Hello,
its me again with another mind exercise* designing a microwave LNA. As I have acquired a bunch of ATF-10136 GaAs fets from some old RF junk, I would like to attempt to make a LNA for the first two radioamateur bands, 1.296GHz and 3.4GHz, so I can increase the chooch factor of my microwave band receiver thing.
(* Because it will not likely end up with anything usable)
Here is the datasheet for the ATF-10136:
https://www.qsl.net/n9zia/trans/ATF10136.pdfHere is an interesting app note I have found about designing an S-band LNA with just this transistor:
http://rfelektronik.se/manuals/Datasheets/ATF-10136-TR2.pdfPlease mind you, I have no formal knowledge/education in RF, it is just where I like to poke around more than I should. I have only basic understanding of what is happening there, so please bear with me.
I am now trying to replicate the input matching circuit for operation in the 3.4GHz band. So in stage one, I have plotted the available GammaOpt values and obtained the GammaOpt values for 3.4GHz, which are 0.463 and 130 degree, according to the smoothly interpolated plot. The graph I have made is attached. So the optimal reflection coefficient calls for 21.9ohm + 20ohm inductive Which I have calculated using z = (1+Γ)/(1-Γ).
Following the application note, I have calculated I need a quarter wave TRL of 33ohm impedance to match the real part of 21.9ohms:
Zo = sqrt(Z1*Z2) = sqrt(50*21.9) = 33ohm
Okay, so far it sounds good to me.
Now I need to match the 20ohm inductive. That wants me to use a series inductor of 0.94nH (X
L=2πfL). Unfortunately and exactly as written in the appnote, this is difficult to make, so I followed the appnote further to replace the series inductor with a shunt capacitor:
Xc = Zo
2 / X
L = 33
2 / 20 = 54.5ohm = Xc
The capacitor needed is therefore 0.86pF (Xc = 1/2πfC) . Which is a pita because I do not know how this could be implemented in real circuit either.
What should I do in this case? Is there any other possibility to make the impedance to mach? So far the 33ohm quarter wave TRL is pretty easily manufacturable, unfortunately I do not know what to do about the series inductor (0.94nH) or shunt capacitor (0.86pF). Both values are very small and I think very impractical for any kind of lumped element component .
Thank you for help
Y.