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I am a newbie in LTspice so the question can be seemed to be bit stupid, but I have build an LNA and now I want to calculate the noise figure of the system. I have run the .noise statement to calculate the noise density but is stuck with how to calculate the noise figure of the system. Can anyone help me out with that?

winny
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Abhishek Chakraborty
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  • Do you know how to calculate NF? – Andy aka Mar 24 '21 at 10:28
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    NF=snr(input)/snr(output) – Abhishek Chakraborty Mar 24 '21 at 10:30
  • So the simulation gives you the absolute noise level. How does that relate to the SNR? You could assume (no need to simulate it) a certain input signal, since you know the noise, you can determine SNR at the input. – Bimpelrekkie Mar 24 '21 at 10:33
  • I am trying to calculate the ratio of total noise at the output of the amplifier to the noise caused by the source to calculate NF. How can I calculate the noise due to source? – Abhishek Chakraborty Mar 24 '21 at 10:45
  • You can set the source to have no noise: make it a current source with a parallel resistor whose value includes the `noiseless` keyword. Example netlist `R1 in 0 50 noiseless`. – a concerned citizen Mar 24 '21 at 13:28
  • But in the output noise, noise of mosfet will be added then how do I get solely output noise due to source. – Abhishek Chakraborty Mar 24 '21 at 14:21
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    Do you want to calculate the noise of the *system* (which is what you say in your question) or the *source*? If you're interested in the system, overall, then perform the `.NOISE` analysis as usual. If you don't want the source to contribute to it, either make your source a voltage source, ideal, or a current source with a parallel resistor which has the `noiseless` keyword. If you want only the source, then just run an analysis for the source, only. So which one is it? Also, use the ping `@` to notify users when there is a reply. – a concerned citizen Mar 24 '21 at 14:34
  • I will calculate the noise figure using the formula- -> total noise at the output/output noise due to the source only. so for total noise I will use the .noise statement to calculate the noise density and from that I will calculate noise rms voltage. but when we want the output noise due to the source only what to do? I will make the resistor noiseless but how to make the mosfet noiseless or else while calculating output noise due to source mosfet noise adds up. how to eliminate the mosfet noise? – Abhishek Chakraborty Mar 24 '21 at 14:52

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Make sure the models of your components are suitable for noise analysis, that is, they contain the appropriate noise sources.

Enrico Migliore
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