Kai,
Thanks for such a detailed explanation.
It gave me many information.
also, for the analysis for BC 556 part,
I tried as such.,
once the port changes to logic low state,
the current flowing through the 10K resistors is = 70uA (.7 V drop in each 10K resistor)
At this instant, the transistor is about to turn ON.
Once turned ON, the current through the 10K (in base) = (4.3/10000)+ 70uA = .5mA

So only at .5mA of base current of BC 556, its collector current becomes 3.58mA.

Is this correct?


Suresh.