When the input voltage vI increases from zero, the base voltage of Q3 increases,
and the output voltage vO increases. The load current iO is positive, and the emitter
current in Q3 increases to supply the load current, which causes an increase in the
base current into Q3. Since the base voltage of Q3 increases, the voltage drop across
R1 decreases, resulting in a smaller current in R1. This means that iE1 and iB1 also
decrease. As vI increases, the voltage across R2 increases, and iE2 and iB2 increase. A
net input current iI is then produced, to account for the reduction in iB1 and the
increase in iB2..