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op amp question

Discussion in 'Electronic Basics' started by jim, Jan 6, 2004.

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  1. jim

    jim Guest

    In spite of all the info my texts have on op amps,I'm still unclear on
    one aspect. When I configure an inverting amp say with the old 741,
    the gain is feedback resistance divided by input resistance. However
    ,several combinations of resistances give the same ratio. Aside from
    input impedance,does the choice of resistances that give identical
    ratios effect the gain bandwidth curve? And if not what would be
    effected by these different choices? Thanks for any help. jim
     
  2. Joel Kolstad

    Joel Kolstad Guest

    For an ideal op-amp, no. For a real op-amp, yes. The input of a real
    op-amp often looks rather capacitive (if it's a FET based amplifier, you're
    looking into what's often the largest gate in the device), so the
    gain-bandwidth product as you use larger resistors will, at some point, be
    reduced by the low pass filtering effect of your large resistor and the
    input capacitance of the op-amp. Additionally, large resistors will
    decrease the noise immunity of your inputs (as their impedances are no
    longer necessarily large in comparison to those of induced noise sources).

    Likewise, if you make the feedback resistor very small you could end up with
    the op-amp's output becoming slew rate limited even for a 'reasonable' load,
    and this can be deleterious as well (especially since slew rate limiting is
    a non-linear effect). And as you're clearly aware, small resistors may
    significantly load down whatever it is that's feeding the op-amp, which may
    be undesirable.

    The Art of Electronics has a good discussion of real-world op-amp
    considerations, as does Op-Amps For Everyone, available as a free download
    at TI's web site here:
    http://focus.ti.com/docs/apps/catalog/resources/appnoteabstract.jhtml?abstractName=slod006b

    ---Joel Kolstad
     
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