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I'm in the process of making some small amplifiers and using bjt's in the small signal realm. I have used bjt's as switches for quite a while, so I am quite familiar with their basic operation. I was reviewing small signal analysis trying to refresh my memory about how to do the analysis when I came across a couple variables that I'm not sure how to incorporate them into my analysis in a practical sense.

First, thermal voltage of a pn (diode) junction - V

_{T}- which my book describes as a 'constant', which is a function of temperature. V

_{T}is defined as

V

_{T}= k*T / q

where

k is Boltzmann's constant = 1.38 x 10^-23 joules/kelvin,

T= temperature kelvin,

q= magnitude of electronic charge = 1.6 x 10^-19 coulomb

Is this true for all pn junctions - or does this change with doping, diode types, etc?

Second - small signal output resistance, r

_{o}- my book does an awful job explaining how to arrive at this value, practically speaking. They give the following equation for the output resistance r

_{o}= (V

_{A}+ V

_{CE}) / I

_{C}. I have never seen r

_{o}or V

_{A}in a datasheet. I'm not sure what V

_{A}even is. I do understand that r

_{o}is a function of collector current due to v

_{ce}, so I understand why it's used, I just don't understand when I have a circuit I'm analyzing, how to come up with a value of r

_{o}!

True to form - my old electronics book tells me how to do all the calculations, but doesn't explain how to arrive at the value in a practical sense. A quick google search also turned up nothing.