How to deal with heat-sinking in an VHF transistor with case-collector connection?

Discussion in 'Wireless & RF Design' started by Spottymaldoon, Feb 13, 2016.

  1. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
    I am trying to use a VHF transistor whose collector is internally connected to the case to provide a ~2W output. Now, running at that sort of power, it's clear that you need a heat sink on the case yet, so far as I can see, any decent heat sink would add enormously to the collector-to-ground capacitance and, working up in the 100MHz+ range (still less for the 400MHz the transistor is rated at), that looks pretty prohibitive.

    Here is the setup recommended by the manufacturer:

    2N3866 schematic.jpg

    and here are the specs they give.
    Untitled-2 copy.jpg
    Ideally I'd want the case to be isolated as in the now-discontinued 2N3375 and so use a nice case-ground.

    What would be the usual workaround here?
  2. bertus


    Apr 5, 2008
  3. KL7AJ

    Senior Member

    Nov 4, 2008
    Another common solution is to use the collector to heat sink capacitance as part of the matching network.
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    newer types of transistors have the case tied to the emitter, allowing the case to be grounded. they typicly have higher output than the older types.
  5. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
    Thanks for the suggestions/observations. Would I be right in saying that for VHF transistors operating much over 1W the general move has been to use FETs? Apart from the high input capacitance and possibly the cost, is there any other disadvantage to using a FET?