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how op amp differntial input is zero?

Discussion in 'Electronic Basics' started by munish, Jul 10, 2007.

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  1. munish

    munish Guest

    how op amp differntial input is zero?
    i need basic explanation(how input impedence high?)
     
  2. The differential voltage between the inputs can be
    approximated as zero, if the opamp is in a stable. negative
    feedback situation, because its gain is very large. only a
    differential voltage very near zero, multiplied by a very
    large gain could produce an output voltage between the power
    supply rails.

    The input impedance is high for very independent reasons.
    Some opamps do not have high impedance on the inverting
    input (current feedback opamps).
     
  3. Eeyore

    Eeyore Guest

    Because it uses a long-tailed pair input configuration. With perfectly matched
    devices this has inherently zero offset.
    http://en.wikipedia.org/wiki/Differential_amplifier
    Do you know how to calculate the input impedance of an active stage ? In which
    case it should be obvious.

    Graham
     
  4. neon

    neon

    1,325
    0
    Oct 21, 2006
    a differential amp has two very closely laser matched transistor bases and a steady current source on its emititers. because of the hi gain 90db or more 1 uv offset can be mv on the output. so the two input are sitting at a virtual same voltage. not ground but the same potential.
     
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