Calling for some advices for generating high voltage RF signals

Discussion in 'General Electronics Chat' started by Matjelo Naleli, Jun 13, 2016.

  1. Matjelo Naleli

    Thread Starter Member

    Oct 7, 2015
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    I have an application which requires a 20MHz high voltage (up to 500V amplitude). I already have a 20MHz signal source I just need to amplify the voltage to reach the desired value. Operational amplifiers seem easy to analyze and design with compared to transistors however I haven't seen any op amp that can work with voltages as high as 500V. I only came across some few transistors so far which can work in high voltage regimes (IGBTs & MOSFETs in sepcific).

    I find myself wondering if building a 20MHz (resonant) amplifier is easier than building a 20MHz RF transformer.

    I also wonder if building a high voltage 20MHz oscillator from scratch is more easier & cheaper.

    I would appreciate some advices on some 'known' components which could be useful for building such an amplifier taking into account the high voltage tolerance and sufficient gain-bandwidth to accomodate 20MHz while still offering sufficient step up from about 10V to 500V.

    Thanks
     
  2. Marley

    Member

    Apr 4, 2016
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    500V at 20Mhz! Could be dangerous!
    What is the application?
     
  3. BR-549

    Well-Known Member

    Sep 22, 2013
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    Nothing can be suggested without a power requirement. How much current needs to be pulled from the 500 volts?
    And do you have a high voltage supply?
    Does the rf need to be clean? ......are you going to be transmitting into the air?
    It will depend on WHAT you are doing.
     
  4. Matjelo Naleli

    Thread Starter Member

    Oct 7, 2015
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    Yeah that's true it could be dangerous. It's for Physics experiments. We need use high RF voltage to drive Linear Paul Ion Trap. In our lab we trap Ytterbium ions and hope to 171 isotope to make qubits. Based on the mass of Ytterbium, dimension of our trap and the stability requirements from Mattheu equations describing the dynamics of ions in an RF trap, we find ourselves haviing to supply up to 800V peak to peak across the trap electrodes in order to generate a steep enough potential to confine the ions.

    No as things stand we don't care at all about how low the current could be. The only major thing required is the voltage to be that high. So this is why I couldn't be specific on what current should be drawn at what junction. I guess as far as current goes, it's a free variable in design.

    The voltage should be clean as well. At the moment we use a hellical resonator as our bandpass filter but tuning it is a pain. We actually want an alternative narrowband (resonant) filter to replace it. The op amp / transistor based resonator that I proposed couldn't work because we couldn't find high voltage components with large enough gain-bandwith product..When we implemented positive feedback loop in simulations we could get very high Q values but the closed loop gain becomes too high resulting in a too small bandwidth for 20MHz application.

    Ohh yes we have high DC voltage supplies, (a couple of kilovolts)..
     
    Last edited by a moderator: Jun 13, 2016
  5. nigelwright7557

    Senior Member

    May 10, 2008
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    You will get thermionic valves that will go to 500 volts and 20MHz easily.
     
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  6. Matjelo Naleli

    Thread Starter Member

    Oct 7, 2015
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    I don't know about Thermionic valves, so I will have to read about them first. Thank you nigelwright7557
     
  7. Marley

    Member

    Apr 4, 2016
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    Does the accuracy and stability of the frequency matter? What about the amplitude?

    This is not an electronics project for "those with only a little knowledge"!

    There are also safety issues here. If you touch the wires carrying this RF voltage you will not feel much. But you will notice a burning smell as your skin disappears!
     
  8. Matjelo Naleli

    Thread Starter Member

    Oct 7, 2015
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    Given that I manage to get a high voltage transistor, do you guys think tuned (or double-tuned) amplifier can be implemented and work well? I have only implemented the transistor-based amplifier for simple audio signals in the past so I am don't trust my skills in high voltage, medium frequency applications

    Yes the frequency stability matters but that's mostly the job of the resonators. Thanks for pointing out safety issues too. They should come first.
     
    Last edited by a moderator: Jun 13, 2016
  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Something to read...
     
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  10. Marley

    Member

    Apr 4, 2016
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    Well I'm thinking of a 800-1000V power supply, an 813 tube (look here) running in class C, a nice big coil made of copper tubing, a high voltage variable capacitor, etc. The whole thing driven by a high power signal generator. Could be self-oscillating but will not be stable or accurate.

    I'm signing out of this thread!
     
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  11. Matjelo Naleli

    Thread Starter Member

    Oct 7, 2015
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    That's a good suggestion actually Marley. I will look into this 813 tube. Thanks

    Wow fantastic paper Kermit2..how did I miss this one!! Thanks a lot..
     
    Last edited by a moderator: Jun 13, 2016
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    Possibly the simplest solution is the casc-ode circuit used in video output stages for hi-res CRT monitors. They usually ran somewhere around 110V so you'd have to scale up the device ratings for your design.

    The casc-ode is a common emitter driving a common base - the common base section can be very fast for a given transistor spec. The really good high res monitors had a complementary pair of emitter follower transistors after the casc-ode, that was to drive the highly capacitive CRT cathode.

    The grounded input section of the casc-ode can be a MOSFET - types rated from 600 to 900V were often specified in flyback direct off mains SMPSUs. Current ratings upto 9A exist. High voltage emitter follower pairs might be harder to source - especially the PNP, but if you're not driving a capacitive load, you might get away with just the casc-ode.

    You don't need a high voltage oscillator - the input stage of the casc-ode is just the base of a bog standard common emitter.
     
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  13. Matjelo Naleli

    Thread Starter Member

    Oct 7, 2015
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    Sadly the load in my case is a trap which is essentially a capacitor (15pF to 30pF). But your suggestion sounds great, I am already looking into cas-ode now to see if I can use it for my case. Thanks
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    At 20MHz; that much capacitance isn't a huge problem.

    One thing to watch - the common base section of the casc-ode has slightly less than unity current gain. Its collector current will be slightly less than the collector current of the common emitter stage driving it. Ie = Ic + Ib. If you use a MOSFET; there's no Ib to worry about.
     
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  15. Matjelo Naleli

    Thread Starter Member

    Oct 7, 2015
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    Thanks for the tips ian field, I appreciate sharing your thoughts and ideas.
     
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