorGraph.jpg

Just my thoughts. I could be way off.

 

[NascentOxygen]

I'm hoping to see your plot of base current needed to maintain VBE=1.0 v.