Help with analyze a simple analog cirucit

Discussion in 'Homework Help' started by eladta, Apr 20, 2013.

  1. eladta

    Thread Starter Member

    Apr 20, 2013
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    Hi all,
    I have a pretty simple circuit but i'm not sure how to analyze it. Can anyone give me a hand ?

    Thanks !
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
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    Do you know how ideal op amp work? And you could start your analysis with assumption that Vin =1V.
     
  3. eladta

    Thread Starter Member

    Apr 20, 2013
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    Yes i do. this is a negative feedback op amp, so i can asume v-=v+
    that doesn't make sense to me cause if the BJT is on the emitter potential (which is close to Vout) equals to Vin
    Am i wrong ?
    What i need is the transfer func. (ie. Vout/Vin).
     
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Hello eladta.

    The op amp will adjust its output voltage so that the - input equals the + input.
    So, what effect does the bjt have?
     
  5. eladta

    Thread Starter Member

    Apr 20, 2013
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    if the bjt is on than the emitter potential is close to 5v (Vc) right ?
    in addition we have - input equals the + input as u said. so Vin = 5v always ?!
    Clearly i'm not realizing it right but what ?
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
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    In this circuit the bjt is operating as a voltage follower, not as a switch.
    Have you studied voltage followers ?
     
  7. eladta

    Thread Starter Member

    Apr 20, 2013
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    i did but i don't remember it well
     
  8. eladta

    Thread Starter Member

    Apr 20, 2013
    41
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    can u tell me the principal of it ?
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Emitter voltage = base voltage minus base to emitter voltage drop.
     
  10. eladta

    Thread Starter Member

    Apr 20, 2013
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    thanks. so if i'm reading it right the circuit is a buffer. why do we need the bjt for ?
     
  11. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Exactly.. What does the buffer do ? ;)
     
  12. WBahn

    Moderator

    Mar 31, 2012
    17,715
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    At what point do you become responsible for relearning things that you have been taught but have forgotten?

    It sounds like you have a very weak grasp of the fundamental concepts. If that's the case, they you really need to take a step back and gain that grounding, because it's only going to get worse from here and become harder and harder to make progress.
     
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