builtinashed wrote:
Apologies for more questions, and please feel free to correct me if I'm wrong, or if I'm looking at it completely wrong (if you have the time and inclination), but if we look at the parallel impedance (using Zo as 0.7 +/- j0 for convenience), it looks like 0.7R in parallel with something like 130pF. If we perform a parallel to series conversion, at 98MHz, the series equivalent gives a resistance just under 0.7R, in series with almost 42nF. If we want to absorb 42nF by series resonance (again at 98MHz), we get something like 63pH - rather than 20ish nH. This would mean we could use the figures calculated from Zo alone - pretty much unchanged (rather than adding 20ish nH to L3) ?
I guess - if we wanted to, there are other ways to deal with COSS. One might be to make the output match a pi-match, and absorb COSS into the input capacitance (ie subtract it from the capacitance at the input of the pi-match) ?
Another option could be to absorb COSS by parallel resonance (with a parallel inductor; possibly L2) ?
You're absolutely right, I've just checked the maths and your workings are correct; due to being lazy I used an app to calculate the resonant inductor and managed to get about an extra 15nH. It still resonates with 162pF but the resistance is higher.
You might ask why 162pF and not 130pF for Coss? Well, you need to apply a scaling factor (I generally start with 1.2x Coss but you'll see anywhere from 1.1 to 1.5 times Coss in literature) before working out the resonant inductor. I read the data sheet at 135pF at 12.5v and so that times 1.2 is 162pF. This is one of the reasons you need to make L3 variable.
There are lots of ways to compensate for output impedance. If you want to do a pi match the drain capacitor actually has the effect of increasing the ouput impedance (a tiny LC match where the L is so small it can be ignored or is in the transistor lead itself).
I might have a go at building one of these amps for the guy that asked for a schematic; it's not really fair to put 2-3 versions of a theoretical design and expect someone with limited experience to build all of them and be a ginuea pig. When I get some time later I'll update the diagram and post it up here. Thanks for spotting the error!