Base Current vs Emitter Base voltage

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.


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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Transistors