A 'mystery' oscillator that uses a 2N3375 - but how does it work?

Discussion in 'General Electronics Chat' started by Spottymaldoon, Dec 4, 2015.

  1. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
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    I have used this oscillator as an approximately 100MHz driver for an electrodeless lamp. It is supremely reliable and puts out several watts of power:

    https://drive.google.com/file/d/0B1ZlKa1Knz2cWmR6UEdiOEtvd2M/view?usp=sharing

    However, although grateful that it worked (using a transistor that is now only made in Russia), I could never figure out what actual type of oscillator it is and where on earth the tank circuit is lurking.

    I have asked some quite learned EEs this question and they have all given answers but none really convincing - please assist.

    I have also posted elsewhere for advice on a replacement circuit using an MRF136 - the Russian transistors can't handle the same power as the old 2N3375s.
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    This was actually a very popular design ages ago. It's basically a "grounded grid" oscillator, to borrow from tube technology. The emitter voltage is in phase with the collector voltage, all that's necessary is to have a tiny step up with the feedback capacitor.
     
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  3. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
    51
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    Thanks, KL7AJ - this is fascinating input and, doing a bit of searching, it's not something commonly used these days yet I should imagine such an arrangement with a FET would be even more practical (I am eyeing the MRF136 at this moment). Does the oscillator rely upon the internal inductance of the valve/transistor to find its resonant frequency?
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Actually it relies on that small choke in the collector lead in combination with the capacitor between emitter and collector. Tis a SERIES resonant circuit, which is why it is somewhat unusual.
     
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