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Op-amp closed loop gain question

Discussion in 'Electronic Design' started by Dave Boland, Apr 26, 2004.

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  1. Dave Boland

    Dave Boland Guest

    It has been a lot of years since I have done anything with op-amps, but
    I seem to recall that some devices have a min. suggested closed loop
    gain. I have also seen an app-note that has a gain of about .303
    (inverting input). My question is what are the guidelines on:

    1. Gain range. Can it go from less than one to 10 without stability
    problems. The Fbw is DC to 1000 Hz, so I'm not worried too much about
    the AC aspect (unless I should be).

    2. My recollection is also that I should not select resistor values much
    below 10K for either Rf or Ri. I don't see this as a problem now (fixed
    gain), but doesn't it present a problem with variable gain designs?

    3. Finally, I believe that using a pot. as Rf is frowned on because of
    the noise of the wiper. If I need to ad some trim capability to the
    design, should I do this with a pot connected to Ri?

    Thanks for your time and any helpful answers.

    Dave,
     
  2. John Larkin

    John Larkin Guest

    Most opamps are stable for any inverting or non-inverting gain. A very
    few are "undercompensated" and don't like low closed-loop gains, but
    their datasheet will say so clearly.
    If the R's are very low, they'll gobble signal current, but no harm
    otherwise. If they are *very high*, the resulting frequency rolloffs
    with stray capacitance could reduce amp stability.

    Nothing wrong with a pot. Wipers aren't very noisy these days, and
    only while in motion at that. And it doesn't matter whether it's in
    the Rf or Ri path.


    Since these statements are unarguably true, I expact lots of arguments
    and flames.

    John
     
  3. Tim Wescott

    Tim Wescott Guest

    "Expect" is spelled with an 'e' in the middle. Otherwise I totally
    agree. Is this enough of a flame?
     
  4. Minimum gain less than 1 is not a problem for a unity gain stable
    opamp (an opamp that can handle a direct connection from output to -
    input).
    You need to keep the feedback resistor current well below the maximum
    current rating of the output. You also want ot keep the resistor
    signal currents much higher than the opamp bias currents, if possible,
    so that the input to feedback divider is dominated by signal current
    not opamp imperfections.
    Connect the wiper to the opamp input, and the input and feedback
    resistors to the ends. That way, the only current passing through the
    wiper is the opamp bias current. You also get a bigger gain swing for
    a given pot resistance, since swinging the wiper moves resistance from
    feedback to input or vice versa, instead of changing just one of these
    resistances.

    Say you wanted a gain of .1 to 10. With a 10k input resistor and a
    10k feedback resistor connected to the ends of a 100k pot, the wiper
    to the - input, you would have 110k input to 10k output at one extreme
    (gain of less than .1) and an input of 10k, feedback of 110k (gain
    greater than 10). Remember that most pots have a 10% or 20% overall
    tolerance.
     
  5. John Larkin

    John Larkin Guest


    Excellent work. I can see that you have a bright career ahead as a
    spell checker.

    John
     
  6. Tim Wescott

    Tim Wescott Guest

    Oddly enough my spelling is fairly poor. I can catch the odd misplaced
    vowel, and I am (hopefully) good at choosing the correct word from the
    list when the spell checker tags one for me, but that's about as far as
    I go. Left to my own devices I can make some pretty embarrassing mistakes.
     
  7. John Larkin

    John Larkin Guest

    Lately I find that I'm also misspelling via substitution of words that
    are close in sound to the one intended. I don't bother to spell check
    ng posts because so many of the words aren't in the dictionary, and
    I'm not getting paid anyhow.

    John

    typed with a chicken nugget in my hand.
     
  8. Jim Thompson

    Jim Thompson Guest

    On Mon, 26 Apr 2004 14:39:45 -0700, John Larkin

    [snip]
    I *try* to always spell-check since, with my klutzy typing, I tend to
    skip letters :-(

    ...Jim Thompson
     
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