I like to use SiGe microwave transistors for all sorts of off-label things, as some SED veterans may remember.
The Infineon BFP640 and BFP650 parts aren't the absolute fastest any more--they top out around 40 GHz f_T, whereas the BFP8xx series get up to over 80 GHz.
However, they have other amazing properties that make up for their slowness. ;) These include flatband noise voltages of 0.5 nV in 1 Hz; betas around 250, which is amazing for an RF device; a low 1/f corner; and, best of all, an Early voltage so large that it's hard to measure.
(It's 250V or so, AFAICT--you have to measure quickly or the self-heating of the device makes the curves go the wrong way!) That lets you run high gain in a single stage, without worrying much about linearity. They're also _amazing_ cascode devices.
However, they do have a tendency to oscillate if you look at them crosswise, e.g. give them more than a few mA of collector current.
That's fine for lots of things, but lately I've been wanting to make biased-cascode SPAD(*) front ends for time-of-flight applications.
SPADs have a fair amount of capacitance for their speed, so you have to terminate them in a low impedance, ideally only a couple of ohms.
Packaged parts have inconveniently high inductance for that, but we should be able to get emitter impedances below 10 ohms up to 3 GHz or so. Besides sub-nanohenry inductances, that requires fairly high collector currents, like 10 mA or thereabouts, putting their f_Ts above
20 GHz. (The resistive part of the emitter impedance is the usual r_E ~ 25 mOhm/I_C.)Naturally they want to oscillate like crazy, so they need base stoppers(**). For really fast transistors, I like to use the Murata BLM15BA005 and BLM15BA010, which are 0402 beads with impedances of 5 and
10 ohms at 100 MHz, respectively.Those ones have a nice low-Q impedance peak at around 3 GHz, which works pretty well with ordinary fast things. However, they're a bit wimpy for these SiGe BJTs, which I've seen oscillate at 12 GHz.
Soooooo, imagine my delight when I found that there are ferrite beads specified by their impedance, not at 100 MHz, but at _5 GHz._ I'm sure they're old hat to cell phone designers, but I don't pal around with any of them, so they're new to me.
For instance, the Murata BLF03VK221SNGD has 220 ohms impedance (mostly resistive) at 5 GHz, and is still over 100 ohms at 10 GHz. It's 0201 size, of course, but LCSC has them, so I can get JLCPCB to solder them down for me.
Fun.
Cheers
Phil Hobbs
(*) Single-photon avalanche diode
(**) Something resistive-looking that you put in series with the base of a transistor to keep it from oscillating. Extra points if the resistance is mostly up at frequencies you don't need for your measurement. (That's where ferrite beads come in.)