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Posted by tiantsmart on December 4, 2008, 8:23 am
hi,I have one question:how to determine the bjt current in bandgap reference?I mean the current in PTAT
Have somebody the experience how to choose the collector current of PNP? and how's the mismatch?
Thanks

Replies to This Discussion

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Reply by Carlos Gamero on December 5, 2008, 6:15 pm
Hi, the range of acceptable values of current depends on the design requirements. IPTAT = Vt*ln(n)/R so if the BJT ratio n = 8 and R = 2k then IPTAT=(26E-3)*(ln(8))/2E3 ~ 27 uA. If that's too much current then you need larger resistors, which might take more area than you want if you don't have high-sheet-res resistors available. For matching use common centroid arrangements for the resistors and the BJT's. Interdigitate the resistors and if you are using n=8 for the BJT's maybe use a 3x3 pattern with the one guy in the center and 8 devices surrounding it.
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Reply by John Wetherell on December 12, 2008, 9:08 am
Current is often chosen based on noise requirements, or area minimization, or power requirements. More current usually means lower noise and larger area. Some applications require bandgap current less than 1uA. In such applications, the resistor sizes become much larger, in which case lower current means more area. Mismatch requirements are easy to meet for uncalibrated bandgaps. Normal process variations cause the bangap voltage to vary 3-5%*1.2V~48mV; mismatch effects should be sized less than that. Some reports show common centroid layout to have the same matching for modern processes. Because of this, and worries about systematic offset from edge effects, I've steered away from the 3x3 pattern to patterns like 5x2, with great success.
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Reply by Jeff on December 18, 2008, 2:04 pm
I ran into this video. It's wonderfully specific to the subject of bandgap references.http://www.archive.org/details/APaulBro1989
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Reply by wuchun on August 19, 2010, 10:27 pm
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