A little bit, thanks.
For example, I hobbled together a 20~50MHz differential cascode stage using
ancient 2N3646's -- the mushroom shaped things. They happen to have a date
code of 71xx or so. Evidently they are somewhat fast, 350MHz compared to
the 100-300MHz of 3904, etc. So that's about 1/10th fT, which I would
certainly consider fast (especially for a mere breadboarded circuit).
A shame I have to make estimations of bandwidth based on rise time using a
square wave that's not particularly fast, so my estimate is 20-50...
I can push 4401/4403 into the 30ns range, but that's no good if I'm to
explore something like the FM band. It'd also be fun, probably more out of
bragging rights than anything remotely practical, to say I can make discrete
logic in the single digit nanosecond range, too.
I've pulled some UHF transistors from a bunch of high-def monitors. These
things used a cascode/emitter follower video amp stage: PH2369 / Philips
2N2369 in TO-92 for the bottom, BFQ225 (I think), a TO-202, 100V transistor,
and a complementary (two diode biased
emitter follower of BFQ221/241 to
drive the grid or cathode or whatever it was. I've tried making a similar
circuit out of these things, wiring as carefully as I can, point-to-point,
but they're a bitch to stabilize, and it's hard to even see how they're
oscillating -- I suspect they are fond of 500MHz or so, which is a shame as
my scope is only 200MHz!
So yeah, I could look up other transistors as these, but what if I want
other ratings, or packages, or pricing? That's why I want a good