This fixed 1-mA current source delivers a fixed current to a load connected between Ql`s collector and ground; the
load can be anywhere in the range from 0 to 14 . The circuit is powered from a regulated 15-V supply, and the R1/R2 voltage divider applies a 14-V reference to R3. The op amp"s output automatically adjusts to provide an identical voltage at the junction of R4 and R5. That produces 1 V across R5, resulting in an R5 current of 1 mA. Because that current is derived from Ql"s emitter, and the emitter and collector currents of a transistor are almost identical, the circuit provides a fixed-current source. The output current can be doubled by halving the value of R5.