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RobG Community Fellow Offline Posts: 569 Bozeman, MT	Re: BGR PSRR Reply #15 - Sep 15^th, 2014, 7:38am I still haven't had a chance to look in depth at this - it could be a great idea - but wouldn't you get the same effect if you used a PMOS output stage with Miller compensation driving M2 and M3? (Or better, from the gate of M2/M3 to Vdd) Perhaps with an LDO you would be load compensated.
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analog_rf Junior Member Offline Posts: 15 Universe	Re: BGR PSRR Reply #16 - Sep 15^th, 2014, 8:59am As mentioned already ,haven't used this in case of Bandgap. For BG PSRR i would prefer using a Folded cascode dominant pole compensated and also add small load cap of maybe 10pf at output will give good psrr no's @ac.
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analog_rf

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Re: BGR PSRR
Reply #17 - Sep 15^th, 2014, 9:14am

RobG wrote on Sep 15^th, 2014, 7:38am:

I still haven't had a chance to look in depth at this - it could be a great idea - but wouldn't you get the same effect if you used a PMOS output stage with Miller compensation driving M2 and M3? (Or better, from the gate of M2/M3 to Vdd) Perhaps with an LDO you would be load compensated.

Miller would be a killer for ac PSRR, since dominant pole would be internal and adding more cap would cause the internal pole to be pushed further inside for stb(you could still use cap to supply,but will not be better than fc opamp).

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RobG

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Re: BGR PSRR
Reply #18 - Sep 15^th, 2014, 2:16pm

analog_rf wrote on Sep 15^th, 2014, 9:14am:

Miller would be a killer for ac PSRR, since dominant pole would be internal and adding more cap would cause the internal pole to be pushed �further inside for stb(you could still use cap to supply,but will not be better than fc opamp).

Miller is what they are using now, except they are using an NMOS mirror so it is referenced to the bottom rail which will make PSRR bad because the output device is basically diode connected at AC. That is why I as thinking the shielding of the Miller "diode" from the gate of M2/M3 was the real advantage.

The weather is too nice for me to do the math right now

.

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: BGR PSRR Reply #19 - Sep 16^th, 2014, 3:58pm generally your dominant pole in the control loop is going to be the output capacitor on the bandgap voltage. it may not seem elegant to some but its worked pretty well for many years, and if the output drive is high impedance (not a source follower) it generally will not be terribly big in size.
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