input and output impedance why we should find them in the way told ! any specific reason

Discussion in 'General Electronics Chat' started by srikar sana, Jul 6, 2016.

  1. srikar sana

    Thread Starter New Member

    Feb 3, 2016
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    i was revising the hybrid parameters in the transistor ac analysis . i was reading boylstead and my question is not related to the book but it was given that h11 or hie= Vi/Ii | Vo=0 i.e., input resistance is equal to input voltage by input current when output is shorted why should we do that i mean why should we short output when finding input resistance and why should we open input when finding output resistance is there any specific reason for doing so ...........?

    i know that there is a reason for everything and some one pls tell me that thank you .
     
  2. DGElder

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    Apr 3, 2016
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    How do you determine the impedance of a voltage source and resistor in series? How do you determine the impedance of a current source and resistor in parallel? Now look at your H-parameter model.

    There you have it.
     
    Last edited: Jul 6, 2016
  3. srikar sana

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    Feb 3, 2016
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    sir DGElder could you pls be more specific because i am a slow learner thank you
     
  4. DGElder

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    If you post the drawing of the transistor model to which you refer - with labels, I will walk you through it.
     
    Last edited: Jul 6, 2016
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Homework?
     
  6. srikar sana

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    Feb 3, 2016
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    haha home work now there is no one to give me home work because i have completed my BE but there are some doubts and said as i am slow learner i trying to learn
     
  7. DGElder

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    Apr 3, 2016
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    "Homework?"

    It's not a problem, it's a theory question
     
  8. srikar sana

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    Feb 3, 2016
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    DGElder here is the diagram forget about the capacitance values
     
  9. DGElder

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    You were asking about the hybrid parameters in the transistor ac analysis. That is a common emitter amplifier, it is not an H-model of a transistor, e.g. where is hie and hfe?
     
    Last edited: Jul 6, 2016
  10. srikar sana

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    Feb 3, 2016
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    could you do that for the above circuit sir pls because that way it helps me better and pls note what you are going to say is recorder into a word document for my future use

    or else you it self take a hybrid parameter circuit for a CE amp and pls expalin me these
     
  11. srikar sana

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    Feb 3, 2016
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    dgelder sir pls put an answer sir why did you remain silent sir ? or else tell the things that are to be done by me so that i can figure it out in the way you told
     
  12. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    The 6 two-port parameters (z, y, h, g, a, b) define relations between currents and voltages at one port to currents and voltages at the other port. The choice of which variables to consider independent and which to consider dependent leads to the conditions imposed at the output port when evaluating the parameters.

    See this for full details: https://en.wikipedia.org/wiki/Two-port_network

    h parameters were originally chosen to use for transistor circuit analysis because transistors in the common emitter configuration generally have a low impedance at the input and high impedance at the output at low frequencies. Because of that, it's easier to set up the terminal conditions for accurate measurement of h-parameters of a transistor then one of the other parameter sets.
     
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