aaron_do wrote on Aug 2nd, 2009, 6:40pm:just wanted to point out that the figure above only generates a reference voltage. The current can only be used as a reference with the help of an off-chip resistor.
Sorry I was not clear, but I thought everyone could pick up on the concept I described without re-drawing the circuit. As shown, the MOSFET on the far right is biasing a resistor with a current. You can obviously bias something other than the resistor using this far right resistor.
(As an aside, you can't generate both the bandgap voltage reference AND the 0 ppm/C current reference at the same time unless the resistors have zero temp-co.)
I think you would agree that there are currents in the MOSFETs. The temp-co of the MOSFET currents can be changed from roughly +3000ppm/C to -3000ppm/C by varying the relative size of R1 and R2 (R2 being the sum of R2A and R2B). Accordingly, you can generate a 0 ppm/C current using only internal resistors.
Regarding tolerances, you can generate an accurate
voltage reference using internal components, but current references will be on the order of +/- 30% since that is the tolerance of the sheet resistance.