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Thermal noise power

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wakoko79

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Hi, I'm just confused with the thermal noise power.
We just had an exam.
the question was something like this:
Given an amplifier with a fixed noise figure of 15.6dB, which antenna will give more SNR at the output?
a 50ohm antenna or a 60ohm antenna? (antenna -> input of amp -> amp -> output of amp to next stage) **assuming same bandwidth and same input signal power

So I thought I need to figure out first what is the thermal power of the antenna so that i can get the SNR at the input.

This is where I got problems. in this slide that we used (refer to PAGE 8). it says noise power = variance = 4kTRB, where: k=boltzman constant, T=temp in K, R=resistance, B=bandwith(Hz)


I consulted with my classmates before the class, they told me to do as in the slides.. so I did, but it really gets to me since i've read in other sources that power is independent of resistance since you have to divide the variance with 4R.
Now Pnoise=kTB... Is this the one that should be used in the calculation of input SNR?

Enlighten me please... THanks!
 
Look up noise figure circle calcuations. You need equivalent noise resistance of amplifer in addition to given information (also statement of temp for reference measurement point).

Just saying it has a given noise figure at a particular input impedance gives you insufficient information to predict what the noise figure will be at a different input impedance.

Your instructor may be trying to use a simple black box amp. In this case you can calculate the equivalent noise temperature of a 15.6 db black box amp. To this number the equivalent input noise temperature is added based on input termination. Under this simplification the 60 ohm termination would yield a slightly worst noise figure because resistance in higher.
 
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