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
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    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.
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    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
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    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
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    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
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    I simulated my design in ltspice (with the stock 2n2222 model):

    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
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    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
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    We're all proud of you.
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    your instructions called for a 2N3904.
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    I'm glad to hear it.

    Notice what I did to find the problem and also what you might have done to make finding the problem easier. Namely, I looked not at the voltages, but at the transistor currents, which I had to calculate from the voltages given. So what you could do in the future is also indicate the simulated currents on the schematic so that you can review them very easily to see if they make sense.

    It's all part of the learning process and gaining the experience needed to consider a problem and decide what pieces of information will best help your efforts to resolve it.
     
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