H parameter analysis

Discussion in 'Homework Help' started by cooded, Oct 25, 2010.

  1. cooded

    Thread Starter Member

    Jul 20, 2007
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    0
    In the Common emitter configuration of BJT for small signal analysis using hybrid parameter, why are hre and 1/hoe parameters ignored?
     
  2. yoamocuy

    Active Member

    Oct 7, 2009
    80
    0
    1/hoe is equal to VA/icq where VA is 100. This number is always going to be extremely large and since it is hooked in parallel, hardly any current is going to enter 1/hoe so it can be ignored. I can't exactly remember the reasoning for why hre is ignored but I'm thinking that if you do solve for it then it will have such a small value that it will act like a short.
     
  3. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    \frac{V_{\tiny{A}}}{I_{\tiny{CQ}}} is the output resistance of the small signal model. It is symbolized by r_o. It is a resistance put in paralel with the dependant current source of the model. It usually is very large and this is the reason it is ingored. However a detailed analysis must take this parameter into account.

    Just because the simplified small signal models don't have those parameters incorporated, doesn't mean that there aren't more detailed versions of them.
     
  4. cooded

    Thread Starter Member

    Jul 20, 2007
    28
    0
    Hi all,

    Thanks for the quick replies.I did get the answers.
     
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