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What kinds of Op Amp should I use?

Discussion in 'General Electronics Discussion' started by loukouk, Nov 11, 2013.

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  1. loukouk

    loukouk

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    Nov 11, 2013
    Hello

    This may be kind of a newbie quesation but bear with me.
    I want to create this circuit http://i.imgur.com/5MTQp4e.png to mod an xbox controller I own, where Vout is accross the rightmost resistor. This should add the voltages of all 2.7 sources together to give me a Vout (summing amplifier). The thing is that I'm not sure what kind of Op Amp I should get that could work with a voltage supply of 5V, but still output a voltage of 16 V. Is this even possible? And does this schematic make any sense at all?

    Thanks for the feedback :)
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    You're right to doubt the possibility of this. No OpAmp can deliver more output voltage than the supply voltage. If you need 16V output voltage, you need at least 16V supply voltage or more, depending on the OpAmp. Only so called rail-to-rail OpAmps can deliver up to the supply voltage. "Normal" OpAmps can drive an output voltage a few volts less than the supply voltage.

    The schematic as it is doesn't make too much sense unless you've simplified it and V3 is in reality 6 different voltage sources, is it? Then this will work as a summing amplifier with a gain of -6 (5kOhm/810Ohm). Summing 6*2.7V with a gain of -6 will require an output drive of -6*2.7V*6=-97V. That's ridiculous and no standard OpAmp will be capable to deliver that much.

    Tell us more about what you want to achieve in terms of input signals and expected output signals and we may come up with a suitable circuit.
     
  3. loukouk

    loukouk

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    Nov 11, 2013
    Oh I thought the gain would be equal to 5000/810 and that we could just times that by Vin to get Vout, and get 16.7 V.
    I was trying to use an Op Amp because I'm modifying an existing circuit board I have, and there are 6 2.7V outputs that I want to tie together so that I can use an element that requires a heavier voltage to function. The thing is that this circuit board is connected via usb, so I only have a 5V supply :/

    thanks again for the feedback :)
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
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    Nov 17, 2011
    It is 5000/810=6.172... and the sign changes, so gain=-6

    Almost right, for one input, so 2..7V*(-6.172)=-16.7V
    But: you add 6 inputs, that's the purpose of a summing amplifier, so 6*16.7V=100V (97V in my first reply because I had rounded the gain down to 6).

    What's that supposed to mean? Do you want to generate 16V from 2.7V? An OpAmp is not the right component for this. You'd need a step-up or boost converter.
     
  5. loukouk

    loukouk

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    Nov 11, 2013
    ok thanks I'll look into that :)
     
  6. BobK

    BobK

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    Jan 5, 2010
    What are you trying to do? I suspect you are trying to step up a 2.7V power source to get 16V. For this, you need a boost converter.

    Bob
     
  7. loukouk

    loukouk

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    Nov 11, 2013
    Well I don't have just one 2.7V source, I have 6 of them. I know it isn't very clear on the diagram, but all 6 of the 2.7 V sources are separate, and I want to add all of their voltage together. A boost converter seems to be for only a single sources? Like a BJT amplifier?
    Also I realize that that 5k resistor creates an impossible gain (as Herald Kapp noticed). Just assume all the resistors are of equal value in the schematic >:) so that the output voltage is around 16V
     
    Last edited: Nov 12, 2013
  8. BobK

    BobK

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    Jan 5, 2010
    Being cryptic does not help to get your question answered.

    Why don't you tell us what these 2.7V sources are and what you are trying to do with them?

    Bob
     
  9. loukouk

    loukouk

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    Nov 11, 2013
    I have an controller board that has six 2.7V outputs which all output the same logic. I wanted to use that logic with an element that requires a bigger voltage load. What I thought I could do was add the voltage at these 6 outputs and not use a boost converter on a single one because that would probably create too big of a current draw on a specific part of the board. Sorry if I'm not making sense. I realize it seems kind of silly but I'm trying to mod an existing circuit I have, which gives me some limitations.
     
  10. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    These 6 output most probably have the same ground. So you cannot add up their voltages.
    It seems that you want to use (at least one of) these outputs to control a load (on/off?) which requires a higher operating voltage. It still isn't totally clear from what you write.
    The easiest way is to use a common emitter transistor circuit where the load is connected in the collector circuit. See this example using LEDs. You can replace the LEds by another load. You'll have to ensure that the transistor is suitable for driving the current's load.
    You supply the higher operating voltage (Vcc in the example) from any available source. If this voltage is not present in the circuit you are going to mod, use a boost-regulator to create it from the circuit's power supply voltage.
     
  11. loukouk

    loukouk

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    Nov 11, 2013
    Okay thanks. That makes perfect sense
     
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