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Design >> Analog Design >> what's the difference between biasing and referenc
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Message started by jjsnail on Mar 9^th, 2006, 2:37am

Title: what's the difference between biasing and referenc
Post by jjsnail on Mar 9^th, 2006, 2:37am

hi all

i know that the biasing circuit is used for biasing a transistor to afford stable current and reference for stable voltage .and the bias voltage is often generated by reference circuit so the biasing current is PTAT.

why use PTAT current for bisaing?what about the zero TC current?

Title: Re: what's the difference between biasing and refe
Post by ACWWong on Mar 9^th, 2006, 2:54am

depends on the circuit to be biased and the fact that PTAT (current related to Vt (=kT/q)) is easy to generate.

for example high speed Source-Coupled logic dividers would benefit from more current at high temperature to maintain divider speed/operation so PTAT bias may be better here than zero TC current.

so it is "horses for courses" ... the designer using the bias characteristic that suits their circuit, be it PTAT, CTAT or somewhere inbetween.

Title: Re: what's the difference between biasing and refe
Post by jjsnail on Mar 10^th, 2006, 3:25am

thanks,ACWWong.

Let's talk about an OPA circuit.the zero TC biasing means the gain is stable when the temperature changes,right?

how can we generate a Zero TC biasing ciruit?do we have to copy and sum up the complementary current in the bandgap ref to get it?

Title: Re: what's the difference between biasing and refe
Post by ACWWong on Mar 10^th, 2006, 9:40am

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.

Title: Re: what's the difference between biasing and refe
Post by jjsnail on Mar 10^th, 2006, 10:07pm

thank u:) ;)