Anybody have any idea what the upper frequency limit is on a 2N2222?
Well, if you put 1GHz in you'll get *something* out... transistors
are usually specced by their "transition frequency" fT where AC small-signal-
current gain has declined to one. Generally fT has to be bigger than one if
you expect to get gain. fT itself is a function of several parameters
(notably collector current). And fT is usually implied by the gain-
bandwidth product at a frequency a few times lower than the measured fT
(with good reason, you usually want a gain of at least a few...)
If you're using the transistor as a saturating
switch then other parameters become more important, particularly base
capacitance. Many manufacturers publish "turn-off time" curves related
to this. In certain switch applications a jellybean NPN wouldn't be good
above a few MHz.
Datasheet lists "typical freq (min)" as 200 MHz, but that's all. Any help
For current PN2222A parts, fT = 250MHz or 300MHz at collector current
of 20mA. It declines for both higher and lower currents. I don't know what
the original JAN specs said.