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Bandgap Reference Voltage

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diarmuid

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Hello,

I know that the bandgap voltage is equal to Vbe + delta(Vbe) * scaler, where the scaler is sized so as to cancel the
-ive temperature co-efficient of Vbe.

Eg. Vbg = Vbe + (Vtln(n))(1+(R2/R3)) .... Ref: Razavi page 385

The possible values for Vbg seem to be always around the 1.2V mark.

However, I have seen 1.024V as a commonly quoted bandgap voltage.

1.) How can you be so precise with this voltage?
2.) Why is it so common?
3.) What type of bandgap circuit can generate it (example of a schematic would be very helpful here)?

Thanks in advance!

Diarmuid
 

checkmate

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1) You can't. Precision bandgap references are almost always trimmed for accuracy.
2) I don't think there is any reason, other than it being a nice rounded numbers that industry partners thought was good for standardization of ADC/DAC references and regulator feedback margins.
3) Vbg = Vbe + k * delta(Vbe) will always give you around 1.2V.
Low voltage bandgaps make use of Vbg = k1 * Vbe + k2 * delta(Vbe) to give you voltages lower than that.
This also allows it to use supplies below 1.2V.

You can refer to Fig 9, which is the most famous of low voltage references.
https://repository.ust.hk/dspace/bitstream/1783.1/2348/1/200410CICC_Vrefdesign.pd..
 

diarmuid

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Thanks for the quick reply.

1. How exactly do you trim the bandgap voltage i.e. do you switch in/out incremental resistors until the desired voltage is achieved?

2. Im very interested to see your attached file. However it doesnt work. The URL doesnt show anything.

https://repository.ust.hk/dspace/bitstream/1783.1/2348/1/200410CICC_Vrefdesign.pd

Would you mind uploading as .pdf.

Thanks and all the best,

Diarmuid
 

godfreyl

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Here you go.
The link worked for me.:???:
 

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  • Vrefdesign.pdf
    172.8 KB · Views: 45

diarmuid

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Yep, works for me this morning. Dont know what was up with my computer yesterday!
 

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