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

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    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

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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

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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

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    The most typical case is to just use dual supplies.

    For low currents, charge pumps are often used.
     
  5. MrChips

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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

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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

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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

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

    Here are your three options:

    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

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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

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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

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    Mod edit: snipped abusive reply. JohnInTX
     
    Last edited by a moderator: Nov 14, 2017 at 12:54 PM
  12. MrChips

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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

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    The problem is, you need to ask questions about the answer if you don't understand the answer. Please don't keep repeating the question until you get an agreeable answer because agreeable answers don't suddenly appear.

    Mod edit: snipped abusive quote - JohnInTX
     
    Last edited by a moderator: Nov 14, 2017 at 12:43 PM
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  14. MrChips

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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

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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

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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

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    TS has been given a timeout.
     
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  18. cmartinez

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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

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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

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    Last edited: Nov 14, 2017 at 6:01 PM
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