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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> Does CMFB loop bandwidth has impact on signal path bandwidth? https://designers-guide.org/forum/YaBB.pl?num=1232441332 Message started by jlee on Jan 20th, 2009, 12:48am |
Title: Does CMFB loop bandwidth has impact on signal path bandwidth? Post by jlee on Jan 20th, 2009, 12:48am Hi guys, I have a question as the subject. A fully differential folded opamp is used with negative feedback to amplify the input signal with bandwidth of 50M. A CMFB is needed for this opamp. My question is how to confirm the bandwidth for this CMFB? Does this CMFB loop bandwidth has impact on signal path bandwidth? Regards |
Title: Re: Does CMFB loop bandwidth has impact on signal path bandwidth? Post by Berti on Jan 20th, 2009, 3:25am Whether the CMFB has an impact on the differential-mode bandwidth depends on its implementation. In a SC-CMFB for instance, high common-mode bandwidth comes at the price of reduced differential-mode bandwidth. There is a trade-off. However, you should give more details. Cheers |
Title: Re: Does CMFB loop bandwidth has impact on signal path bandwidth? Post by jlee on Jan 20th, 2009, 4:52am Thanks for your input. The CMFB I used is sensing common mode voltage by two large resistors parelleled with fully differential output nodes and an opamp is used to force the sensed common mode voltage to a referced voltage. This opamp output is connected to the gate of tail current source of fully differential opamp. Regards |
Title: Re: Does CMFB loop bandwidth has impact on signal path bandwidth? Post by thechopper on Jan 20th, 2009, 6:22pm It looks to me you are using a high swing CMFB circuit. IF that is the case actually is the differential signal path the one mainly setting the CMFB BW. In my statement I'm assuming your CMFB signal is fed back at some intermediate node in your differential path beore the differential compensation stage (for example a Miller stage) Regards Tosei |
Title: Re: Does CMFB loop bandwidth has impact on signal path bandwidth? Post by jlee on Jan 21st, 2009, 12:02am Hi Tosei, thanks for your comment. Yes, the CMFB feedback path is really the same as your statement. However, I cannot understand why "the differential signal path is the one mainly setting the CMFB BW". ::) Regards |
Title: Re: Does CMFB loop bandwidth has impact on signal path bandwidth? Post by vivkr on Jan 21st, 2009, 1:37am Berti wrote on Jan 20th, 2009, 3:25am:
Hi Berti, What do you mean when you say that high common-mode bandwidth comes at the price of reduced differential-mode bandwidth with an SC-CMFB? Could you elaborate please? Regards, Vivek |
Title: Re: Does CMFB loop bandwidth has impact on signal path bandwidth? Post by thechopper on Jan 21st, 2009, 6:38pm jlee wrote on Jan 21st, 2009, 12:02am:
As I tried to describe before, I'm assuming your CMFB signal is fed back to some intermediate point in your differential signal path, for example somewhere in the first stage of a two-stage Miller compensated opamp. Therefore such CM fed-back signal will have to go through the differential compensation stages, therefore it will be limited by the differential path speed. Hope my explanation is clearer now. Regards Tosei |
Title: Re: Does CMFB loop bandwidth has impact on signal path bandwidth? Post by Berti on Jan 21st, 2009, 10:10pm Quote:
Vivek, The bandwidth of a SC-CMFB depends on the size of the two capacitors connected between the outputs and the controlling node. Larger capacitors will give larger common-mode bandwidth but increases the loading of the amplifier, therefore decreasing the differential-mode bandwidth. The following paper is worth reading: Ojas Choksi and L. Richard Carley , Analysis of Switched-Capacitor Common-Mode Feedback Circuit ; IEEE Transactions on Circuits and Systems II ; 2003 . Regards |
Title: Re: Does CMFB loop bandwidth has impact on signal path bandwidth? Post by vivkr on Jan 21st, 2009, 11:31pm Berti wrote on Jan 21st, 2009, 10:10pm:
Hi Berti, Thanks for the link. I will check it out. But I think you are speaking not about the size of the capacitors used in the SC-CMFB but the ratio of the sampling to the integrating caps (it is an integrator). Am I right? So, if the smaller of the 2 caps is constrained to a certain size, then increasing CMFB loop bandwidth implies increasing the loop feedback factor (larger cap). Regards, Vivek |
Title: Re: Does CMFB loop bandwidth has impact on signal path bandwidth? Post by Berti on Jan 22nd, 2009, 7:44am Actually no. I mean the capacitor of the SC-CMFB which is connect to the output of the amplifier. Cheers |
Title: Re: Does CMFB loop bandwidth has impact on signal path bandwidth? Post by vivkr on Jan 22nd, 2009, 11:34pm Berti wrote on Jan 22nd, 2009, 7:44am:
Yes! I mean the same one, the one which is connected permanently. That is the feedback cap for the integrator formed in the CMFB loop. If i make that one larger, I increase the feedback factor for that integrator (=> larger loop BW), but at the cost of larger load. Maybe I will try to get hold of the paper and read it first. Thanks for the link. Regards, Vivek |
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