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.

Datasheet noise figures are provided at optimal source impedance. Optimal impedance is provided in the datasheet Table 8 as Gamma_opt.

In your case, formula 4.5 is provided for connection with grounded base and it is not relevant for datasheet NF measurement condition.

You can do the similar calculations for the datasheet conditions but you must use more complete expression for noise that includes the noise of the source. Please, refer to expression <A>(38) from the document below. It differs from (4.3) provided in your document:

<A> ftp://ftp.dii.unisi.it/pub/users/vignoli/old/elettronicaiii/low_noise.pdf

>the value of R_F...

R_F value does not matter. In the expression (4.3), you can divide by R_F^2 to get mean-square output current.

It is more reasonable to convert expression to equivalent input noise and it will not depend on the collector load. Expression <A>(41) will provide this value. Vni^2 is propotional to noise power. So all you need to obtain noise figure is to divide this expression by input noise 4kTR1*dF.

NF = 10 log [(1+rx/R1)] // if R1 is optimal source resistance at optimal collector current.

Have a great day,

Pavel

NXP TIC

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