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Design >> Mixed-Signal Design >> SC-CMFB of the opamp
https://designers-guide.org/forum/YaBB.pl?num=1149568231

Message started by baohulu on Jun 5^th, 2006, 9:30pm

Title: SC-CMFB of the opamp
Post by baohulu on Jun 5^th, 2006, 9:30pm

I build a folded cascode amplifier with a common 4-capacitors-and-6-switches SC-CMFB. I use two phase non-overlap clock to drive the switches. in the simulation, I choose "tran", "pss"."pac" analysis. in the simulation, the input signal is continuous sin waveform, the output of the opamp is directly chosen as the output of the simulation (not after a SHA) . so you can see that, I use a continuous input signal go through a opamp with a SC-CFMB,is it rignt?
and in the result of pac, the differential gain of the opamp increse as the frequency of the clock decrease, I don't know why ,theoritically, the differential mode gain should have no relation with the SC-CMFB. Is there anyont having met this case?
besides, in the continuous CMFB, we can calculate the gain of the CMFB, but in the SC-CMFB, what's the correspondence of the gain in the SC-CMFB, or how do we evaluate the effect of the SC-CFMB?
thank you

Title: Re: SC-CMFB of the opamp
Post by ywguo on Jun 6^th, 2006, 7:36am

Hi, Baohulu,

Don't forget that the SC-CMFB is a part of the differential load, too. Obviously that the equivalent impedence of the SC-CMFB becomes small as the clock frequency increases. So the differential gain becomes low when the clock frequency increase. :)

Best regards,
Yawei

Title: Re: SC-CMFB of the opamp
Post by baohulu on Jun 6^th, 2006, 10:29pm

hi,ywguo

I also realize that the capicitor with two non-overlap switches emulate a resistor. and this type of SC-CMFB can be viewes as a load composed of a capacitor parallel with a resistance, am I right?

it I am right, does it mean that if I use the sc-cfmb, I should get a higher open-loop gain of the opamp in order to get a equal gain as with a continuous cmfb?

thanks