Low noise buffered reference circuit for precision reference

This circuit provides 3 preset currents that are always fixed despite changes in ambient temperature and line voltage. ZD1 generates a temperature-stabilized reference voltage for the non-inverting input of IC1. 100% feedback from the output is available on the inverting input to keep the emitter of Q2 at the same potential as the non-inverting input. Therefore, the load current is only controlled by the transistor selected by S1.