some problems about bandgap circuit

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winsonpku

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I am designing a bandgap circuit, but i have some prolems about this bandgap. The bandgap is op amp based.
1. the output voltage of the bandgap is sensitvie with bipolar. i mean when the biaploar is changed from TT to SS, the voltage drift is about 40mV, how to reduce the voltage drift on biploar model.
2. how to reduce the offest of the op amp, becuase the offset of the op amp will be amplified about 10 times, so how to reduce the offset is very imprortant.


Thanks all

winson
 

1. a) try with another bipolar
b) try collector current compensation
c) If it didn't help, introduce trimming network. Usually when we shift zero TC point to desired value, voltage drift decreases too.

2. Offset of amplifier changes PTAT curve. Offset usually depends from threshold voltage mismach of input differential pair. To reduce offset increase matching (increse area of input devices). Another way use N=24..48..80. With increasing N offset of amp loses significance.

good starting point reference
 

You can study the paperesign considerations of Recent Advanced Low-Voltage Low-Temperature-Coefficient CMOS BangdapVoltage Reference. In that paper, you could find some help.
 

I will appreciate if you could explain it in details:
a) how to compensate the collector current
b) trmming network means the more bipolar transistors?
c)N=24 means what?

Thanks again

 

a) Follow reference link and see fig.7
b) Trimming means the set of resistors shorted by fuse(poly or metal, ...). By burning fuse u can change resistor ratio in bandgap and fit it output voltage to desired value.
c) N - ratio between bjt in PTAT generator. Iptat = (Vos+Vt*ln(N))/R. Increasing N u make the second term in summation lager then the first. Vos - offset, Vt = kT/e - termal voltage, R - resistor value.

All of this is evident. Before designing bandgap u should to be familiar with this kind of circuits. Follow my reference link and find many good papers about voltage references.
 

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    winsonpku

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Thanks!
 

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