# Creating negative rail supply with op amps ?

Discussion in 'General Electronics Chat' started by MathematicianKing, Nov 14, 2017 at 11:47 AM.

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1. ### MathematicianKing Thread Starter New Member

Nov 10, 2017
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Hi there,

is it ok practice to generate negative voltage supplies using a simple op amp with neg feedback ?

If I have a single +ve supply and I need a -ve supply of some value, can I just build this opamp circuit to generate it?

I've seen solutions using diodes and caps but which is best ? Also using more elaborate circuits with opamps and such.

2. ### dl324 AAC Fanatic!

Mar 30, 2015
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An opamp can only produce voltages between it's V+ and V- supplies. Post a schematic of the circuit(s) you're referring to.
Best depends on current and voltage requirements.

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3. ### MathematicianKing Thread Starter New Member

Nov 10, 2017
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The voltage would be lower than V+.

Suppose the current requirements are satisfied by the opamp, is this a solution that is used by anyone ? Or this is just an unusual solution ? In other words, where does this fit in the ultra grand godly scheme of things?

4. ### dl324 AAC Fanatic!

Mar 30, 2015
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The most typical case is to just use dual supplies.

For low currents, charge pumps are often used.

5. ### MrChips Moderator

Oct 2, 2009
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As Dennis says, you can't get a negative voltage from the opamp if you don't have a negative supply.

(There are exceptions, of course. If you AC couple a signal it will go negative. You can also use a charge pump to create a negative voltage from a positive supply.)

If you must use a single supply opamp then you can create a pseudo-ground that is half-way between the supply rails.

6. ### OBW0549 Well-Known Member

Mar 2, 2015
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Please post a schematic diagram of this "solution" you're referring to; I can't make heads or tails out of what you're saying.

Op amps don't generate negative supply voltages; they use them, and can only create a negative output voltage if they're given a negative supply to operate from. As @dl324 said, an op amp can only output voltages lying between the supply voltages applied to its V+ and V- supply pins.

You seem to be operating under some misconception or other, though I can't figure out what it is...

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7. ### MathematicianKing Thread Starter New Member

Nov 10, 2017
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I have an LM358 here which operates from a single supply. Can I use this opamp in an inverting feedback config so as to generate a negative voltage that I could use as a negative rail in my circuit ? I don't have schematics or a circuit, it's something I want to know.

8. ### MrChips Moderator

Oct 2, 2009
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Why do you keep asking the same question even after we told you it would not work?

1) use a dual power supply
2) use a charge pump to generate a negative supply
3) use a pseudo-ground halfway between GND and the positive supply.

9. ### OBW0549 Well-Known Member

Mar 2, 2015
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No.

One more time: an op amp can only output voltages lying between the supply voltages applied to its V+ and V- supply pins. Period.

10. ### GopherT AAC Fanatic!

Nov 23, 2012
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Read about the phrase "clipping". It means, even if your op amp has a gain of 1200, it won't output a 1200v signal for a 1v input. The output won't go more than supply voltage (for an op amp with rail to rail output). It won't go to more than 7 volts for a 9 v supply if you have a traditional op amp.

Also, just because you are configuring an inverting output, it won't generate a -1200 volt signal. It won't go below 0v on a single supply op amp (rail to rail) and it won't go below 2v for a traditional op amp. Understand?

11. ### MathematicianKing Thread Starter New Member

Nov 10, 2017
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Mod edit: snipped abusive reply. JohnInTX

Last edited by a moderator: Nov 14, 2017 at 12:54 PM
12. ### MrChips Moderator

Oct 2, 2009
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<snipped abusive quote> JohnInTX
I am not confronting you. You have been told repeatedly that an opamp cannot generate a voltage beyond its power supply rails. It is simple as that. There is nothing special to understand.

Last edited by a moderator: Nov 14, 2017 at 12:42 PM
13. ### GopherT AAC Fanatic!

Nov 23, 2012
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Mod edit: snipped abusive quote - JohnInTX

Last edited by a moderator: Nov 14, 2017 at 12:43 PM
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14. ### MrChips Moderator

Oct 2, 2009
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<snip>
For a new member, I am sorry to say you are off on the wrong track. This forum does not tolerate belligerent behavior, period.

Mod edit: snipped abusive quote - JohnInTX

Last edited by a moderator: Nov 14, 2017 at 12:44 PM
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15. ### ian field AAC Fanatic!

Oct 27, 2012
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It can be pretty simple, you need an inverter (common usage puts "inverter" into an unrelated context). A lot of people use a 555 to drive a capacitor/diode charge pump, or there's the off the shelf 7660 chip. You can do it with a single transistor, inductor and a few various components, but its pretty much trial and error which can get a bit fiddly.

16. ### cmartinez AAC Fanatic!

Jan 17, 2007
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<snip>
If you wish to generate a negative voltage using a single positive voltage source, then I suggest you take a look at page 9 of this datasheet..

Mod edit: snipped abusive quote - JohnInTX

Last edited by a moderator: Nov 14, 2017 at 12:45 PM
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17. ### MrChips Moderator

Oct 2, 2009
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TS has been given a timeout.

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18. ### cmartinez AAC Fanatic!

Jan 17, 2007
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Good... it's unfortunate that some people think they can get free advice from others whilst insulting them at the same time...

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19. ### OBW0549 Well-Known Member

Mar 2, 2015
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Ummm... I think it's you, not me, who has "gone impatient"; I've just been trying to patiently explain to you that you can't get something for nothing-- i.e., that an op amp can't output a negative voltage if it isn't provided with a negative supply voltage in the first place.

Don't believe me? Try it yourself: get an op amp, configure it as an inverting amplifier, power it from a single +V supply, and give the circuit a positive input. What output do you get? Try it.

Yes, I know; I got that from your very first post. And, like the others here, I've been telling you that you CANNOT get an op amp to output a negative voltage given only a single, positive supply; yet you don't seem to want to accept that simple, obvious fact.

Mod edit: snipped abusive quote - JohnInTX
No comment.

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20. ### JohnInTX Moderator

Jun 26, 2012
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Last edited: Nov 14, 2017 at 6:01 PM
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