On Sat, 2 Jul 2016 19:00:24 +0200, you 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.
On Sat, 2 Jul 2016 19:00:24 +0200, you wrote:
>On Fri, 1 Jul 2016 12:28:44 -0400
>Bob Camp <kb8tq@n1k.org> 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.
