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 _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
