"Roy J. Tellason" bravely wrote to "Greg Mayman" (08
Jan 04  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.   :-)

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

 Mike
 ****

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