Re: emitter resistance
I cannot deduce the formula, but:
k=Boltzmann's constant; k=1.380658E-23 [J/K]
T=absolute temperature T=273.15+t[°C] [K]
q=electron's charge q=1.6E-19 [C]
So at room temp (300K), this quantity kT/q (thermal voltage) works out to about 25.89mV. So if the temperature is less than 300K (27°C), it will be closer to 25mV.
Usually, I consider it 26mV.
The formula you are referring to represents the input impedance of a transistor in CB. It it calculated as the ratio of voltage (thermal voltage) over current.
If you read carefully the paper posted by MRFGUY, you will see where it comes from.