I was recently gifted with a tube of MRF2628's so I figured I would take a walk on the wild side away from my usual stuff and give an RF project a shot for fun.

I am perhaps lucky that the transistor datasheet has a reference circuit. I have been looking at that and pulling my hair out trying to figure it out. I have spent a lot of time with chatgpt asking it questions and working through the values of the various inductors. I now have a fairly flushed out schematic.

I have downloaded the latest version of simnec, which is the latest "version" of symsmith and I have the input matching network laid out, but I do not see how to model the base of the transistor. In the datasheet this is a smith chart, and they broke off the resistance and reactance at a few different frequencies. So, how in this program do I model terminating into a complex impedance when I know both parts of it?

Also, looking at the chart, over the frequency range it covers at any rate, the resistance seems to be pretty stable, and low, as I would expect. It is the "real" part so the fact ir changes at all is a bit surprising, and it is low as I would expect going into the base of a bipolar transistor. I think I understand that part fairly well. One thing, on the smith chart, it states that Zo = 10 Ω, does this mean the "normal" 50 ohm smith chart is now normalized to ten ohms? And is that only for the output? Also, does that have any bearing on the chart they broke out with some values at different frequencies? This is all new to me. Most of my "education" is from this YouTube video:

I have built amps from kits before, the last one being based on an MRF186 and the biasing on that I totally understood. I do not understand the biasing on this at all, unless this reference design is running class C. I am assuming that most linear RF amps would be class B ish.

Any insights on this would be appreciated. I want to move more from gee, that looks about right, to having a decent model and understanding of this. I am also not opposed to a more complex but intuitive biasing scheme. I also, perhaps wrongly, suspect that the complexity of the input and output matched have to do with the wide spectrum the reference amp covers. So when I get it modeled, that may change into something simpler.

The images, the amp is the reference out of the databook, the smith chart is out of the databook, and the last one is my simnec simulation so far. If you are interested, it also containes the inductance values I came up with for the coils.