Attached is an image of a typical BJT current mirror. The reference current is established by the resistor. We can calculate R with: R = (V.CC - V.BE) / I.REF Now what I don't understand: I know I.C = I.S * e ^ (V.BE / V.T). The collector current is quite sensitive to changes in the base-emitter voltage yes? Then why is it I'm able to choose the almost arbitrary value of 0.7V for V.BE and end up with the calculated reference current? Could explain to me how the circuit ends up with a precise V.BE (causing the correct current to flow through Q1 and Q2) despite our rough calculations? Or maybe my whole thought process is wrong, I'm not really sure at this point.