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if you read any textbook on noise figure, you can see that the noise figure euals to loss.
As all components, source, load and DUT are under the same ambient temperature, no more conditions apply.
You can see it as the increase in total noise (expressed in dB) at the output of an amplifier due to the noise that is added by the amplfier. The source is assumed to produce kT W/Hz at ambient temperature (that is about -174 dBm/Hz).
Example: I have an amplifier with 20 dB gain (factor 100). If that amplifier is noise free, you would measure -154 dBm/Hz at the output of the amplifier.
The actual measured output noise is -151 dBm/Hz. So there in 3 dB increase w.r.t. the noise free situation. So for this amplifier the noise figure (F) = 3 dB.
For this example, the amplifier added same noise power as the noise generated by the source resistor.
When your amplifier is at a different physical temperature, the conditions for the source's noise production do not change (so noise remains about -174 dBm/Hz).
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