Is there any way to trick an op-amp?

Discussion in 'General Electronics Chat' started by Khalid Abur-Rahman, Mar 27, 2009.

  1. Khalid Abur-Rahman

    Thread Starter Active Member

    Dec 25, 2008
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    to make it think its getting both the negative and the positive that it needs...but its actually getting its voltage from one source...a 741C to be specific
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Hopefully this is what you have in mind.....

    Some op-amps have been designed specifically to operate from a single supply. Take for instance the LM124.

    http://www.national.com/JS/searchDocument.do?textfield=LM124

    Otherwise, you can create a "pseudo-ground" by center-tapping the power rails with a resistive divider, with the tap point bypassed back to the rails with sufficient capacitance. Another technique is to use a spare op-amp to provide a driven center-tap - which can give a lower impedance at the common [ground] point.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    741 is a bad candidate for this, because it requires around 15 V minimum (there is no minimum voltage given in the data sheet).

    I wrote an article on such a problem...

    http://forum.allaboutcircuits.com/showthread.php?p=84026#post84026

    There are other types of op amps though, such as a LM324, which can go down to 2V with no problem.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    A 741 is 41 years old. Bury it and use a better more modern opamp.
     
  5. PRS

    Well-Known Member

    Aug 24, 2008
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    I 'tricked' an op amp into thinking there was a dual supply by making a voltage divider of 2 resistors of equal value. I dubbed its center point 'ground' and the negative part of the supply -Vcc. It worked just fine.
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    I think you might be missing the point of the exercise. The 741 is IDEAL for learning about op-amps because it IS so simple and universal. :)

    eric
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    You might as well use an opamp that works. The lousy old 741 opamp was designed for a plus and minus 15V supply and a DC or low frequency signal.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    Its limitations is what makes it ideal to learn from. You don't have to go far to bump into them, which is perfect for students. Every op amp has limitations, but if you don't see them clearly you won't understand them as well, and all of a 741's problems are the same as other op amps, just a lot more obvious. Took me a little while to understand this. When you need better op amps they exist.
     
  9. flat5

    Active Member

    Nov 13, 2008
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    "Good enough is enemy of the best."

    Funny slogan for your argument :)
     
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