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How to find the input resistance of an amplifier?

  1. Aug 28, 2012 #1
    1. The problem statement, all variables and given/known data
    Here is a picture of the problem

    http://i1226.photobucket.com/albums...-8A04-C5855FF971D8-87001-000004D91E955189.jpg


    2. Relevant equations

    Im guessing Vo = Avo VL (RL/(RL + Ro))



    3. The attempt at a solution

    The solution is already beneath the problem. I just can't seem to figure out how to get to it. Could someone please explain?
     
  2. jcsd
  3. Aug 28, 2012 #2
    Exercise 1.21.
     
  4. Aug 28, 2012 #3

    gneill

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    Staff: Mentor

    Write KVL for the left hand loop. Note that ix is the same as ib.
     
  5. Aug 28, 2012 #4
    Ok, that worked out pretty nicely! Thank you very much. So just to clarify, when Im doing this type of exercises I need to use KVL's all the time or does it depend? Im really new to amplifiers so Im trying to get where all this is coming from.
     
  6. Aug 28, 2012 #5

    gneill

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    Staff: Mentor

    You use whatever circuit analysis method suits the given circuit arrangement -- choosing which method is simplest to use comes from practice. Here you wanted to find the ratio vx/ix, and those would be principle variables in the KVL for that loop.

    I suppose you could apply KCL at node E to find the node voltage there, and then use that and vx to find ix, and then finally determine the ratio vx/ix, but it seems like more work to me.
     
  7. Aug 28, 2012 #6
    So I treat it as if I was analyzing any other circuit. Then what exactly does the blue highlighted area mean/represent???
     
  8. Aug 28, 2012 #7

    gneill

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    Staff: Mentor

    It happens to be a simple equivalent circuit (or model) of a transistor.
     
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