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Design >> Analog Design >> PSRR of bandgap reference
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Message started by subgold on Oct 21^st, 2009, 8:06am

Title: PSRR of bandgap reference
Post by subgold on Oct 21^st, 2009, 8:06am

I have a question concerning the PSRR of the bandgap.

I have a sub-1V current summing bandgap topology. I use a cascode transistor in the output branch to improve the PSRR.

then I add a regulation loop around the cascode transistor (gain-boosting) to further improve the PSRR. Then the problem arises. The PSRR improvement can only go up to a certain level, and can go no more higher, even if I increase the gain of gain-boosting amplifier, which would theoretically increase the output impedance of the cascode branch.

Also, the loop composed by the opamp has a rather high gain and PSRR, which doesn't seem to be the limiting factor here, does it? i am not so sure how this loop will eventually impact the PSRR.

Can anybody give me a clue on what is limiting the PSRR in this case?
Thanks a lot.

Title: Re: PSRR of bandgap reference
Post by thechopper on Oct 21^st, 2009, 7:45pm

Hi subgold,

In theory your opamp should provide infinite output impedance at the drain of your cascode device (and thus infinite PSRR); which obviously is not attained in real ckt.
The opamp has a finite gain and therefore it will not allow to reach such high impedance and PSRR levels, the latter also limited by the op-amp PSSR.
In addition CGS of your cascode transistor will start - at high frequencies- to couple noise from the power supply directly onto the gate; overriding in this way the op-amp gain boosting feature.
Both things then (opamp finite gain and CGS coupling) will end up degrading your PSSR with respect to ideal levels.

Best
Tosei

Title: Re: PSRR of bandgap reference
Post by Mayank on Oct 22^nd, 2009, 8:11am

Hi subgold,
I dont know whether u are talking about schematic level design OR Post-Layout Simulation, But in any case, Take care while PNR to keep the capacitance between the opamp o/p node and the vdd/vss rails to be negligible(this cap will directly introduce coupling, hence degrading supply rejection). Especially in BG ckts, Layouts are v critical...

thanx,
Mayank.

Title: Re: PSRR of bandgap reference
Post by subgold on Oct 23^rd, 2009, 12:40am

thanks both for your inputs.