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Why JFET Noisless

Discussion in 'Homework Help' started by sunny`, Apr 22, 2012.

  1. sunny`

    sunny` New Member

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    Hi all, I am new to this forums so I am not really sure if this is the right place to post this. I tried searching the same question in google but i was unable find a proper answer.

    I just want to know why JFET is Considered NOisless and Why BJT has Noise.

    I understand that as JFET is Unipolar it has less noise but what does its unipolar character has to do with noise creation, that I donot know.Please help me.


    Thanks in Advance
    Sunny.

    (Also I am sorry if I am not suppose to post topics like this in this forum)
     
  2. duffy

    duffy New Member

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    It's not noiseless, but it has less noise because the current doesn't flow across a junction like it does in a bipolar - it just flows through a channel.
     
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  3. crutschow

    crutschow Well-Known Member Most Helpful Member

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    To augment duffy's explanation -- Current flowing through a junction creates shot noise which is generally higher than the Johnson (thermal) noise created by the channel resistance of a JFET for typical transistor operating currents.
     
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  4. dave

    Dave New Member

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  5. jimy245

    jimy245 New Member

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    JFET is a unipolar device and operate only with one type of charge carrier so have some little noise but no noiseless ,beside in BJT is a biploar device operate with both electron and hole carriers so having nosie much as JFET...
     
  6. sunny`

    sunny` New Member

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    Thanks for the answers but can you tell me what is this Shot noise?The Noise Created at the Junctions? If so, then Why ?

    And in BJT is there no Thermal noise?Only shot Noise?
     
  7. crutschow

    crutschow Well-Known Member Most Helpful Member

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    If you click on the reference to "shot noise" in my previous post you will get numerous hits on an explanation. Basically it's due to the discrete charge of each electron as it crosses a barrier, such as a semiconductor junction. The statistical variation in the number of electrons crossing the barrier at any instant in time creates a variation in the voltage created by the sum of all the electrons, which we call noise.

    Shot noise has nothing to do with whether a device is unipolar or bipolar. A vacuum tube (valve) has shot noise even though it is a unipolar device that uses only electrons.

    A BJT will have Thermal noise for any resistance it has between terminals, but that is normally much less than the shot noise.
     

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