OK, nice and simple and was given the derivation of it so I understand how we got there,

**but**where do I find r'e? Not on the datasheets! I did find that Shockley derived it working on diodes and was given that

**r'e=25mV/IE.**Huh? I'm assuming that Google mangles subscripts as this editor does and that it should be I(sub)E? At least V/I conforms to Ohm's law.

In the examples and problems, at least it provides the value but no reference as to its origin. Is it even in common usage and what is the typical source?