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The transistor current equation is

Ie = -(Ic+Ib)

Why is the negative sign sometimes used?

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1 Answer 1

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Using the "Standard Referencing Method" for circuit analysis, the current through any terminal on an electrical component is measured going into the terminal like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Now KCL (Kirchoff's Current Law) states that the sum of the currents into a junction like this equals zero, like so;

Ic + Ib + Ie = 0

Now we can rearrange to get Ie;

Ie = -(Ic + Ib)

This makes perfectly good sense since if the currents didn't balance out it would mean that current was entering or exiting the transistor through thin air.

It might seem counter-intuitive to reference the emitter current in the opposite direction to the one it would typically be positive in, but this is actually a very useful way of referencing currents because it avoids ambiguity, you know that the current is always referenced into the terminal.

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