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Calculating Base Spreading Resistance from NF Data

Question asked by Stephen Pflanze on Nov 6, 2017

I am attempting to calculate the base spreading resistance from noise specifications for the BFT25A bjt:

https://www.nxp.com/docs/en/data-sheet/BFT25A.pdf 

The NF specification is for noise power, which means if I convert dB to a ratio, and multiply that ratio by 4kT where k is Boltzmann's Constant and T is temperature in Kelvin I should get an output noise power density. I expect that T should be the 25C given for the NF spec on page 3 of the spec PDF.

 

Now I need to apply that noise density output to Equation 4.5 on page 2 of:

https://leachlegacy.ece.gatech.edu/ece6416/Labs/exp04.pdf

I expect that the value of R_F that should be used in equation 4.4 should be the load used to make the output noise power measurement in the specification. But this is not given. Should this value be 50 ohms?

 

I need the base spreading resistance to make noise calculations that are specific to the circuit I am designing the BFT25A into.

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