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Better op-amp buffer configuration?

Discussion in 'Electronic Design' started by billcalley, Jun 25, 2005.

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  1. billcalley

    billcalley Guest

    Hi All,

    I realize that using a single-supply op-amp as a buffer may be one
    of the easiest circuit configurations in the world; just tie the
    chip's output back to the inverting input, and insert the signal into
    its non-inverting input. That's how the books show it anyway, but
    are their any real-life op-amp issues that I have to look out for with
    this basic circuit? Anything I can do to make this circuit operate
    better under temperature, along with the unavoidable op-amp variations?
    Perhaps to help with any stability or voltage offset issues? Or is
    this circuit simply the way op-amp buffers are done, even in real life?

    Thank You,

    Bill

    Cross posted to sci.electronics.design and sci.electronics.basics
     
  2. For many (but not all) opamps that configuration will work fine. Quirk
    I had with LM324 as a follower, fast rise or fall times exceeded speed
    of the amp and cross coupled (through the bias system?) to other amps
    in the package. TL084s fixed that. Current feedback opamps can be used
    this way but would require a feedback resistor to prevent oscillations.
    Best bet, check manufacturers data sheets to verify usage.
    GG
     
  3. Check whether the chosen op-amp needs external compensation to be
    unity-gain stable (the NE5534 does).

    Check whether a resistor may be needed in series with the output to
    prevent problems if you are driving a capacitive load such as a long
    screened cable.
     
  4. Check whether the chosen op-amp needs external compensation to be
    unity-gain stable (the NE5534 does).

    Check whether a resistor may be needed in series with the output to
    prevent problems if you are driving a capacitive load such as a long
    screened cable.[/QUOTE]
    Put the isolating R in series with the output of the opamp, and take
    the feedback to the -i/p After the resistor. Generally works, and
    compensates for attenuation that you would get when you drive a lowish
    Z load


    martin
     
  5. billcalley

    billcalley Guest

    Thanks for all of your help guys!

    Cheers,

    Bill
     
  6. excellent article, thanks


    martin
     
  7. Chris Jones

    Chris Jones Guest

    Put the isolating R in series with the output of the opamp, and take
    the feedback to the -i/p After the resistor. Generally works, and
    compensates for attenuation that you would get when you drive a lowish
    Z load


    martin[/QUOTE]

    Actually, if you take the feedback from after the resistor, the stability
    will normally be even worse than without the resistor at all. If you need
    to take feedback from after the resistor for accuracy reasons, then put a
    capacitor directly from the op-amp o/p pin to the inverting input, and a
    resistor between the load and the inverting input, and of course the
    original low value between the op-amp output pin and the load.

    http://www.analog.com/library/analogDialogue/archives/31-2/appleng.html

    Chris
     
  8. Ol' Duffer

    Ol' Duffer Guest

    Instead of just connecting output to inverting input, use a resistor
    to approximate impedance of input to minimize offset drift due to
    input bias current.
     
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