MIKE ROSS wrote in a message to Roy J. Tellason:

 MR> "Roy J. Tellason" bravely wrote to "Greg
Mayman" (08 Jan 04 
 MR> 04:06:32)  --- on the heady topic of "*BIG* TRANSISTORS"

 RJT> Greg Mayman wrote in a message to Roy J. Tellason:

 RJT> True,  but gain isn't that much of an issue with power devices.

 GM> I dunno about that. If I'm using a power device to pass 10 amps or
 GM> so, I'd like to be able to drive it with a considerably lower
 GM> current. So a reasonably high gain -- say 20 or higher -- is
 GM> definitely an advantage.

 RJT> Yeah,  I'd be a bit disappointed if the gain were less than 20.   :-)

 MR> HFE isn't everything. For power transistors what we really we want
 MR> is a "POWER" gain, i.e. since P=VI we want lots of V for the same
 MR> I. For instance many power output transistors have gains of 7 to
 MR> about 10. Interestingly the common base configuration, often used
 MR> in RF amplifiers, has a current gain "alpha" of just under 1 or
 MR> about 0.95 etc so straight current gain is irrelevant in this mode.
 MR> Even power switching uses don't need much gain, for example TV
 MR> horizontal output transistors.

I think that the horizontal output types do have some nontrivial gain, 
though the main feature there is their ability to withstand some serious
collector voltages.  I have a number of those kinds of parts in my salvage,
 don't yet know what I'm going to do with them.  Collector current
capability is a couple of amps,  typically,  but voltages go *way* up there
-- to as high as 800V if I'm remembering right.  The only other application
I can see those being useful in would be switching power supplies,  and
those are generally higher-current devices,  so I don't know how useful
those parts would be in that application.

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