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Special op amp "tricks"

Discussion in 'Electronic Basics' started by Jon Slaughter, May 10, 2007.

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  1. Are there any tricks I need to be aware of for practical op amp circuits?
    Mainly stuff to do with audio?

    Just wondering how close the textbook stuff is to reality?

    Thanks,
    Jon
     
  2. John Fields

    John Fields Guest

    ---
    Yes.

    1. Consider the opamp inputs holy and _ALWAYS_ have the ends of the
    resistors connected to the opamp inputs as close to the opamp as
    you can get them. Sacrifice almost everything for that.

    Ditto for what's driving the resistors through which the opamp is
    being driven, but the opamp input end of the resistors takes
    precedence.

    2. Always roll off any bandwidth you don't need.
     
  3. Is this mainly for HF stuff or does it matter. What is the purpose of this
    too? lead inductance?
    huh? Not sure what you mean here? How can you do this if the signal is, say,
    comming from a guitar or mic?
    And this is because of oscillations?

    Thanks,
    Jon
     
  4. John Fields

    John Fields Guest

    ---
    It's for everything.
    ---
    ---
    long leads are antennas, so you wind up picking up all manner of
    crap including, possibly, signal from the output and Voila!
    oscillator.
    ---
     
  5. Chris

    Chris Guest

    Amen to #1. Possibly that could be expanded to include the feedback
    loops -- keeping them as small as possible and putting the terminating
    leads as close as possible to the input pins also works miraculous
    wonders.

    In addition to #2, a couple more lesser caveats might be in order:

    * Always bypass the power supply for anything faster than an LM358
    (and sometimes even then), and keep the AC impedance of the bypasses
    to GND as small as possible. Memorize the venerable Analog Devices
    appnote AN-202, "An IC Amplifier User's Guide to Decoupling,
    Grounding, and Making Things Go Right for a Change". This appnote was
    written in the days when dinosaurs ruled the earth and AD still made
    modular op amps, but its advice is still just as valid.
    http://www.analog.com/UploadedFiles/Application_Notes/135208865AN-202.pdf

    * Always look carefully at fault conditions, and make absolutely sure
    the input voltages will never exceed absmax under any circumstances.
    Again, careful planning with GND impedances is necessary. An op amp
    input can be grievously damaged, but still kinda function, causing no
    end of headaches.

    Textbook stuff is great as far as it goes. But the OP might just want
    to open up good audio equipment, especially older stuff from the
    seventies and eighties, and look carefully at their schematics. The
    additional practical methods to make things work well and reliably are
    right in front of him, if he takes the time and effort to learn from
    them.

    Cheers
    Chris
     
  6. Thanks again, I'll try to keep that stuff in mind next time I play with them
    but chances are I'll forget ;/

    Jon
     
  7. yeah, I never thought to do that. But then again, its one thing to see what
    they were doing and totally different to understand why they did it.

    Thanks,
    Jon
     
  8. I've seen op amps used in parallel to reduce noise. Neat trick.

    Dave.
     
  9. Bob Masta

    Bob Masta Guest

    One trick that you are likely to need for audio use is the
    ability to drive a cable. Many op-amps are poor at this
    due to the capacitance of the cable. The trick is to
    put 100 ohms between the amp output and the cable,
    and connect the amp feedback resistance from the cable side of the
    resistor instead of from the amp output. You also need a
    small feedback cap (10-50 pF) wired in the usual way
    from output to - input.

    Best regards.


    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!
     
  10. John Fields

    John Fields Guest

     
  11. Thanks guys. I'll try to keep all this stuff in mind but atleast I know some
    of the details.

    Jon
     
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