gEDA-user: Current source rail blocking

[DJ Delorie]

> Do you have any recommendations how to block the current sources
> from picking up some garbage from the air and causing oscillation?
> Should I block the rail with a single big capacitor against the
> ground, or use individual capacitor for each transistor placed
> between emitter and base?

In my m3a expansion board, I did both.  I put big caps near the
board's power feed, medium caps on the ends of the rails, and small
caps right at each power pin.  In your case, you'd want a medium cap
at the end of each rail to provide low frequency bypassing, and a
small-package cap right at each base for high frequency bypassing.
This is based on my guess that smaller packages plus less traces means
higher inductance; you'd have to figure out how the trace/package
inductance figures in to your RF circuits.

http://www.delorie.com/pcb/m3a/board.pcb