Thanks to all those who gave suggestions.

Timothy Gray wrote:
>I used to make a tip31 have more gain by driving it with a 2n2222 a little
>more circutry but cheaper than the hi-gain hi-power transistors. Just a
>thought,

Thanks Timothy, but a darlington configuration has both too high a
saturation voltage as well as too high Vbe turn on voltage for my
application.  Thanks for the suggestion, though.


Tom McGahee wrote:
>Have you considered using a Direct Coupled Compound (DCC) arrangement? This
>has the gain advantage of a Darlington, but has only one forward
>diode drop (Vbe drop).

Thanks Tom.  Yes, I had considered a compound transistor (thats what we call
them here) but the saturation voltage was higher than I liked.  Great
suggestion though.

I guess that I should mention what this is for.  I'm trying to come up with
a low dropout series pass device for a gell cell charger.  My current
designs use LM317 regulators as pass devices - these dropout at about 2.5V.
They work great for line powered chargers (I've built LOTS of them) but are
a problem when the charging supply comes down a very long data cable (part
of my Pyro control system).  Every volt of dropout that I eliminate adds
several hundred feet of cable.

The newest design has a pass transistor driven by a LM10.  The whole thing
vaguely emulates a LM317, except that the reference voltage is 0.2V instead
of 1.2V.  It would take a total redesign to use mosfets (I've been
considering them, though).

Thanks for all the suggestions.

dwayne


Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(403) 489-3199 voice     (403) 487-6397 fax