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

Thread Starter

Spottymaldoon

Joined Dec 4, 2015
63
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?
 

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alfacliff

Joined Dec 13, 2013
2,458
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.
 

Thread Starter

Spottymaldoon

Joined Dec 4, 2015
63
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?
 
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