Comments on Why Ib=const. for BJT output characteristics Ic=f(Vce)
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Why Ib=const. for BJT output characteristics Ic=f(Vce)
Introduction: In some books and other technical papers (also from universities) it is - surprisingly - still claimed that the bipolar transistor (BJT) would be a current-controlled element. This is simply stated - without any explanation or proof (which I think is impossible).
But of course, the other representation (voltage-controlled) can also be found in many knowledge sources - just as it is considered in the SPICE models.
For me and also for many students this is an unsatisfactory situation. Therefore, I consider the distinction (current vs. voltage-control) to be very important to avoid contradictions between theory and practice. Because many circuits and observable effects can only be explained with voltage control Ic=f(Vbe).
As one argument pro current control often the output characteristic Ic=f(Vce) is referred to, where the base current Ib is considered as a fixed parameter (Ib=const). And - as a matter of fact: Although the collector current Ic is determined by the voltage Vbe, the characteristic curves Ic=f(Vce) are practically always given only for different (fixed) base currents Ib.
My Question: Why ?
EDIT: In response to Elliot Alderson`s comment I enclose a short contribution from the great Barrie Gilbert.
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I fully agree with OP considerations about measuring the transistor output curve. We can very easily and precisely set successive current values with constant increment. Thus, the IV curves will be evenly spaced vertically (equidistant from each other).
Let me generalize these observations into a "golden rule":
Golden rule 1: If you want to set precisely the voltage across and current through a voltage-stable non-linear element (diode, base-emitter junction of a transistor), drive it by a current source. Otherwise (if you control it with a voltage source), the current will change rapidly and it will be difficult to set the operating point. It is interesting that, in other applications (such as a differential pair), this effect (aka "current steering") is useful.
We can formulate another but dual "golden rule":
Golden rule 2: If you want to set precisely the voltage across and current through a current-stable non-linear element (collector-emitter junction of a transistor), drive it by a voltage source. Otherwise (if you control it with a current source), the voltage will change rapidly and it will be difficult to set the operating point. As above, there are applications (e.g., the so-called "dynamic load") where this effect is useful.
Finally, we can combine them into a more general "golden rule":
Golden rule 3: If you want to set precisely the voltage across and current through a non-linear element, drive it by the opposite kind of source.
Are there further arguments for publishing the output characteristics for Ib=const and not for Vbe=const.
Maybe the Earley effect? Or the temperature dependence?
I remembered that I asked such a question in RG network a few years ago - Why do we use Ib instead Vbe as a parameter when measuring common-emitter output characteristics?
Maybe the discussion there can help...
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