# Non ideal behaviour of a current mirror

Voltage across emitter and collector of QA is 0.7V.

In a current mirror, why is it such that when the voltage across the collector and emitter is different for Qb than QA the value of Iout increases whilst Iin is constant?

• Have you studied and understood the Early effect: en.wikipedia.org/wiki/Early_effect ? Note how this effect makes $I_c$ dependent on the value of $V_{ce}$. This is explained in every decent textbook about Bipolar current mirrors. – Bimpelrekkie Mar 16 at 21:59
• Aside from the Early Effect (true enough), there is also the fact that one side is supplying the base currents for both sides. So that input current on one side has to subtract away 2 base currents before it becomes a collector current and sets the VBE. So the other side isn't the same for that reason, too. – jonk Mar 17 at 2:23