Thermal noise power

[wakoko79]

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!

 

[RCinFLA]

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.