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Dipankar Community Member Offline Posts: 59	very low beta value of pnp to be used in BGR Nov 09^th, 2009, 7:59am Dear Experts, Can anybody share his/her experience regarding BGR design with pnp device having very low beta (1.5-2.5) ? Any special care/techique to adopt ? or will you suggest to go with MOS-only BGR. Spec : BGR out = 0.5 V with 12.5 ppm (:() accuracy over -40 to 120 C.
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Mayank Community Fellow Offline Posts: 334	Re: very low beta value of pnp to be used in �BGR Reply #1 - Nov 9^th, 2009, 10:53am hello, All CMOS Trans-Linear BGRs are a new concept --> more complex, still developing...& though famous academically...Industrially, Bipolar BGRs are still in vogue... Try with BJT's first...standard achitectures...maybe some senior with ex in BGR design can suggest better.... --mayank.
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HdrChopper Community Fellow Offline Posts: 493	Re: very low beta value of pnp to be used in �BGR Reply #2 - Nov 9^th, 2009, 5:40pm I agree with Mayank... Go with BJTs....low beta will be an issue though. One option I can think of now would be to use MOS beta helper, so that the low beta effect is mitigated... Best Tosei
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Mayank Community Fellow Offline Posts: 334	Re: very low beta value of pnp to be used in �BGR Reply #3 - Nov 9^th, 2009, 8:29pm Thanx Tosei, Can you please provide some info / material on MOS beta helpers.... --Mayank
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: very low beta value of pnp to be used in BGR Reply #4 - Nov 9^th, 2009, 10:07pm hi mayank, i feel some how if you can supply base current through some means its fine..i donno abut beta helper but please refer fig 7 in the following paper.. http://repository.ust.hk/dspace/bitstream/1783.1/2348/1/200410CICC_Vrefdesign.pd... Thanks, rajasekhar.
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vivkr

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Re: very low beta value of pnp to be used in �BGR
Reply #5 - Nov 10^th, 2009, 12:39am

Dipankar wrote on Nov 9^th, 2009, 7:59am:

Dear Experts,

� �Can anybody share his/her �experience regarding BGR design with pnp device having very low beta (1.5-2.5) ? Any special care/techique to adopt ? or will you suggest to go with MOS-only BGR. Spec : �BGR out = 0.5 V with 12.5 ppm (:() accuracy over -40 to 120 C. �

One very good way of avoiding any issues with finite beta is to simply use the PNP as a diode. The PNP available in a process are mostly lateral and chances are quite high that you anyway do not have a floating PNP available, i.e. the collector is probably anyway tied to substrate. Have you cross-checked this point?

I don't know about beta of 1.5-2.5 but I have seen beta in the range of 4-5 for PNPs and seen these used in bandgaps. And in my design experience, bipolar bandgaps always outperform MOS ones in terms of precision.

Regards,

Vivek

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Dipankar

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Posts: 59

Re: very low beta value of pnp to be used in �BGR
Reply #6 - Nov 10^th, 2009, 8:55am

Dear All,
I'm using PNP (collector in p-sub) and it helps. Thank you all for your inputs.

Another query is : should we care about the opamp's psrr used in BGR ( traditional BGR topology) very much or just making loop gain high will be enough. This will effect my choice of opamp topology - two-stage or folded-cascode.

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With Thanks and Regards,
Dipankar.

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Mayank

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Re: very low beta value of pnp to be used in �BGR
Reply #7 - Nov 10^th, 2009, 11:32am

Hi,
As for your query, yeah, the PSRR of the opamp, and for that sake the whole BandGap, should be high....For high PSRR in BG ckt, we go for cascode structures....But as Jerry once suggested, cascode structures increase Noise at the output....There's a trade-off here.

Along with the PSRR, input OFFSET of the opamp is also a critical figure...Pls. keep this also in mind while designing...

--Mayank

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: very low beta value of pnp to be used in �BGR Reply #8 - Nov 11^th, 2009, 8:33am I am with Vivek on this one - the standard PNP in CMOS (lateral device, collector tied to substrate) is most commonly used in a diode connected configuration to create a bandgap. It's out there in millions of chips. -- jerry
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