[time-nuts] Isolation amp transistors

[ed breya]

Gerhard wrote:
"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."


Gerhard, what sort of damping are you using? I think the simplest is a small series base resistor on any common-base stage like the cascode upper Q. The same thing should help on the lower transconductance converter Q. If added base R degrades LF/MF performance too much, maybe lossy ferrite beads would do instead.

My favorite VHF Q is the good old 2N5179 or similar, but it appears you want something in surface mount, and not obsolete. I'm not familiar with the modern SMT stuff. If your present transistors are working, but just need a bit more stability, it seems it should be OK with the right scheme, and not the transistors' fault.

Ed