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Differential Pair Input Noise Issue

Discussion in 'Electronic Design' started by [email protected], Aug 18, 2008.

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  1. Guest

  2. Eeyore

    Eeyore Guest

  3. MooseFET

    MooseFET Guest

    I suggest you look at the LSK170 if you want lower noise levels and
    can stand the input capacitance.

    The basic rule for resistors in low noise circuits is to make them
    either as large as you can or as small as you can. The one on the
    gate of the JFET should be small or perhaps zero.
  4. Guest

    Noise current is proportional to the square root of the total current,
    and resistor (Johnson) noise is proportional to the square root of the
    resistance value; by going for a higher current and lower resistors at
    that point, the designer has minimised the noise contribution atthat
    particular point.
    Small enough the op amp doesn't swing to the rails (and saturate its
    internal gain paths) under normal operating conditions.
  5. Guest

    thanks, your explanation is the best even though I read your post for
    a couple of times.
    2k ohm is a big value based on my understanding of thermal noise
    that's why I posted this question here. Right now I understand the
    author's words-he thinks 2k ohm is small enough to minimize thermal
    noise. In stead I think the author should pick a op-amp running under
    lower supply voltages then pick a smaller resistor.
  6. Phil Allison

    Phil Allison Guest

    ** Do not top post !!!!!!!!!!

    ** But the voltage noise of those 2N5486 fets is much higher - and it rises
    sharply at low frequencies.

    ** Did you come here to ask questions or give lectures ?

    ..... Phil
  7. Guest

    It's usually not as simple as that; You need enough gain in the first
    stage to make sure that the op amp input noise is lower than the noise
    from your long-tailed pair of FETs; these are two independent noise
    sources so the total noise is the square root of the sum of their
    squares, which means that it s rarely worth getting the lesser of the
    two noise sources below half the bigger one.
    Don't mind Phil - he's obnoxious, but he does know his electronics.
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