op amp, +ve and -ve supply

Discussion in 'Homework Help' started by bug13, Nov 22, 2012.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Hi guys,

    While I was going through my note about op amp, a question pop into my head: why op amp need a +ve and -ve supply? beside the reason that it need to give positive and negative swing??
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    It does not need +ve and -ve supply. For example, if an op amp is using -12V and +12V, you can shift the supply by +12V and use a single 0V and 24V supply. You can create a pseudo ground reference level at +12V.
     
  3. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    38
    how can you do that? a simple basic circuit would be nicely:)
     
  4. MKCheruvu

    Member

    Nov 20, 2012
    30
    6
    You need +ve and-ve supply for an Op-Amp ,if you want the Output to be a Bipolar ac signal .
     
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  5. MrChips

    Moderator

    Oct 2, 2009
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    You create a pseudo ground by creating a voltage divider using two resistors of the same value.
     
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  6. episode66

    New Member

    Nov 24, 2012
    5
    1
    I agree with the other posters that what you label the supply terminals does not matter to the op amp. +12/-12 or +24/0 is all the same to the op amp.

    However... the op amp references everything internally to the midpoint of the +/- supplies (often called rails). The textbooks always show +/- supplies with resistors returning to ground (which is the midpoint of the +/- supply rails). The trouble starts when you build a circuit and call it single supply (+24/0) and start connecting one end of all your passive components to the 0 , forgetting that this is really the negative rail.

    psuedo-grounds constructed with voltage dividers can work, but you have to keep in mind that they are 1) usually high impedance return paths (your circuit may not tolerate that) and they 2) bring the power supply ripple right to the input of your op amp (this defeats the PSRR or power supply rejection ratio of the op amp which is an important feature of an op amp circuit).

    Op Amps for Everyone by TI is a good place to start and it is free for download: http://www.ti.com/lit/an/slod006b/slod006b.pdf
     
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  7. MrChips

    Moderator

    Oct 2, 2009
    12,421
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    You do not need +ve and -ve supply for an op amp to work.

    I intentionally kept it simple. The devil is in the details.

    If you are going to use a 2-resistor voltage divider, calculate the resistance so that they draw at least ten times the current requirements of the op amp.

    A better solution is to use a TLE2426 Precision Virtual Ground.

    If the inputs and outputs are coupled through capacitors this would work for AC bipolar signals.
     
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