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BJT analysis question

  1. Oct 31, 2016 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    This is the solution given:
    VB = (VCC*30k)/(70k+30k) = 3V
    Why is a voltage divider being used to find Vb?
  2. jcsd
  3. Oct 31, 2016 #2


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    Staff: Mentor

    Your question should be in the problem statement, not the attempt.

    Are you sure that VB is the same as Vb? Looks to me like a determination of a Thevenin equivalent for the base voltage supply. Was there also a Thevenin resistance calculated (paralleling the 30 k and 70 k resistors)?
  4. Oct 31, 2016 #3
    VB is Vb.
    Thevenin resistance was calculated for 30k and 70k.
  5. Nov 1, 2016 #4


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    What is your task? To find the DC voltage VB at the base?
    Don`t forget that you have a loaded resistive voltage divider - loaded with the transistor base current.
  6. Nov 1, 2016 #5


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    Staff: Mentor

    This is what I imagine the author of the solution was going for:
    With ##V_B## and ##R_B## being determined as the Thevenin equivalent of the base voltage divider.
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