William Chops" Westfield" 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, and I remember being rather ... stunned.

Just look at it as two independent transistors and it should all make sense.
Look where the base current of the second transistor comes from.  It all
comes from the emitter of the first transistor.  And where does that come
from?  Mostly from the collector of the first transistor, which is also tied
to the collector of the second transistor.

Now add up the voltages required to keep the second transistor on.  The base
will need about 700mV.  Even with the first transistor saturated, figure it
will drop 200mV.  So that means the output (the common collectors) needs to
be at least 900mV to keep the whole darlington "on".


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