op amp negative supply input

Discussion in 'General Electronics Chat' started by count_volta, Jan 12, 2011.

  1. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    Hi, I am planning to use an op amp to amplify a DC voltage. The voltage is positive and will never be negative. Do I need to connect the negative supply pin on the op amp to anything? Ground maybe?

    Thanks guys.
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Yes you need to connect the negative terminal to a suitable voltage which may include the supply common rail - or ground. However not all op-amps work properly with a unipolar supply. Only certain types are suitable - in particular those that are designed for unipolar operation, have a guaranteed capability to drive the output to ground and which include ground in their common mode range.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    You should post a circuit of, or at least explain a bit better what you're planning on doing; what you need for inputs and outputs.

    As T_N_K mentioned, you'd get a surprise if you tried to operate an opamp like a TL072/TL082 from a single supply; as it can't "see" within 3v of the negative rail nor within 1.5v of the positive rail.
     
  4. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    I am planning to use an L165V high power op amp. I want to put it into voltage follower configuration. My input voltages will be 8.5V-13V, and I want the same voltages on the output. Basically I'm going to use this as a buffer to drive 1.5A. The input will come from a DAC which will be controlled by a PIC. The power supply I have to use is +18V.

    I am wondering also, many circuits use op amps. Obviously those circuits don't use a lab power supply for power. The 1 dollar radio you buy at the dollar store doesn't need a lab power supply to power it, yet I am pretty sure it has many op amps. How does one create a negative supply voltage when all you have is a positive voltage?

    This is for a design at my job by the way. The specs are set in stone, cannot change them.
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
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    You cab use the 79XX series of regulators.
    The 7905 is the compliment to the 7805.

    7805 is the +5v regulator
    And the 7905 is the -5v regulator.

    [ed] **** I was wrong, the 79xx does not convert a positive voltage to negative***


    Linked, is the 7905 datasheet. There are also 7912 and 7915 available.

    http://www.national.com/ds/LM/LM7905.pdf

    Also, you can "cheat" by using 2 batteries in series. 9v battery for instance:
    Consider the point where the two batteries connect to be your circuits "GND". The remaining battery (+) is the +9v supply, the second batteries (-) terminal is your -9v supply.

    http://forum.allaboutcircuits.com/showthread.php?p=228725#post228725
     
    Last edited: Jan 12, 2011
  6. SgtWookie

    Expert

    Jul 17, 2007
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    You should be OK, if you watch the power dissipation in the opamp. You might have some problems with that. I don't have the time to calculate it.

    Is it just sourcing current, or will you be sinking current as well? IE: what type of load is it?

    Make sure to use the Boucherot cell on the output like they show in the datasheet (1 Ohm in series with 0.22uF) to dampen any tendency to oscillate. Make sure you use caps across the supply pins; I'd suggest at least 0.1uF and 100uF or larger.

    See "Figure 13, Split power supply" schematic on page 7 in the datasheet for this IC.

    If the DAC will have a low frequency output, you will probably do OK with it - but see the caveat above.

    More details will help avert possible disaster. I'm out of time for this evening.
     
    Last edited: Jan 12, 2011
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