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

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    \$\begingroup\$ 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? \$\endgroup\$
    – Andy aka
    Aug 2, 2019 at 17:28
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    \$\begingroup\$ Idk abt that but "voltage is the cause and current is the effect" feels a bit better.. I maybe wrong though \$\endgroup\$
    – nn08
    Aug 2, 2019 at 17:31
  • \$\begingroup\$ 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. \$\endgroup\$
    – cm64
    Aug 2, 2019 at 17:43
  • \$\begingroup\$ Nullbyte_just one sentence: You are completely right (with your commen above. \$\endgroup\$
    – LvW
    Aug 2, 2019 at 20:07
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    \$\begingroup\$ @LvW Can you give me the name of a Barrie Gilbert paper? \$\endgroup\$
    – G36
    Aug 5, 2019 at 17:13

3 Answers 3


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 Ib flows into the base, while a VBE 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 VBE varies with the base current. The ratio of them is still useful, engineers call it the transconductance or gm 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×Ib expression is most useful for collector current. When using a biased transistor as an amplifier, the gm×VBE expression is frequently used.

  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. Any conclusions reached should be edited back into the question and/or any answer(s). \$\endgroup\$
    – Dave Tweed
    Aug 5, 2019 at 12:20

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.

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    \$\begingroup\$ 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. \$\endgroup\$ Aug 3, 2019 at 11:35
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    \$\begingroup\$ 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. \$\endgroup\$ Aug 3, 2019 at 11:47
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    \$\begingroup\$ @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! \$\endgroup\$ Aug 4, 2019 at 15:51
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    \$\begingroup\$ @LvW: ever heard of diffusion current; which is current not caused by an electric field (voltage)? Diffusion current is the key element making a BJT work. \$\endgroup\$
    – Curd
    Aug 5, 2019 at 12:51
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    \$\begingroup\$ @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. \$\endgroup\$
    – Curd
    Aug 5, 2019 at 14:30

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.


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