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Op amp supplies

Discussion in 'General Electronics' started by Nir Goren, Jul 30, 2003.

  1. Nir Goren

    Nir Goren Guest

    I'd like to generate a stable negative voltage for my Op amp. The best I
    could come up with is using a “Low Noise, Switched Capacitor-Regulated
    Voltage Inverters”, but as far as I can see this low noise is pretty noisy…



    Is there a better way of doing things?



    Nir Goren

    VLSI center

    BGU University

    Tel : 08-6477154

    Fax : 08-6477620

    Email :
     
  2. Robert Baer

    Robert Baer Guest

    One could use either the switched capacitor scheme, or a switched
    inductor, or a dual voltage supply - if a negative voltage is absolutely
    needed.
    If you do not like the noise at the output of the switched supplies,
    then use a filter and/or a second regulator.
    One could also use a single supply and either "tap" it with resistors,
    or use a device that TI announced years ago that does the same thing
    (only a lot better).
    If the signal in and out of the op-amp will never be negative, then
    use a rail-to-rail device and no negative supply.
     
  3. Rich Grise

    Rich Grise Guest

    What's the current demand? Would a 9V battery work?

    Good Luck,
    Rich
     
  4. Yes. Either way, though, the original poster wanted to be able to
    produce a negative-voltage supply, for an opamp.

    I assume that he already HAS a positive supply, or a single-output DC
    power supply. National's website has an appnote that has a design for
    a nice "power supply SPLITTER", which takes a single voltage and
    splits it into positive and negative voltages of half the original
    voltage each. (And the former ground becomes the neg supply rail, with
    a new ground reference provided by the circuit.)

    One nice thing about that one is that it can push equal maximum
    currents through both sides. And it can do 1 to 1.5 amps through both
    sides when running at + and - 15 volts (or 2 to 3 amps through one
    side at a time). (And it can go up to +/- 30 or 40 volts, IIRC.)

    It's in a National Semi appnote that refers to either the LM675T or
    LM1875T audio amplifier IC (which it uses). I think the appnote had
    it using the LM675T. But I built one with an LM1875T, which is about
    the same (except no free samples). And it works great!

    I added the splitter to a variable-output boost-type switching power
    supply that I had designed (using a 78S40 IC and an external TIP41C
    power transistor), which takes 12V in and produces anywhere from about
    8 to about 70 volts DC out, controlled by a pot. With an added LC
    filter on each output (and a small "snubber" circuit around the
    TIP41C), the switching noise on the outputs, when running under full
    loads (resistive) at +/-15V (1.5A each), is about 10 mV peak-to-peak.
    And the whole thing fits on about 2x4 inches of PCB. (But the
    splitter, alone, should work with any power supply or battery that has
    a voltage within its maximum range.) I also added a 7805 regulator,
    fed by a two-resistor voltage divider across the main positive output,
    so I could have a +5V (but relatively low-current, in mine) output
    available, too, to power CMOS ICs, or whatever. I use this power
    supply board in my Curve Tracer product. (See
    http://www.fullnet.com/u/tomg/gooteect.htm . It's cool!)

    The exact same kind of power supply splitter could be made using most
    opamps, I think, changing mainly the maximum total output current (and
    the maximum input voltage), depending on the opamp.

    Cheers,

    Tom Gootee

    tomg AT fullnet.com

    http://www.fullnet.com/u/tomg
     
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