Scaled +6.6V Reference Using 741 Opamp
To maintain constant current of D1 at 2 mA, this circuit uses combination of negative and positive feedback, unaffected by ambient temperature variation and unregulated input voltage inconsistency. Reference voltage of 6.6 V is scaled up to give higher volatage level at output of A1. This reference voltage is determined by breakdown voltage of 1N4611 zener. The scaled output has better characteristic (lower impedance) and able to supply enough current while keeping the reference voltage accuracy. For reliable starting, a single ended should be used for this circuit.