# Transistor as Current Source. Forward Active?

1. Mar 11, 2012

### fallen186

1. The problem statement, all variables and given/known data
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.

2. Relevant equations

3. The attempt at a solution
PSpice Code:

#### Attached Files:

• ###### CurrentSourceTransistor.jpg
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2. Mar 12, 2012

### Staff: Mentor

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.

Last edited: Mar 12, 2012
3. Mar 17, 2012

### Staff: Mentor

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