Roy Lewallen wrote:
Tim Wescott wrote:

. . .
The "saturation" in bipolar transistor terminology means "current
saturation", but it could just as well mean "carrier saturation".
When the transistor is saturated the base region is stuffed full of
carriers (holes, for an NPN transistor). It takes a while for those
carriers to go away, during which the transistor stays on. This is a
very nonlinear effect, and can be very slow. The old 74Sxx series
logic put a schottkey diode from collector to base on the transistors
to keep them out of saturation, and sped them up considerably.


The very slow saturation recovery you see in saturated switch
applications (unless using a gold-doped transistor) is largely absent in
typical RF power applications. The reason is the bipolar drive usually
employed -- there's typically a large amount of negative base current
available to suck the stored charge out of the base region in a hurry.

. . .
I didn't think of that -- in spite of having applied it in a small
switching regulator application, per an ap note by Zetex.

D'oh.

Thanks for pointing it out -- perhaps I'll remember this second
application. Perhaps when the third one comes around I'll put one and
one together...

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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