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Design >> RF Design >> LNA SP simulation
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Message started by baohulu on Jul 13^th, 2016, 7:35pm

Title: LNA SP simulation
Post by baohulu on Jul 13^th, 2016, 7:35pm

hi, all
when I simulate a LNA with input port 50ohm and output port 50ohm, so sp sim. then the results main form has NFmin, Gmin. the Gmin description in the spectre is
Gmin: optimum source noise reflection coefficient at NFmin
thus, I think it is (zopt-zs)/(zopt+zs),
when I change the matching network (simple L type or pi type), NFmin has the same curve, but Gmin is changed with the matching network. I can't understand this change. if it is the reflection coefficient at NFmin, it will keep the same if the NFmin is not changed. right?
is there any wrong about my opinion?

Title: Re: LNA SP simulation
Post by dog1 on Sep 12^th, 2016, 9:11am

Hello baohulu,

Sorry for my late reply.

my understanding is like this:
Gmin is related to the port impedance you choose, Zs, and the input impedance at NFmin, Zopt. While NFmin is only the minimal noise figure. NFmin unchanged doesn't mean that Zopt is not changed.

NFmin changes with the input refer current source and the input refer voltage source (and their correlations) in the picture. When the matching network change, if it is noiseless, then NFmin will not change, only Zopt will be shifted. This is because for each frequency, you can always find a shift in source impedance to counter-act the change in the matching network, and arrive at the NFmin. And because your Zopt now change but Zs=50ohms not, your Gmin now changes.