Hi Gerhard, On 06.07.22 18:28, Gerhard Hoffmann via time-nuts wrote: > Am 2022-07-05 18:04, schrieb Bob kb8tq via time-nuts: > >> If you need that sort of isolation, it certainly can be done. >> NIST has papers on very simple / DIY compatible cascode >> amps that will do the trick. ( chain of common base stages >> driven by a common emitter). Some folks on the list have >> gone a lot further in terms of complexity than NIST did. >> >> Device wise, the cascode amps seem to work pretty well >> with some very humble transistors ( 2N3904 etc ). There >> likely are fancier parts out there, but some of the really >> old stuff appears to be “good enough”. > > I have made a new isolation amplifier but I'm absolutely not happy with > the available transistors. Anything in sot-89 is either to slow ( > Zetex/Diodes Inc, the 2N3904-alikes) > or is much too hot. > > I want at least 200 MHz to have no phase shift at 100. BFQ19s gave me 1 > GHz of BW. The version in the plot is already heavily sandbaged but > still has quite an S21 overshoot on the high frequency end. The > input-voltage to cascode current converter is especially problematic in > that the smallest capacitive load on the emitter tends to make it more > unstable. That spoils S11, of course. I even took the feedback from a > tap of the emitter resistor. Backward isolation is 120 dB over most of > the useful range but changes depending on the damping methods. > > Any ideas of more friendly transistors? BFQ31 were quite well-behaved > but are extinct now. I still have a reel, but stuff from the secret > drawer is unfair. And it's PNP. > Well, what kind of parameter are you after? I suspect, you're looking for sufficiently high fT and beta. Your mentioning of too hot devices seems to imply you'd like to have these at low collector currents, probably not more than a few milliamps. Anything else to look for? Best regards, Florian