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Noise contribution in LNA?

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joehwang

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calculating noise contribution

Hi,

I have a question about the noise contribution in the following LNA:
Ls has a parasitic resistor:rs

M1 or Ls(rs), which one contribution more noise?

In me opinion, if Ls is large, and thus, the rs is large, the noise contribution in Ls is bigger than M1. Otherwise, if Ls is small so that rs is also small, the noise contribution in M1 is bigger than Ls.
Is it correct?


Thanks!

 

post lna contribution to system noise

of course it is correct
 

noise contributor

I am not sure if I entirely agree with you.Note that the Rs introduces negative feedback.Hence the gm of the bottom transistor is lowered.For the purpose of the analysis of noise contributions we can treat the circuit as a single stage Common Source LNA(cascode by itself add minimal noise).I have tried to derive the noise figure of such a structure for an examination and it was not a neat expression.I suggest you to derive the noise figure expression and see the result or do a quick analysis in ADS.
 

    J

    joehwang

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you may refer to Thomas H. Lee's book for further understanding.

Basically i agree with your statement. from my past experience, i think M1 is a major noise contributor as well.
 

    J

    joehwang

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Yeah I also think that it is a major noise contributor.
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