Emitter Follower problem, please help!

Discussion in 'Homework Help' started by upopads, Dec 21, 2007.

  1. upopads

    Thread Starter Active Member

    Dec 18, 2007
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    Given the attached circuit find (a) The DC bias point and small signal midband AC model for the transistor. Draw the small signal midband AC equivalent circuit.
    (b) Find the small signal midband AC voltage gain vo/vi and the overall voltage gain vo/vs (c) Find the small signal midband AC input and output resistances.


    The answers are given in the back of the book.
    (a)
    Ic = 5.6mA, gm = 215mA/B
    r(pie)=232 Ohms
    (b)

    vo/vi = 0.88 and vo/vs = 0.44

    (c) Ri = 1k Ohm and Ro = 15 Ohms

    Anyone know how to get these answers? I started off by getting Rth and Vth

    VBB = [R2/(R1+R2)]Vcc = (6.8kohms/9.8kohms)5V = 3.47V

    RBB = R1||R2 = 2081.6 Ohms

    but I don't know how to do the rest and I really need to know how. Thank you.
     
  2. upopads

    Thread Starter Active Member

    Dec 18, 2007
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    Rs in that circuit is 3k Ohms not 5 sorry.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    That transistor has no base connection. Even after you add one, it is will not be an emitter follower. It looks like it could be intended to be a common emitter amplifier, but the bias will have the transistor saturated. I suspect it is drawn wrong.
    Edit: I think maybe you meant to draw a PNP, with the emitter where your collector is now. Then it would be an emitter follower, after you add the base connection.
     
  4. upopads

    Thread Starter Active Member

    Dec 18, 2007
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    you're right...this is what it is supposed to look like, the book says its an emitter follower
     
  5. upopads

    Thread Starter Active Member

    Dec 18, 2007
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    Beta is equal to 50
     
  6. upopads

    Thread Starter Active Member

    Dec 18, 2007
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    If I knew how to get the collector current I could find the other two values.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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  8. upopads

    Thread Starter Active Member

    Dec 18, 2007
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    My textbook is wrong then...but i still need to know how to do this problem
     
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