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H parameter model question

  1. Aug 12, 2013 #1
    1. The problem statement, all variables and given/known data

    calculate current and voltage amplification

    3. The attempt at a solution
    Conversion to h parameter model,

    and this is my working step,
  2. jcsd
  3. Aug 12, 2013 #2
    there are something i can't understand, in my calculation, i use equation 3 to find Ai, i get -132..... But if i use equation 6 instead of 3, i get 132...., same number but different sign. I suppose there is something went wrong in my equation 6...
    Last edited: Aug 13, 2013
  4. Aug 13, 2013 #3


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    Staff: Mentor

    Your eqn(2) doesn't correctly show how hfeib divides between the two resistances.

    I suggest that, in future, after you create something as a .docx file that you take a screenshot of your effort and post it as a jpeg instead of the .docx file. This will allow those who rely on a basic tablet to still follow the thread, using software add-ons no more sophisticated than a jpeg viewer.

    A further suggestion: I think it's a good idea to informatively label dimensionless parameters, e.g.,
    hfe = ... amps/amp
    hre = ... volts/volt

    It helps you to stay correctly focussed, and makes checking easier (especially by those of us who may have forgotten some of these. :wink:
    Last edited: Aug 13, 2013
  5. Aug 13, 2013 #4

    The Electrician

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    Gold Member

    Your equation 2 is Iout = hfe Ib + hoe Vout

    Equation 3 is Iout = -hfe Ib () What are these parentheses?

    Why is there a minus sign in front of hfe in equation 3 but not in equation 2?

    Shouldn't equation 2 be Iout = hfe Ib + hoe Vout + Vout/Rc?

    Also, I don't see anywhere where you have taken Rs into account. Rs has a very large effect on Av.

    I would think that Av would be defined as Vout/Vs. Is your problem defining it as Vout/Vin, which is the same as Vout/Vb?
  6. Aug 13, 2013 #5
    yeah sorry messed up a few thing up there, this is my second attempts

    so is this the correct ways?

    I think the arrow in the diagram is telling us the Vin is Vb
    Last edited: Aug 13, 2013
  7. Aug 13, 2013 #6
    This is for the voltage amplification, but i can't really figure out how to include the Rs in the calculation

    Do you mean by putting amps/amp behind the term like a unit?
    Last edited: Aug 13, 2013
  8. Aug 13, 2013 #7


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    Staff: Mentor

    For your Iout/Iin expression, the algebra looks right, though there's a transcription error: you show RB hie where it should be RB + hie. I haven't checked your arithmetic, and haven't checked your voltage equation.

    You may need a sign change. The arrow direction for iout is indicating iout = - vout / RC
    Last edited: Aug 13, 2013
  9. Aug 13, 2013 #8


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    Staff: Mentor

    If VB is correctly denoted as Vin, then as you observed earlier, Rs does not enter into your calculations.
  10. Aug 13, 2013 #9

    The Electrician

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    In post #5 and #6, you have used a value of .87k for hie, but in post #1 its value was given as .86k; which is correct?

    An easy way is to calculate the input resistance at the Vin node, and use that impedance plus the 1k Rs to form a voltage divider. The voltage division ratio from the Vs node to the Vin node can be multiplied times the Av you already calculated to get the overall voltage gain from Vs to Vout.
  11. Aug 13, 2013 #10
    Oh yeah, I get what u mean.

    hah sorry hie it is actually 0.86k ohms

    I find it is a bit tricky to find the input impedance even tough Rs is not included because of the HreVout, it causes great pain for me to kill off Ib , Iin and Vb.
  12. Aug 14, 2013 #11

    The Electrician

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    In post #1, you said your problem was to "calculate current and voltage amplification", but you didn't say whether voltage amplification, Av, was defined as Vout/Vs or Vout/Vin.

    Have you decided which it is?

    Do you still need to calculate Vout/Vs?
  13. Aug 14, 2013 #12
    from the

    -hfeIb = Vout (hoe + 1/Rc)

    i should change it to

    hfeIb = Vout (hoe + 1/Rc)

    then at the end, my Ai,

    Ai = RB/ [ (Rchoe + 1)( RB + hie )/hfe + hreRc ]

    With this change, i get positive 132.4439 amplification
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