A couple questions: 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 - VT - which my book describes as a 'constant', which is a function of temperature. VT is defined as VT= 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, ro - my book does an awful job explaining how to arrive at this value, practically speaking. They give the following equation for the output resistance ro = (VA + VCE) / IC. I have never seen ro or VA in a datasheet. I'm not sure what VA even is. I do understand that ro is a function of collector current due to vce, 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 ro! 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.