[sdiy] Gummel plot: was Re: double transistors

[Martin Czech]

I learned from the technology/CAD guys that they use the so called "Gummel"
plot. Base and collector to gnd, emitter pulled, measure emitter current
and emitter voltage, plot the data.
Vcb is zero, so no Early or other collector effect is shown.
It seems to work for them. 

I think most of our circuits depend on collector current versus
Ube. Since it is difficult to voltage control the Ube and
the resulting exponential Ie, it is doen the other way: controlled
current Ie. Force some emitter current with base tied to reference.
Measure Ube, Ie and perhaps Ic, some devices may have a rotten Hfe.
I think for simplicity one can ommit Early effect, so a constant
Ucb -say 5V- would be ok.

m.c.