In transistor specifications such as these:
There is at several frequencies an Rn/50 specification. My understanding is Rn is a resistance that would give the same thermal noise density as a transistor?s input noise:
Vn = SQRT( 4 k T Rn B ) Where B is the bandwidth (area under the frequency response curve).
Equation 43 on page 11 of:
Gives Vn as a function of rx (base spreading resistance), and collector current. Assuming given value for Rn is accurate (I am aware it might not be), is it valid to calculate an rx (base spreading resistance) value by setting the above two equations for Vn equal to each other and solve for rx?
After doing this and squaring both sides this would be:
4 k T Rn B = 4 k T rx B + 2 k T (VT/IC) B Where: VT is the thermal voltage k T / q . IC is the collector current Rn is specified at.
The solution for rx would be: rx = Rn - VT/(2 IC)
I would use the value of rx given for the frequency that is closest to the frequencies the transistor would be used at.
Another question: Why is it that in transistor spec sheets the given value for Rn has been divided by 50?