ACWWong
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Well again it depends on the OPA and the configuration under consideration. Quantities like gm have tmeprature characteristics which can be compensated for by appropriate bias. So in the design of an opamp, one may well start with zeroTC bias and then sweep over temperature to see if the varitaion of the relevant opamp performance parameters are acceptable before considering improvements thorugh bias charactersitic tweaking.
yes zeroTC biasing is a combination of PTAT and CTAT quanties. if you comibine in different ratios you can make any temperture coeffient inbetween as well, not just zeroTC (although zeroTC is usually the target)
For good treatment of this subject have alook at Grey & Meyer "analysis and design of analog integrated circuits" or similar.
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