Mosfet amplifier help

Discussion in 'General Electronics Chat' started by coinmaster, Feb 9, 2016.

  1. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    Hello again, I have a opamp servo that has a max output voltage of about 40v and I need to output more voltage than it's rated for. I need a linear voltage swing of +25v (maybe more) to -150v.
    I'm starting to reach the point where my limited understanding of transistors is becoming a bottleneck.
    From my understanding, mosfets are terrible voltage amplifiers and will need some sort of feedback to linearize them. Also from what I understand I would need a P type mosfet and they don't amplify positive voltages correct?
    Any advice on how I would go about doing this?

    To sum up I need to have my opamp servo which currently swings +/- 40v and amplify its output with a gain of 5 so it can become +/-150v which will be used as a voltage reference for a vacuum tube's grid bias.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    What supply voltages do you have?
    For ±150V output you will need ± supply voltages somewhat higher than that.
     
  3. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    I'll be ordering a custom transformer for this so the supply can be whatever I need it to be. I have bench supplies that can go up to 300v (800v if I really wanted) at the moment.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    How rapidly do you need to change the output voltage?
     
  5. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    .05 miliseconds from the change in voltage on the inverting input of the opamp. It's to keep 0vDC null on the output of an amplifier and it will be applied to the grid of a tube where 20hz-20khz is applied. Then again I wonder if it matters, I'm not sure if a change in voltage as fast as the AC frequency would end up canceling out the signal, I think the better answer would be I don't know.
     
    Last edited: Feb 9, 2016
  6. crutschow

    Expert

    Mar 14, 2008
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    If you want a DC null on the output then you don't want to respond to the AC voltage, just it's average (DC) level.
    So I would think you would want its response to be much less than 20Hz, probably a fraction of a Hz, perhaps with a integrator in the loop so there is no steady-state DC voltage.
     
  7. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    1hz is 1 second. I don't want a 1 second lag for my DC null, that will cause a mess of problems in my design. How fast does an opamp switch voltages?
     
    Last edited: Feb 10, 2016
  8. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    Anyone?
     
  9. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    *poke*
     
  10. dannyf

    Well-Known Member

    Sep 13, 2015
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    bootstrap it.
     
  11. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    You can't bootstrap DC.
     
  12. coinmaster

    Thread Starter Member

    Dec 24, 2015
    350
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    I think you are trying to help me get my servo to work. My servo works fine all I need is to up the output voltage. Surely someone knows how to get 5x gain out of a mosfet?
     
  13. crutschow

    Expert

    Mar 14, 2008
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    You say you don't know how fast you want it to adjust the voltage and then you say a 1 second lag is too slow.
    How will you keep a faster adjustment from affecting the low frequency response of the amp?

    And you can bootstrap DC as dannyf suggested.
    Here's an op amp bootstrap approach for a high voltage output.
    For ±150V output the transistors should be rated for at least 200V.
     
  14. coinmaster

    Thread Starter Member

    Dec 24, 2015
    350
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    I don't know the exact speed but the servo functions fine as is, I've tested it in my amplifier. Perhaps you know something I don't?
    servo-1.png
    It's simplified but you get the idea, the opamp takes its input from the output of the amp and then biases the triode until the output is zero. Is the opamps speed not near instant? Does that 1uf capacitor slow it down?
    I'll take a look at that link, thanks.
     
  15. crutschow

    Expert

    Mar 14, 2008
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    That circuit is an integrator with a time-constant of 100kΩ * 1μF = 0.1s, giving an integrator unity gain frequency of 1.6Hz.
    The output thus does not change instantly.
    What does the signal input circuit to the grid look like? What signal is driving the grid?
    That, along with the integrator response, will determine the amps low frequency response.
     
    Last edited: Feb 11, 2016
  16. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    The input signal is from a triode with a MU of 20 which probably makes a 20v rms AC signal based on typical dac output voltages which goes through a 1uf coupling cap to grid.
     
  17. crutschow

    Expert

    Mar 14, 2008
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    Do you know what the output resistance of the triode is?
    Also what is the gain from the grid to the output load?
     
  18. coinmaster

    Thread Starter Member

    Dec 24, 2015
    350
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    Around 7k atm but it will vary when I'm done with it.
    1
     
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