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Thevenin equivalent

  1. Sep 10, 2014 #1
    1. The problem statement, all variables and given/known data
    I'm trying to understand where the (β+1) comes from.


    2. Relevant equations
    vth=-βiR2
    i+βi+is=0

    vth=-β(-is/(β+1))R2


    3. The attempt at a solution

    vth=-βiR2
    i+βi+is=0

    vth=-β(-βi-is)R2
     

    Attached Files:

  2. jcsd
  3. Sep 10, 2014 #2

    berkeman

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    Staff: Mentor



    Welcome to the PF.

    Without seeing the original circuit that this is modeling, it's hard to be sure. But for a CE BJT amplifier, your collector current Ic = β * Ib. So your emitter current is.....
     
  4. Sep 10, 2014 #3
    i've attached the solution that i have in the book, the value of beta is 150 and R1=100k and R2=39k, the only problem I have is that how do you get to the vth while substituting beta and the resistance in both sides to be equal to -Beta (-is/beta+1), the beta + 1 has to come out from simplying and combining like terms but I just can't see it.
     
  5. Sep 10, 2014 #4

    gneill

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    Staff: Mentor


    So factor out the i in that equation above:

    (β + 1)i + is = 0

    There's your (β + 1) term.

    That doesn't look right. You want to substitute for i in your equation for Vth from above, but what you've stuck in there also contains i.
     
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