I agrre with the last post. The bandgap refrence offers a 4-10 mV change for a range of temperature of -125°C to 100°C. wich its very good for the most aplications involving reference circuits. For most specific detail about it i have a lot of paper about it, so send a private so I can send them
there are many kinds of references like zener, bandgap etc.... but bandgap is used as it uses a compensation technique using Vbe drop to produce a current change and make the reference stable with respect to temperature...
One of the best reference generators is BAND_GAP circuit. the principle of these circuits is to use an element such as BJT which has two parameters dependent to temprature changes and......
REFERENCE: B. Razavi, CMOS CIRCUIT DESIGN