# Designing a circuit using a BJT

Discussion in 'Homework Help' started by MarsG, Sep 28, 2015.

1. ### MarsG Thread Starter New Member

Sep 12, 2015
24
1
I am required to design a circuit using a BJT with the only requirements being that the collector current be 5 mA, emitter voltage has to be 2.5V and the source voltage is 12VDC. I posted below all my calculations, pspice schematics and the project info, Im not sure what im doing wrong.

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2. ### WBahn Moderator

Mar 31, 2012
22,990
6,884
The numbers in your simulation yield a base current of 129 uA, which would be a beta of 27.5. What transistor model are you using? Is it possibly you are using a power NPN?

3. ### dannyf Well-Known Member

Sep 13, 2015
2,196
421
If Ic = Ie (roughly) = 5ma and emitter sits at 2.5v, Re = 2.5v/5ma=500ohm.

I would conservatively estimate beta to be 50 so Ib = 5ma / 50 = 100ua.

I would set the current through the bias network to be 10x of that, or 1ma. That means the two resistors in the bias network to be 12v/1ma = 12k.

The base sits at 2.5v+0.7v = 3.2v. So the lower bias resistor is 3.2v/1ma = 3.2k and the top to be (12-3.2)/1ma = 8.8k.

I typically set the collector at 1/2 Vcc (or slightly higher) to maximize swing. That means your collector resistor is (12-6)/5ma = 1.2k.

Round those numbers to the nearest standard values and you are done.

4. ### Russmax Member

Sep 3, 2015
82
12
You haven't done your calculations wrong, for the most part. You've used some "rule of thumb" assumptions, as opposed to meticulous calculations, but that's OK. That does mean there will be some error, but you'll be in the ball park. Design is usually an iterative process, so don't be discouraged.

You need to check your transistor model. It's DC beta is 26.8. That's what's wrong. Is this the model you're supposed to use for this homework? If you don't know what model you're using, or supposed to be using, check with your instructor before proceeding.

Either get a model that matches your assumptions or redo the calculations with the lower beta.

Regards

5. ### dannyf Well-Known Member

Sep 13, 2015
2,196
421

Ie = 4.89ma, and Ib=24ua.

Ve=2.4v, Vc=6.2v, Vb=3.1v.

Not bad for a back-of-the-envelope design.

6. ### MarsG Thread Starter New Member

Sep 12, 2015
24
1
lol Yea I was using the wrong transistor model, all is well now thank you

7. ### MarsG Thread Starter New Member

Sep 12, 2015
24
1
Thank you. The model I was using was wrong, it all worked out after I switched it out.

Russmax likes this.
8. ### Russmax Member

Sep 3, 2015
82
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We're all proud of you.

9. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,909
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your instructions called for a 2N3904.

Mar 31, 2012
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