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Discussion in 'Electronic Basics' started by raghu, Dec 14, 2005.

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  1. raghu

    raghu Guest

    why the input resistance and output resistance of opamp is high(or
    infinite) and low(or zero) respectively? wat is the advantage if
    input(and o/p) resis. is high(is low for o/p resis)? Wat is loading
    effect? thank a lot in advance.
  2. I'll take homework questions for $200 please, Alex.

  3. Andrew Holme

    Andrew Holme Guest

    You might need a better understanding of voltage, current and
    resistance before tackling op-amps. Try Googling for: "Ohm's Law" and
    "potential dividers"
  4. Bob Monsen

    Bob Monsen Guest

    Think of the following situation:

    input signal -------.
    o--- output

    Your input signal has some resistance of its own, which is modelled by
    R1. The input of the opamp measures voltage across some resistance,
    which is modelled by R2.

    Now, what is the relationship between the input signal and the output
    signal? The output will be 'near' the input only if R1 is much less
    than R2. So, opamp designers ensure that this is true by making inputs
    have a VERY high impedance.

    As to the output resistance, the situation is now reversed. The opamp
    wants to have the SMALLEST output resistance, because it doesn't know
    how bit the resistance the next element in the signal path will have,
    and so wants to ensure that the signal the next guy sees is as big as

    Bob Monsen

    Kiss girls all you want to - it beats the hell out of card games."
    ~ Jubal Harshaw
  5. KruSat

    KruSat Guest

    The most simple words that can clear ur doubts is as follows....

    The input resistance must be very high and output resistance most be
    low for any amplifiers and instrumentation and measurement circuits.

    input resistance must be very high so as to absorb as much less power
    as possible from the previous stage...

    and output resistance must be very very low so as to deliver the
    maximum possible power to the next stage...

    high ip. res. high sensitivity.
    low op. res. low power losses within the stage.
  6. raghu

    raghu Guest

    thanx a lot.u r as clear as mirror.
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