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How can I generate a negative supply voltage for an Opamp?

  1. Aug 31, 2015 #1

    goodphy

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    Gold Member

    Hello.

    I really don't get a concept of a way to generates negative voltage as a one of the supply voltage to typical OP-AMP. (Operational Amplifier)

    The output voltages is supposed to swing to more than 10 V thus supply voltages to OP-AMP should be +15V and -15V.

    I'm thinking of supply these voltages from typical 220 VAC home power line.

    Could you please give me some ideas? The basic concept of generating negative voltage or simple way of doing this job would be either helpful for me.
     
  2. jcsd
  3. Aug 31, 2015 #2

    rbelli1

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    Have you tried searching the web for such a supply? There are many ways of getting what you want but the details of what you are trying to accomplish will determine what is needed to get the proper voltages, currents, and noise levels.

    BoB
     
  4. Aug 31, 2015 #3

    berkeman

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    Staff: Mentor

    Unless you have a *lot* of EE experience, you should not be trying to build projects that involve tapping off of the AC Mains supply. It is best to start with an SELV power source (safety extra low voltage, typically less than 60V). You can buy safety-approved "brick" power supplies that plug into the AC Mains and give you a safe voltage you can use to power your project.

    So for this project, I would start with a 220Vac brick to make +15V, and then use an inverting DC-DC circuit to make -15V, followed by a low-dropout linear negative regulator to make about -13Vdc with low noise. You could also use a positive low-dropout (LDO) regulator to make a +13V that has low ripple and noise. What kind of power supply noise tolerance does your opamp circuit have?
     
  5. Aug 31, 2015 #4

    Baluncore

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    I think you are looking for an isolated DC to DC converter with +15V, 0V and -15V outputs. You may get away with +/-12V with newer design op-amps.
    You will need to check the input voltage range to suit your DC source and the output currents you require.
    Here is a link to a selection table. It is the Australian page of a US supplier. Scroll down to see devices.
    http://www.digikey.com.au/product-search/en?FV=fff40042,fff800df,1c0008,1140050,15c0003,2dc1bff,17d4002c,17d40096,17d80006,17d80016&mnonly=0&newproducts=0&ColumnSort=1000011&page=1&stock=0&pbfree=0&rohs=0&quantity=&ptm=0&fid=0&pageSize=25
     
  6. Aug 31, 2015 #5

    rbelli1

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    When you want to swing 10V do you mean +-10V or 10 V total (+-5V)? This is important for device selection because with modern rail to rail op amps you get to within a volt or two of the rail at full spec and then get lower spec from there to the actual rail. Older devices might need several volts to not malfunction completely.

    Even a 741 will be able to get +-5V out of +-12V rails without even sweating but won't be able to do +-10V well if at all. Then you need to consider PSRR for crappy ebay power supplies and then you need LDO's as berkeman suggests and it all goes pear shaped from there.

    All of these considerations may be moot if the requirements are lax.

    BoB
     
  7. Sep 16, 2015 #6
    Hi,

    You need a lab DC source and two to be exact which you can get on ebay. You can construct your own, but that is a project in itself, so you're better on buying a dual source one of which you can connect in a opposite manner to create the negative rail.

    Alternately you can connect the negative terminal to ground and use a single supply op amp if you can get away with it in your application if the need for a bipolar output isn't a must.
     
  8. Sep 17, 2015 #7

    donpacino

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