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I'd like to use a simple non-inverting amp circuit in a small project I am working on. The project is battery powered, so naturally the only voltage I have access to is 3.3v. My question is: how should I get the voltage for the -ve rail? Of course I can add a DC-DC converter but I'm sure this is already a solved problem so I figured I'd ask first.

Thanks!

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  • \$\begingroup\$ Do you need a negative voltage? couldn't you just manipulate the 0v for what you need with a not gate. \$\endgroup\$
    – Dean
    Nov 16, 2010 at 8:08
  • \$\begingroup\$ @Dean, how does one use a not gate to manipulate 0V? (I suppose a charge pump might be an option, but there are simpler ways of solving the problem.) \$\endgroup\$
    – Thomas O
    Nov 16, 2010 at 16:03

2 Answers 2

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It's common in modern battery-powered audio electronics not to reference audio to 'true' ground, with +/- supply rails. Instead, you make an audio reference half way between ground and your voltage rail (often with a pair of resistors and a buffer of some kind). Then you arrange your audio signal to swing either side of this mid-rail reference - one tends to end up with quite a lot of series coupling capacitors to let you shift the DC point of the signal around.

Alternatively, if you don't like that (and there are reasons not to - getting the reference quiet enough and stiff enough can be a pain, as can the series caps, and at 3v3 you're fairly short of headroom once you've cut it in half), then a -ve rail can be produced using a charge-pump device like a MAX660 (there are lots of other sources for basically the same part) - that's a lot simpler than a full-up DC-DC converter.

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  • \$\begingroup\$ You don't need to buffer the bias voltage for an inverting amp. You're just connecting it to the non-inverting input, which is a high input impedance. What you need is a lot of capacitance between the bias voltage and audio ground, to eliminate any power supply noise. \$\endgroup\$
    – endolith
    Nov 16, 2010 at 15:34
  • \$\begingroup\$ @endolith, that doesn't apply for noninverting amplifiers. \$\endgroup\$
    – Thomas O
    Nov 16, 2010 at 16:04
  • \$\begingroup\$ @Thomas Oh, I misread the question. Do this for non-inverting single supply: analog.com/library/analogDialogue/archives/35-02/avoiding/… \$\endgroup\$
    – endolith
    Nov 16, 2010 at 16:11
  • \$\begingroup\$ Be careful with how close to the rail you can output. \$\endgroup\$
    – Kellenjb
    Nov 16, 2010 at 16:47
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Will is correct, but on the other hand if this project isn't audio or simply doesn't require positive and negative voltages then you just need to use an op-amp that can operate with unbalanced supplies such as (pulls random op-amp out of digikey) LT1494IN8. You just connect -ve to ground and +ve to your rail and you're good. You are of course limited to your voltage range for output (0-12V or whatever your battery is) so if you are using audio you'd need to do what Will says, but if you just want to amplify something you can use this.

I'm sure this is a typical question BTW. When I was in school they never let on that single-supply op-amps existed. We always used +/-15V and I always figured that was the only way to use op-amps. But positive and negative supplies are so uncommon in the real world that it pays to learn about these things.

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    \$\begingroup\$ If you are using voltages in between the positive and negative rails, you don't need a special op-amp to operate with unbalanced supplies. The single-supply op-amps are named that because their common mode range includes the negative supply. In any case, check the minimum operating voltage carefully! \$\endgroup\$
    – markrages
    Nov 16, 2010 at 15:57
  • \$\begingroup\$ To operate with unbalanced supplies you need a rail-to-rail op-amp, and it usually helps to check the data sheet to ensure they say they'll operate with a single supply. I've seen both on several data sheets so it's not an odd thing and seems fairly standard. \$\endgroup\$
    – AngryEE
    Nov 16, 2010 at 17:04
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    \$\begingroup\$ you have made two slightly incorrect statements. (1) You do not need a rail-to-rail amp for unbalanced supplies, although with only 3.3V you probably will find rail-to-rail input and output useful. (2) Because no op-amp requires dual supplies, looking for the phrase "works with single supply" on the datasheet is unnecessary. (I think the phrase was coined when the first amplifiers that included the negative rail in the common-mode input range were introduced.) Instead, look at the datasheet for the common-mode range and the output swing to decide if it will work with your supply. \$\endgroup\$
    – markrages
    Nov 22, 2010 at 3:26

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