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Amplify opamp output voltage

  1. Feb 11, 2016 #1
    Hello, I need an opamp that can output +/- 150v without paying $500 for a high voltage opamp.
    I need a workaround solution, I'm thinking of a 5x DC gain of the opamp output voltage but I am totally clueless on how to achieve this.
    Any ideas?
     
    Last edited: Feb 11, 2016
  2. jcsd
  3. Feb 11, 2016 #2

    Averagesupernova

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    We will need some more specs than this. What type of signal are you amplifying? DC? AC? If AC, what frequency range? What type of load are you driving?
     
  4. Feb 11, 2016 #3
    DC, I'm using it to bias the grid of a tube. It is for DC offset on the output of an amplifier.
     
  5. Feb 11, 2016 #4

    donpacino

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    will the voltage have to be changed?
    What range should the voltage have?
    How much current will be drawn (power)?
     
  6. Feb 11, 2016 #5
    Not sure what you mean by "will the voltage have to be changed" It's a DC offset servo so the offset needs to be adjustable and the servo itself adjusts its voltage. The servo itself works fine I just need to boost its output voltage, unless you have another plan.
    The voltage range should ideally be +/- 150v but I don't technically think I will have to go very far into the positives in actuality, I need to stretch down to -150v though.
    The current draw is inherently 0 since the grid of a tube is an infinite impedance.
     
  7. Feb 11, 2016 #6

    donpacino

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    IF the application is LOW power, you can use something like this. you will have the change the part numbers, the current pn are just there to get a design in schematic form

    The basic premise is you use an opto isolator to control the power coming from an hv supply. the opto isolator take the voltage drop. you need special opto isolators to do this. You can also drop the power across a powerfet or something similar. again the is for LOW LOW power.

    you use a LARGE value resistor in a voltage divider (or resistor network depending on the equation you want to implement). this will allow you to put the feedback to the negative terminal of an op-amp. The positive terminal can be used as your control voltage. The bjt on the output of the fet controls the opto-isolator.
     

    Attached Files:

  8. Feb 11, 2016 #7

    donpacino

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    Keep in mind there are a lot more design considerations and safety considerations. this is just an architecture concept.
     
  9. Feb 11, 2016 #8
    Aren't opto-isolators slow acting?
     
    Last edited: Feb 11, 2016
  10. Feb 11, 2016 #9

    davenn

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    your image is totally unreadable ... try again
     
  11. Feb 11, 2016 #10
    I'm not going to rearrange the entire thing to look subjectively readable, forget I posted it, it's not that important anyway. The opto-isolator does seem to be an interesting idea but I've read that they have a lag when changing resistance, is this true?
     
  12. Feb 11, 2016 #11

    Averagesupernova

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    It's DC. How much lag is a problem?
     
  13. Feb 11, 2016 #12

    davenn

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    what's your definition of slow ? .... 100's of kHz ?

    what changing resistance ?
    opto's are semiconductor based
     
  14. Feb 11, 2016 #13
    It depends, the operating points of the stage will be digitally adjustable and I plan to have dynamic operating points based on stored track data to maximize power efficiency so it could change fairly rapidly.

    Well, conductance, or whatever.
    Slow is probably in the low single digit hz.
     
  15. Feb 11, 2016 #14

    jim hardy

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  16. Feb 11, 2016 #15

    jim hardy

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    Aha i gave up too soon

    here's the one i was looking for, 2 pages further in.
    upload_2016-2-11_16-58-58.png

    same link
     
  17. Feb 11, 2016 #16

    rbelli1

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  18. Feb 11, 2016 #17
    That 1000v-300ma booster seems pretty cool, except for that fact both of those circuits are positive voltages only :( 99.9% of the voltages I'll be dealing with are negative. A DC-DC converter does seem like an interesting idea though, perhaps if I can convert DC into ac and then run it through and transformer and convert it back into DC I could have the multiplied voltage I need.
    Thanks for that, I'm currently in LTSpice trying to get Donpacino's circuit to work. I can't seem to get the output voltage to exceed the input voltage, I think my crappy understanding of transistors and opamps is getting in the way here.
    Will I be able to multiply voltage and not just offset it?
     
  19. Feb 11, 2016 #18
    power_supply-png.95676.png
    Wait so is R2 the input or? I can't seem to get it to function the way I want it.
     
  20. Feb 11, 2016 #19

    rbelli1

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    What are the values of your HV_supply and LV_supply?

    BoB
     
  21. Feb 11, 2016 #20

    donpacino

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    the voltage divider R1 and R4 will determine the gain of the signal, the transfer function being output/desired_voltage. the R2 input would be your desired voltage. It can be an output from a microcontroller, a potentiometer with a voltage divider, whatever. There will be a linear transfer function between desired_voltage and output dependent on the value of R1 and R4.

    Even though there are non linear elements that function will remain linear as long as a circuit element does not saturate. like I said before I just used LT spice to draw a quick and dirty schematic. You'll need new part numbers.
    The HV_supply will just have to be a dc voltage source that can source a high enough voltage.
    LV supply can be 5-15 volts depending on the op amp and opto isolator.


    note: I failed to notice that your supply has to be +/- 150. This will not work for a negative voltage, although you might be able to use similar concepts to get something to work.
     
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