EMRFD Message Archive 1209
Message Date From Subject 1209 2007-12-02 15:38:15 Wes Hayward Hybrid Cascode IF Amplifier Stability Issues Hi all,
I just finished updating the part of my web page that deals with the
IF amplifier that WA7MLH and I described in the December 07 QST. In
getting things ready for Roger's board and kitlet sales, I discovered
what I had suspected might be there: A problem with stability.
After all, this circuit has three cascaded stages that used the
hybrid cascode topology followed by a differential amplifier. One
part of the diff-amp drives the output line to a detector while the
other drives an AGC detector. The new boards use RF chokes for
most inductors, and these produce resonances.
The solution was simple enough. (It's easy to say easy after the
fact.) We changed gain in one stage and added local negative
feedback to the diff amp. All of this stuff is now described on
the web site. Just go to http://w7zoi.net/ and then click on the
picture showing the cascode topology.
Many thanks to Roger (KA7EXM) for his efforts in providing the boards