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Design >> Analog Design >> which BJT (pnp/npn) is better to use in a BGR?
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Message started by yvkrishna on Aug 14^th, 2012, 8:54am

Title: which BJT (pnp/npn) is better to use in a BGR?
Post by yvkrishna on Aug 14^th, 2012, 8:54am

Hi,

Given a choice to use both npn/pnp bjts in a cmos process which one is better to use in Bandgap reference design? Assume both of them have same beta (current gain=ic/ib)

As already known having poor beta causes additional voltage drop in base resistance which could finally limit the accuracy of bandgap. Is it just enough to pick the one which has higher beta?

Area there any other tradeoffs related to inherent curvature of VBE, area, speed etc?

Or are there any better topologies in which only one of them could be employed?

Thanks,
yvkrishna

Title: Re: which BJT (pnp/npn) is better to use in a BGR?
Post by loose-electron on Aug 26^th, 2012, 1:01pm

the collector tied PNP is commonly used.

I have used PN junctions and gotten good results if the models are properly defined.

Title: Re: which BJT (pnp/npn) is better to use in a BGR?
Post by Kevin Aylward on Jul 14^th, 2013, 10:06am

Speed may be an issue in that slower devices may be harder to stabilise in a feedback loop. In a CMOS process I would expect that the npn will be much faster than any pnp. Often the pnp is not a real one. The best bandgaps use bipolars as transistors, not merely as diodes, and use both types throughout. A key issue when Mosfets are used, is the difficulty in trimming to a universal “magic” voltage. If all bipolars are used, all offsets are tuneable ptats. That is, if the bg is trimed to the magic voltage, it will have the correct square shape temperature curve, irrespective of individual errors. This is important as it allows the band gap to be set with one trim at, say, 27 deg. Unless all Mosfets are relatively, absolutely huge, say 200u/40u, a trim to what should be a magic voltage won’t be. In which case production trimming becomes a nightmare.

Title: Re: which BJT (pnp/npn) is better to use in a BGR?
Post by tm123 on Jul 22^nd, 2013, 9:18am

yvkrishna,

If you are using a CMOS process, substrate PNP's are usually available without adding much if any cost to the mask set. Of course the limitation is that the collector terminal of the transistor must be tied to the substrate. Lower beta, slower speed etc. are also inherent with substrate pnp's but you should still be able to make a reasonable bandgap reference circuit. I expect adding NPN transistors in a CMOS process will cost you $$ because of added mask layers. If I were using a BiCMOS process I would certainly choose NPN transistors.

I would recommend the Gray/Meyer text book as a basic reference, but several books have bandgap reference circuits in CMOS processes. I have used Brokaw cells and PTAT generators when NPN's are available.

Hope this helps.

Tim

Title: Re: which BJT (pnp/npn) is better to use in a BGR?
Post by loose-electron on Aug 2^nd, 2013, 6:56pm

this was asked over a year ago...