The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> Fully balanced differential opamp start-up issue
https://designers-guide.org/forum/YaBB.pl?num=1285716395

Message started by thechopper on Sep 28^th, 2010, 4:26pm

Title: Fully balanced differential opamp start-up issue
Post by thechopper on Sep 28^th, 2010, 4:26pm

Hi all,

I'm having problems with the start-up of a fully differential opamp used in a simple first order switched capacitor filter.
The amplifier is a two stage one, the first stage being a folded cascode one with an NMOS differential pair, and the second one a class A Miller stage (NMOS).
Silicon is showing that the opamp is not properly starting up only under certain VCC ramps: for fast ramps (up to 500 us from 0 to 5v) the opamp starts-up correctly, while for ramps between 500us to ~10ms it does not. It seems to work ok again for ramps even slower than 10ms.
The folded cascode stage is ideally balanced: pmos current mirrors at the top push 10ua, the differential pair 5ua and the NMOS mirrors (at which the CMFB is controlling the output CM) another 5ua.
Ideally if during start-up the PMOS mirror pushes more current than NMOS the output NMOS gate will raise its voltage and output CM might not raise, so I'm think of pushing a little bit more than ideal current on NMOS cascodes to guarantee proper start up.

Any ideas and comments are welcome
Best
Tosei

Title: Re: Fully balanced differential opamp start-up issue
Post by subgold on Oct 1^st, 2010, 3:20am

sounds like you just need some more startup current in the NMOS side to activate the CMFB loop, assume there is no startup issue in the CMFB itself.

just a thought:
maybe you put 4uA to be controlled by the CMFB and leave 1uA in the NMOS as normally biased current source, as long as the CMFB is still stable enough. or move the CMFB to the PMOS side, as there is more freedom to manipulate the portion which is controlled by CMFB (10uA vs. 5uA)

Title: Re: Fully balanced differential opamp start-up issue
Post by thechopper on Oct 2^nd, 2010, 6:35pm

Thanks for you comments subgold. I'm fixing this by putting more current in the NMOS current mirror as you are suggesting. However the opamp will be working with an unbalanced CMFB under normal operation (I do not have the ability to implement a start-up ciruit that can be turned off after a certain time or reaching a certain threshold, since I can fix this in metal only)

I guess that working with an unbalanced CMFB is not that important assuming the rejection ratio of such circuit will not strongly affect the differntial signal much.

Best
Tosei

Title: Re: Fully balanced differential opamp start-up issue
Post by vp1953 on Oct 3^rd, 2010, 7:08pm

Hi Tosei,

If it is possible, can you post the schematic?

When you say opamp does not startup properly, is the output of the first stage at vdd/gnd? Does it use any external bias sources and if so, are the bias sources working properly under startup conditions?