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Noise figure vs 1-dB point of LNA

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kamranraja222

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Hello everyone,
If the low noise amplifier(or power amplifier) operates in saturation region or beyond its 1-dB compression point,than how the noise figure of the amplifier will be affected? Thanks
 

NF will be degrated, since gm is also degrated.
Click to expand...
Surely. I wonder if NF is of any practical relevance with signal level near the compression point. Hard to see any noise under the signal and it's distortion and cross modulation products. I guess it's a purely theoretical homework question.
 

FvM said:
Surely.
I wonder if NF is of any practical relevance with signal level near the compression point.
Click to expand...
See equations in attached document.
 

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pancho_hideboo said:
NF will be degrated, since gm is also degrated.
Click to expand...
NF also will be degrated due to noise holding.
Higher harmonics hold down noise around them o desired signal band.
 

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    kamranraja222

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Up conversion noise from around DC also exist.

[Summary]
NF will be degraded by the following two mechanisms.
(1) Decreasing of effective gm
(2) Up and Down Conversion of Noise

Both (1) and (2) increase noise floor around fRF.
 

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FvM said:
Surely. I wonder if NF is of any practical relevance with signal level near the compression point. Hard to see any noise under the signal and it's distortion and cross modulation products. I guess it's a purely theoretical homework question.
Click to expand...

from a pure SNR point of view, no. But in modern systems, the increase in phase noise when you are driving your device nonlinearly can have a big impact. FET devices especially do not like to be overdriven, and have gate induced traps released, etc.
 

biff44 said:
from a pure SNR point of view, no.
Click to expand...
Wrong.
What bandwidth do you assume regarding SNR ?
SNR worsen obviously due to increasing of noise floor, even if we ignore noise folding.
You can easily confirm this by actual measurement using Spectrum Analyzer.

Maybe you think limiter amplifier.
However this is LNA.

biff44 said:
the increase in phase noise when you are driving your device nonlinearly can have a big impact.
Click to expand...
Wrong.
Mainly AM Noise increases.

However about signal which causes gain compression, AM Noise will be a little suppressed as same as limiter amplifier.
 
Last edited:

pancho_hideboo said:
Wrong.
What bandwidth do you assume regarding SNR ?
SNR worsen obviously due to increasing of noise floor, even if we ignore noise folding.
You can easily confirm this by actual measurement using Spectrum Analyzer.

Maybe you think limiter amplifier.
However this is LNA.

Wrong.
Mainly AM Noise increases.

However about signal which causes gain compression, AM Noise will be a little suppressed as same as limiter amplifier.
Click to expand...

Jeez, my eyes must have lied to me! LOL
 

The case is not only of academic interest. A strong out-of-band signal can be driving the amplifier to its nonlinear region. That interference and intermodulation products could eventually be filtered out, but the possible increase of noise floor would have bad consequences.

Z
 
Hello everyone,
Practically, How can I find the noise figure of LNA by using spectrum analyzer for two cases: When operated in linear region and when operated in saturation region. I also want to check how the noise floor will affect? Thanks
 

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