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The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> help in bandgap reference https://designers-guide.org/forum/YaBB.pl?num=1234849541 Message started by xiaodu2001 on Feb 16th, 2009, 9:45pm |
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Title: help in bandgap reference Post by xiaodu2001 on Feb 16th, 2009, 9:45pm Hi, may i ask about current-mode bandgap reference. The current-mode bandgap reference is based on the same theory as conventional bandgap reference. A PTAT (proportional to absolute temperature) voltage is added to a base-emitter voltage which exhibits a negative temperature coefficient, making the output independent from temperature in first-order. The core circuit is illustrated in the figure attached. Question is: R2a and R2b are supposed to be identical, but in practical implementation, mismatch between R2a and R2b is 2.5% on purpose, why? |
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Title: Re: help in bandgap reference Post by ywguo on Feb 17th, 2009, 1:59am I don't think they should be a little different on purpose. |
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Title: Re: help in bandgap reference Post by superken on Feb 26th, 2009, 10:09am yes, I agree with ywguo that I wouldn't introduce mismatch in those two resistors on purpose by design.. unless you want to start playing with second or higher order temp-co compenstation. The other possibility is that the whole structure may have systemetic imbalance (for eg. due to bad opamp design or just bad layout), the designer may start tweaking in that direction.. but I doubt it. Cheers! :) |
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Title: Re: help in bandgap reference Post by thechopper on Feb 26th, 2009, 5:55pm I agree with both comments above. Actually one example of systematic unbalance could be the different beta both BJT might have. If the current density difference is large enough, then base currents will NOT be the same for both BJT and that will add a second order effect (even considering an ideal opamp). Eventually the intentional resistor mismtach could compensate for that as superken suggested. Regards Tosei |
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Title: Re: help in bandgap reference Post by Dan Clement on Feb 26th, 2009, 8:10pm The resistors steal current from the bipolars and that will affect the tempco. I could understand that one would tweak these resistors to get the tempco they want. However, mismatch will be a killer and would probably remove any benefit of the tweaking. Are the resistors even necessary? If not, remove them. The tempco of these resistors also impacts the temperature curve, so if you can get rid of them you should. Regards, Dan |
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Title: Re: help in bandgap reference Post by PaloAlto on Feb 26th, 2009, 11:20pm Dan Clement wrote on Feb 26th, 2009, 8:10pm:
Yes they are. you need them to create the current that is inversely proportional to T Dan Clement wrote on Feb 26th, 2009, 8:10pm:
Sorry to disagree. They don't contribute their tempco in the final VOLTAGE. They introduce their tempco in the current. The current is passed through the ouput resistor to generate the output voltage and then the tempco is removed 8-) |
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Title: Re: help in bandgap reference Post by Dan Clement on Feb 27th, 2009, 12:15pm Oops! I looked at this when I was tired. I thought there was one additional resistor that determined the ptat current. Yes, the tempco of the resistors will cancel but only if the layout is drawn well and the effects of mismatch are minimized (duh, right?). Thanks for pointing out my mistake :). Best Regards, Dan |
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Title: Re: help in bandgap reference Post by xiaodu2001 on Mar 1st, 2009, 11:31pm Thank you for all your help! I also think that the difference of the 2 resistors is used to cancel the offset of the op-amp, or the mismatches between the beta of the bipolar caused by different current density, and so on. but , if it is ture, how can I simulate it and verify my assumption? |
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Title: Re: help in bandgap reference Post by genuineintel on Mar 2nd, 2009, 1:40am Which one has larger resistance? R2a or R2b? |
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