Re: CMOS Bias current generator and Bandgap reference voltag
Hi cuthbert,
this depends very much on the method how you want to generate the PTAT current: if you use the scheme of Fig 8.8. (a) from the PDF above, the temperature dependency of transconductance doesn't play a role for the PTAT current generation: because of the exponential characteristics in weak inversion operation, the PTAT dependency results only from the thermodynamic voltage's Ut=kT/q temperature dependency. You just have to make sure that all involved transistors operate in the weak inversion region.
Fig. 8.8. (b) shows a circuit for nearly temperature independent current generation. Only M6 & M7 must operate in strong inversion. In this case, the (Ut)² dependency of the current is (almost) compensated by the 1/T² dependency of the carrier mobility.
erikl