Base Current vs Emitter Base voltage

Question

Why is it universally stated in so many places that the Base Current Ib controls the collector current Ic in a Bipolar Junction Transistor when I guess it is pretty obvious that its the Emitter Base forward bias voltage that will be responsible for the change in the base current.. Making the Vbe controlling factor of Ic.. It is all very confusing

Which came first, the chicken or the egg? Is it the voltage that determines the current flow in a resistor or, is it the current flow that dictates the voltage dropped? — Andy aka, Aug 02 '19 at 17:28
Idk abt that but "voltage is the cause and current is the effect" feels a bit better.. I maybe wrong though — nn08, Aug 02 '19 at 17:31
According to Shockley transistor equation the collector current is controlled by the voltage on the base.You are talking about the ratio of Ic/Ib. That is a side-effect that base-current flows and that the ratio of base to collector current is somewhat constant in active region. — cm64, Aug 02 '19 at 17:43
Nullbyte_just one sentence: You are completely right (with your commen above. — LvW, Aug 02 '19 at 20:07
Far too many engineers seem to actually believe that the base current controls the collector current. But the physics say otherwise. One better paper to read is W M Webster's, "On the Variation of Junction-Transistor Current-Amplification Factor with Emitter Current," from Proceedings of IRE, 1954. If you get that paper and read it, you'll understand more than enough. But to understand why engineers use base current, see Figure 2.17 located [here](https://electronics.stackexchange.com/a/305720/38098). Required recombination current to sustain collector current holds a ratio over many orders. — jonk, Aug 02 '19 at 20:10
@Nullbyte There is a paper from the famous Barrie Gilbert (I think in IEEE magazine) in which he explains (1) why the BJT is voltage controlled and (2) how it could happen that so many people believe (without any proof) that the BJT would be current.controlled. As you probably know, B. Gilbert has introduced a lot of novel circuit solutions which only work because the BJT is controlled by Vbe. — LvW, Aug 03 '19 at 09:03

score 11 · Accepted Answer · edited Aug 05 '19 at 12:14

Voltage does not cause current, current does not cause voltage, at least for any meaningful understanding of the word 'cause'. They both co-exist.

When the base-emitter junction of a transistor is biassed, an I_b flows into the base, while a V_BE exists across it.

If we now measure the collector-emitter current, we find the ratio to the base current is more or less constant over a very wide range, many orders of magnitude. This is sufficiently useful that engineers call this ratio beta.

The ratio of collector current to V_BE varies with the base current. The ratio of them is still useful, engineers call it the transconductance or g_m of the transistor, but it's valid at only one base current setting. So while the BJT is also voltage controlled, as the relationship is non-linear, it's not useful for doing calculations for the initial biassing of the transistor, which usually involves comparing currents over a wide range.

This means that when biasing up a transistor, the beta×I_b expression is most useful for collector current. When using a biased transistor as an amplifier, the g_m×V_BE expression is frequently used.

Comments are not for extended discussion; this conversation has been [moved to chat](https://chat.stackexchange.com/rooms/97049/discussion-on-answer-by-neil-uk-base-current-vs-emitter-base-voltage). Any conclusions reached should be edited back into the question and/or any answer(s). — Dave Tweed, Aug 05 '19 at 12:20

LvW · Answer 2 · 2019-08-03T08:49:23.693

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Nullbyte, I agree with you, the situation seems to be confusing because some books/articles state that the BJT would be current-controlled and some other say voltage-controlled. This is a very unsatisfying situation - I really cannot understand, why some (many) people still think (no - they only believe) that the BJT would be current-controlled. There is not a single proof for this claim. Can you imagine how two additional charged carriers in the base region should be able to release 1000 additional carriers from the emitter (assuming B=500)?

In contrary - there are many explanations and effects which clearly show that the collector current Ic of a BJT is - of course - controlled by the base-emitter voltage Vbe.

It is really a phenomenon - all designers of classical BJT-based gain stages assume voltage control (low-resistive base voltage divider, voltage-feedback of the emitter resistor Re), but some of them - without realizing what they were doing - still believe in current-control. Don`t ask me why - I cannot answer.

One explanation may be the fact that for CALCULATION PURPOSES it sems to be simple and convenient to ASSUME current control - and, inded, it works! However, we should not mix physical principles with design-oriented methods.

As a background, I like to give you a similar example:

We say that in a simple resistive voltage divider the current would produce a voltage across each of the two resistors in proportion to the resistor values (given in Ohms) - right?

No - physically wrong. A current cannot produce this voltage. It is always the voltage that allows/drives a certain current. And the voltages are a only measure of the electric fields within the two resistors. Hower, for calculation purposes, we are allowed to say: The total current produces a voltage V=IxR. But physically, in the cause-and-effect sense this is wrong.

And the same applies to the base current Ib=Ic/B ...it is a by-product that cannot be avoided. And for calculation/design purposes we may use this equation in the form Ic=Ib*B. But this does not mean that Ic would be controlled by Ib.

edited Aug 03 '19 at 08:49

answered Aug 02 '19 at 19:50

LvW

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Vbe, not Vgs... – Chu Aug 03 '19 at 00:38
Chu_thank you. I`ll correct it. – LvW Aug 03 '19 at 08:48
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This answer is actually just confirming OP's bias without actually stating correct facts. Current and voltage are really just effects of the same thing – an electric field over a load. **The claims made in this answer are plain wrong**. I can only encourage LvW to take a look at the solid-state physics that make the BJT work – there's a net flow of charge carriers necessary; that's a *current* in it's purest form, and hence to: "There is not a single proof for this claim": here you have it, proof in a single comment. – Marcus Müller Aug 03 '19 at 11:35
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this doesn't change the fact that the current won't occur without a voltage – but again, that doesn't make the BJT any more voltage-controlled than it is current-controlled. Both can exist only if the other is present. – Marcus Müller Aug 03 '19 at 11:47
MarcusMueller_ a "proof in a single comment" ??? May I kindly ask you to explain your "proof" in an extra detailed answer. Is this your "proof": "...there's a net flow of charge carriers necessary..." I can´t believe that you are satisfied with this "proof", which simply says: A current is necessary....Are you joking? Are you interested to learn something about "my proofs" for voltage-control? In fact, these are not "my" proofs - they can be found in many knowledge recources. – LvW Aug 04 '19 at 10:36
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@LvW you're clumsily trying to turn my words, and it doesn't work. I clearly said: "look at the solid-state physics, and you'll see a net flow of charge carriers, that's a current is necessary". I didn't imply causality or caused-ness. Again, current and voltage do not share a one-way "one causes another" relationship, which you should really have seen when you've learned the basics of Maxwell's equations. You're wrong, and arguing with you really doesn't help anyone, so I'll stop here. Have an nice day! – Marcus Müller Aug 04 '19 at 15:51
OK - I am wrong. And with me all the specialists from leading US-universities. – LvW Aug 04 '19 at 16:02
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@LvW: ever heard of [diffusion current](https://en.wikipedia.org/wiki/Diffusion_current); which is current **not** caused by an electric field (voltage)? Diffusion current is the key element making a BJT work. – Curd Aug 05 '19 at 12:51
Curd....You can believe me - I have "heard" of this effect. But didn`t we speak about "classical" and usable electric currents? To me, this is not a serious counter example. On the other hand...diffusion current is driven by a sort of pressure (diffusion pressure). I think, we could compare this pressure (which acts as a force) with the force in electronic circuits which we call "voltage". What do you think? – LvW Aug 05 '19 at 13:45
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@LvW: nice try trying to hold up your claim by introducing your own terminology but that dosen't work. It's the very definition of diffusion current, that it is **not caused by a force/field/voltage** (in contrast to drift current). Diffusion current has a purely statistical cause: the fact that there are more electrons/holes in one place (concentration) than there are in a nearby other place. – Curd Aug 05 '19 at 14:30
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@LvW: ...and of course diffusion current is just as classical and usable as drift current. It **is** used in and any BJT:It is what makes it work. – Curd Aug 05 '19 at 14:34
@ Curd...didn`t you notice the term "diffusion pressure" I have mentined ? You can believe me - I know what I am speaking of. I am 75 years old - and I gave electronic lessons for more than 25 years at a university. I am and I was in contact with several experienced hardware specialists (like Barrie Gilbert) as well as profs at leading US universities (Berkeley, Harvard, MIT). I know how a BJT works and I could give you many examples (effects) supporting my claims. But nobody is interested ...this is not a good engineering practice - completely different to the forum "ResearchGate" ... – LvW Aug 05 '19 at 14:41
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I know that - in a certain sense - I am a bit "crazy" not to stop to be involved in this discussion about BJTs. I have learned that for many people the question"current or voltage controlled" is treated like a religion. They are not willing to discuss and hear new arguments....they want to stick to their opinion. Full stop. In this forum and in this dicussion the member "jonk" is really wise - he has the same opinion (voltage control) but he does not discuss anymore. It seems he has learned that it makes no sense trying to convince people from the "curent party". – LvW Aug 05 '19 at 14:50
@LvW: "diffusion pressure" (which I don't remember being used in context of semiconductor physics) is pressure that is **caused by** diffusion (not pressure that is the cause of diffusion). Therefore introducing this term leads completely astray (If you want there is a concept in semiconductor physics similar to "diffusion pressure": barrier voltage at a pn junction **caused by** diffusion current (which in turn cause drift current going the opposite direction until equlibrium of currents(!) (not voltage) is reached). So referring to "diffusion pressure" doesn't help here at all. – Curd Aug 05 '19 at 19:12
@Curd - I must admit that I have translated the german term "Diffusionsdruck" into the english language word-by-word - not knowing if it is 100% correct. In german "diffusion pressure" means: The electrons/holes "want" to establish an equilibrium in concentration - with other words: The follow an "internal" pressure to move into the other region. This makes the diffusion current. Only after this effect comes to an end (due to the diffusion voltage barrier) this "pressure" is zero. In this sense, the term "diffusion pressure" which I have used is the cause of the diffusion current. OK? – LvW Aug 06 '19 at 09:31
@Curd My translation word-by-word seems to be not too bad. In this forum I found the sentence: "The depletion region is created due to a balance of diffusion and drift current. Diffusion pressure causes holes to travel from the P-side and electrons to travel to the N-side and combine with one another around the junction." See here: https://electronics.stackexchange.com/questions/220824/how-the-depletion-region-of-pn-junction-changes-under-bias – LvW Aug 06 '19 at 09:45

score 0 · Answer 3 · answered Aug 02 '19 at 19:45

It's because current is a defining factor in the relationship between the base-emitter junction and collector current and it makes the most sense to use current here. It's effectively a diode and adding a voltage supply greater than it's saturation voltage without a resistor between them is like shorting it. Sure you could use the diode equation to relate voltage and current here but that only makes it more confusing.

Contrast this to a MOSFET where the gate is an insulator and doesn't allow current through it (ignoring capacitance and leakage current, which aren't relevant to the concept). The MOSFET is literally a voltage controlled device, the voltage is the defining factor in the drain current.

Base Current vs Emitter Base voltage

3 Answers3