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BJT output voltage as function of input voltage

  1. Feb 20, 2014 #1

    Maylis

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    1. The problem statement, all variables and given/known data
    The circuit below is a BJT common emitter amplifier. Find Vout as a function of Vin. This may seem hard to parse at first, but it is actually pretty straightforward. Carefully replace the BJT symbol with the model for a BJT we used in class, making sure the base (B), emitter (E), and collector (C) terminals are connected properly, then use any method you want to solve for Vout!


    2. Relevant equations



    3. The attempt at a solution

    My solution doesn't use V0 at all. I am wondering if my connection is correct and if I did this correctly.
     

    Attached Files:

  2. jcsd
  3. Feb 20, 2014 #2
    Looks ok for me. And your BJT model is too too simplified to be able to take into account the Vo influence.
     
    Last edited: Feb 20, 2014
  4. Feb 20, 2014 #3

    rude man

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    Fine, except needs a minus sign.
     
  5. Feb 20, 2014 #4

    Maylis

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    why the minus sign?
     
  6. Feb 20, 2014 #5

    rude man

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    Because if the input goes up, the output goes down.
    If your input were a sine wave, the output would be a sine wave of opposite phase.
     
  7. Feb 20, 2014 #6

    Maylis

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    where does the minus sign come in for the analysis? I don't just want to tack on a negative to the front of my answer based on a concept with no proof
     
  8. Feb 21, 2014 #7
    No, you don't need any "minus" sign. In "normal" CE amplifier the output voltage is taken between collector and ground. But in your circuit Vout is the voltage across Rc resistor. So you don't need "minus" sign.
     
  9. Feb 21, 2014 #8

    rude man

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    That is correct. Why anyone would want to take the output that way I don't understand, but you are correct.
     
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