Yes, it can be done either way.  The advantage of what you saw (mixed
types, all transistors in common emitter configuration) is that
transistor dissipation can be reduced to a minimum.  If all of the
transistors are of the same type, one set will be operated common
emitter (the set with emitters tied to a rail) and the other set will be
operated as emitter followers.  The set operated as emitter followers
will dissipate more power (more than three times as much).
Additionally, less voltage will be available for driving the LEDs.

Best regards,
Dave

"Mitchell D. Miller" wrote:
>
> I saw a sample circuit the other day that made me ask a question I couldn't
> answer.  The circuit was for driving an 8x8 array of LEDs.  Each row and
> each column was interfaced to the driving processor through a transistor.  I
> noticed, however, that the transistors driving the columns were NPN and
> those driving the rows were PNP (or vice versa), which makes column
> selection active high, and row selection active low.
>
> I was wondering if the same circuit couldn't be built using all the same
> transistor (either NPN or PNP), and change the active polarity of either the
> rows or columns so they ALL would be either active high or low.
>
> Am I missing part of this picture?
>
> -- Mitch
>
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