The point is that you cannot obtain an accurate value of resistors, but you can obtain an accurate matching of resistors. Usually you design a bandgape reference where your voltage reference depends on the ratio of resistors and on base-emitter voltages. You can search here for some ideas:
http://www.circuitsage.com/bandgap.htmlaaron_do wrote on Mar 12th, 2007, 2:24am:Hi all,
I've found that the resistor variation from the model to the fabricated one can be enormous. For a biasing circuit like a bandgap reference, resistors are quite necessary (although I've seen some resistorless designs). This causes the results to be very different from expected. What kind of techniques do people use to get around that? Also, what is a good CMOS bandgap for 1.8 V supply?
thanks,
Aaron