At 04:50 PM 6/28/2008, you wrote:

> > We analyzed darlingtons in EE class.  The second transistor never
> > gets into saturation and has Vce much higher than Vce(sat); I forget
> > the exact reasons,
>note: for the purposes of this discription I will call the transistor
>that handles most of the current the power transistor and the transistor
>that connects to the input the signal transistor.
>
>The problem is that the signal transistor acts as an emmitter follower.
>but it can only do that if it's collector is at a higher voltage than
>it's base.
>
>Once the voltage accross the collector and emmitter of the output drops
>below the base-emmitter voltage of the power transistor the signal
>transistor starts acting like a diode and the only current gain is that
>of the power transistor. Of course there is a transistion region between
>good follower action and diode action.
>
>If you drive enough current into the base you can bring the output
>voltage right down but if you do that then there was no point using a
>darlington in the first place.

Not really correct. As the base current of the "signal" transistor
increases, it will begin to flow into the collector of the
"power transistor"  and you'll never get the latter to have a low Vce.
The collector current of the "signal" transistor *reverses* for high
base current.

Roughly speaking, the minimum output voltage of a darlington
with a pullup on the collector (or with more than tiny base current)
will never be less than a Vbe (diode) drop.

>In general given the relatively high current drive of pic outputs
>darlingtons really aren't needed.

An ordinary BJT can be counted on for but 10:1 or 20:1 of current gain,
if low Vce is a requirement, so darlingtons have their uses, as do
MOSFETs.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist