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Transistor equivalent circuit question

  1. Jul 30, 2013 #1
    M.aspx?v=8c69628c5d5fb6b56fad.jpg
    This is the equivalent circuit to a transistor,
    I am to find:
    i )Short-circuit input impedance, H11
    ii )Open-circuit reverse voltage gain, H12
    iii)Forward current gain, H21
    iv)Open-circuit output admittance, H22

    2. Relevant equations

    Vin = (H11)(Iin) + (H12)(Vout)
    Iout = (H21)(Iin) + (H22)(Vout)

    3. The attempt at a solution
    So first i short the output, and the Vout = 0, then i have

    Vin = (H11)(Iin)
    Iout = (H21)(Iin)
    H11 = R1 + R2//R3 = 110 Ω
    H21 = -IR3 / Iin = 2/3

    Then I open Iin source,
    Vin = (H12)(Vout)
    Iout = (H22)(Vout)

    H12 = Vin / Vout
    = (VR2 / Vout)
    = 2/3
    1/H22 = R3 + R2
    H22 = 1/90 Ω -1

    Am i doing this question correctly?

    Reason of editing: Vin = V1; Vout = V2
    Iin = I1 ; Iout = I2
     
  2. jcsd
  3. Jul 30, 2013 #2

    gneill

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    Staff: Mentor

    Your approach is fine.

    Something funny going on with your determination of H21 though. Which resistor in your diagram is carrying Iout? And what's the labelled direction for that current?
     
  4. Jul 30, 2013 #3
    Is it R2?

    I use currendivider rule to find the current flow through R2, and i put negative sign because Iout is opposing the I_R2. So this is wrong?
     
  5. Jul 30, 2013 #4
    Ah didn't notice the typing error, it should be

    H21 = -IR2 / Iin = 2/3

    = 60 / (30 + 60)
    = 2/3
     
  6. Jul 30, 2013 #5

    gneill

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    Staff: Mentor

    Indeed :smile: Now, what happened to the minus sign?
     
  7. Jul 30, 2013 #6
    ah, -2/3..., thank you very much
     
  8. Jul 30, 2013 #7

    gneill

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    Staff: Mentor

    Cheers.
     
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