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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Getting into wars about whether something is current controlled or voltage controlled is pointless. In most cases, there is some of both going on. Which "one" to call it then has more to do with personal biases than physics.
However, the real answer is that we do electrical engineering here. Models of how a bipolar transistor work in our context are for helping us understand transistors for the purpose of using them in circuits. The underlying physics may provide insights, but in the end, it is ultimately irrelevant.
All explanations that go into details of holes, electrons, depletion regions, carrier diffusion, and the like, miss the point. What we are interested in is what a transistor does largely as a black box.
For designing circuits, the first model of a BJT is
A little B-E current allows a lot of C-E current.
Followed closely by
B-E looks like a diode.
which also means that
The B-E voltage is about one diode drop during normal "on" operation.
and there is also
The C-E voltage is about 200 mV in saturation, more when the current is "high".
I've been designing circuits professionally for decades, and I can tell you that the above four simple guidelines get you quite a long way. That's usually good enough for conceiving of the overall circuit topology. Once you get into details, you make sure to select a transistor that can withstand your maximum voltage, maximum collector current, power dissipation, make sure the gain is enough, etc. I'm not trying to minimize those, but they are details in the overall scheme of designing circuits.
Even when you do look under the hood, it is still quite reasonable to think of BJTs as current-driven. You try to move charges from the emitter to the base (create a base current). However, most of those charges that get into the base region get swept to the collector before they can come out the base lead. Those "charges swept to the collector" are a current. What this says is that to get a certain base current, you end up with lots more collector current (when the device is properly biased).
If you want to think of a BJT as voltage-controlled, that's fine. You can argue the physics both ways, since its not black and white. However, for the purpose of circuit design, the current-controlled model of a BJT is a lot more useful in my experience.
Question: Are there further arguments for publishing the output characteristics for Ib=const. and not for Vbe=const.
In most circuits, Ib is what we are varying. That's the signal. Vbe comes along for the ride, and usually doesn't vary much, and the variations aren't usually relevant.
Current is always the result of a voltage, not vice versa. We need an E-field to enable movement of charges (which we call "current"). Am I wrong?
Yes, and pointless. This is just the high tech version of the chicken versus egg argument.
An E-field causes a force on charges, but it takes displaced charges to cause an E-field. You can keep going round and round about which comes first, but it's all pointless. It doesn't help in designing circuits nor in understanding the physics.
There are also other ways to move charges than due to an E-field, like a changing magnetic field or physically moving charged objects. Transformers and the alternator in your car work on the first principle, and Van De Graaff and Kelvin generators on the second. Moving charged objects is also apparently how the large E-fields that cause lightning arise.
Again though, this is all pointless bickering that distracts from understanding the underlying physics, and how to use it to design circuits.
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