On Sat, 2 Jul 2016 19:00:24 +0200, you wrote: >On Fri, 1 Jul 2016 12:28:44 -0400 >Bob Camp <[email protected]> wrote: > >> Real cascode circuits can be built with RF transistors. They also can be >> simulated. >> Simulating them with the standard models is a PIA. The issue is that the >> inductance >> of the package is not de-embedded from the test socket as carefully as it >> might be. >> There is also the somewhat non-intuitive need to stick a low value resistor >> in the base. >> Done properly, they are very reproducible and reasonably insensitive to >> load. > >Thanks! That resistor in the base did the trick! >Am I right in the assumption that the resistor gives the transistor >some negative feedback and thus prevents it from oscillating? > >Attila Kinali
If you can find it, "Preventing Emitter-follower Oscillation" by Michael Chessman and Nathan Sokal has an analysis of negative resistance oscillation in transistors and why adding dampening to one lead is often necessary. There is a copy of this paper in the appendix of 1993 Linear Applications Handbook Volume II from Linear Technology. I do not remember where I read it but apparently Jim Williams made a habit of preemptively adding a dampening resistor wherever he found a transistor with low impedance on at least 2 of its 3 leads just to be safe. W2AEW made a video with an extreme example of this problem: https://www.youtube.com/watch?v=oZ1Dv2dVGkU With RF transistors the problem just gets worse and more difficult to diagnose since the frequency of oscillation may be outside the range of common test equipment. Side effects like mysterious operating point changes may be all that is visible. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
