# Transistor formulas

#### Xander

Joined Dec 13, 2005
6
Hey, i was set this assignment recently and im stumped as to how to work it. Ive looked through notes but each transistor circuit differs so much that i can figure out the formulas to work out each part.

Power rail +10v and 0v. NPN transistor is the circle (im not much of an artist) and the 3 points cbe are collector, base and emitter. Now questions are i have to find the emitter voltage, current, the (approx) collector current and the base current. The gain is also 100.

Now i recall something to do with either Ib = Ic or one of them is so small its negligable. i just cant remember though.
I do remember hfe = Ib/Ic, so finding the base current once others are answered should be fairly simple.

Hopefully some one could explain the ideas behind this. If its not too much trouble, could some one also explain whats different when the resistor is switched to the collector line?

#### hgmjr

Joined Jan 28, 2005
9,029
Take a look at the transistor tutorials here on the AAC website. After you have reviewed the material; take a stab at solving your problem and then get back to us with your efforts. A diagram showing your work will help us assist you with any weakness in your understanding of the problem.

HINT: Your problem involves a common collector transistor stage.

hgmjr

#### Xander

Joined Dec 13, 2005
6
Thanks.

So, The emitter voltage = 8.4 - 0.7 which is 7.7volts.
Ie = vout/RL
Ie approx Ic
Hfe = Ib/Ic so Ib = Ic/hfe

nice and simple.

Thanks.

#### hgmjr

Joined Jan 28, 2005
9,029
Looks like the material in the AAC tutorial did the trick.

I think you have the basic technique down now.

The next time you have problems with an electronic concept and your text book does not help you figure it out; hopefully you will remember the AAC tutorial and the valuable material is contains.

hgmjr