Transistor as a switch

Exercise

A transistor is used in the following circuit. The load resístor R₄ has a value of 22 kOhm. The output voltage of the operational amplífier is 4V when switched on.

1. What is the function of stage 2 in this circuit?
2. Calculate the value of resístor R₃. The overdrive factor ODF should be 2.5.

Extract from the data sheet of the transistor BC 546 used here:
hfe 150 | V_CEsat = 0.25 V | V_BE = 0.7 V

Transistor as an electronic switch - Exercise