How can I create a fixed voltage reference output voltage from a varying input voltage?
Use a voltage reference chip like the REF02. It has : -


WIDE SUPPLY RANGE: 8V to 40V
OUTPUT VOLTAGE: +5V ±0.2% max
EXCELLENT TEMPERATURE STABILITY: 10ppm/°C max (–40°C to +85°C)
LOW NOISE: 10μVPP max (0.1Hz to 10Hz)
EXCELLENT LINE REGULATION: 0.01%/V max
EXCELLENT LOAD REGULATION: 0.008%/mA max
LOW SUPPLY CURRENT: 1.4mA max


There are gazillions to choose from and many that only need a few hundred microamps.