do I need to put a pull down on the base of your garden variety
2n2222? if the base is left floating, no base current = transistor
off, right?
No base current = transistor off, yes.
If you charge up the base, then leave it floating, the base capacitance
must discharge through the base, which takes time. It will be faster if
it discharges through the current-limiting resistor that you may find on
the output of a CMOS logic device, but it'll discharge faster yet if you
put in that resistor to ground.
If your base is sitting there at 0V with lots of capacitance to the
world (or just the wrong conductor) your transistor may turn on from
EMI. The resistor to ground will help prevent this.
If you're running the thing close to it's rated voltage, a resistor from
base to ground will help keep it from leaking excessive collector current.
There are more caveats yet, but these are the biggest ones that I can
think of.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See
http://cfaj.freeshell.org/google/
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at
http://www.wescottdesign.com/actfes/actfes.html