> 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.

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