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
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