Annoyed by the inaccuracy of people saying, "It takes six tenths of a volt to turn a transistor on", I did a graph of a bipolar transistor (2N4250A) to see what happened. Some people say, "sixty millivolts per log" or, twenty some millivolts per doubling". One answer is a graph that shows an almost perfect straight line from 1na to 1 ma on a log/log graph. (I quit at 1 ma to avoid heating the transistor up.) I assume the line continues into the higher current ranges except for heat affecting the results.
Equations can be derived from this graph, but they won't be the same for every transistor in every range of current. They will be close for use as a relative calculation, as in, "new Vbe/ old Vbe = something something Ic2/Ic1".
The most interesting part, to me, is that the results go below a nanoamp while barely changing the slope of the line. Extrapolating would take the Vbe to zero when the current (Ic) is a tenth of a picoamp. That isn't even as high as the advertised leakage for that transistor.
I had to attach the graph as a pdf because it takes over a megabyte to scan it well and there is an upload size limit on this site.
What say you as to the "standard" equations? Can you write them for me in your answer? I'm making a blog of this so I can point people to it when the subject comes up.
Equations can be derived from this graph, but they won't be the same for every transistor in every range of current. They will be close for use as a relative calculation, as in, "new Vbe/ old Vbe = something something Ic2/Ic1".
The most interesting part, to me, is that the results go below a nanoamp while barely changing the slope of the line. Extrapolating would take the Vbe to zero when the current (Ic) is a tenth of a picoamp. That isn't even as high as the advertised leakage for that transistor.
I had to attach the graph as a pdf because it takes over a megabyte to scan it well and there is an upload size limit on this site.
What say you as to the "standard" equations? Can you write them for me in your answer? I'm making a blog of this so I can point people to it when the subject comes up.
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