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Design >> Analog Design >> Query on folded cascode ota with switched capacitor cmfb
https://designers-guide.org/forum/YaBB.pl?num=1378556694

Message started by RaDex on Sep 7^th, 2013, 5:24am

Title: Query on folded cascode ota with switched capacitor cmfb
Post by RaDex on Sep 7^th, 2013, 5:24am

hi all ,
I have designed a folded cascode ota with both ideal and switched capacitor cmfb. With ideal cmfb open loop gain is 67dB but when used with sc cmfb the gain decreases to 33dB .
What is the reason for this ?
in sc cmfb non-switched capacitor value is 200fF and switched capacitor value is 100fF. clock frequency is 10.24M

Title: Re: Query on folded cascode ota with switched capacitor cmfb
Post by harpoon on Sep 11^th, 2013, 3:39am

Hi RaDex,

Firstly, I am guessing that you are referring to your CMFB loop gain.

How are you simulating the SC-CMFB loop ?

Transient + stb ?

Title: Re: Query on folded cascode ota with switched capacitor cmfb
Post by RaDex on Sep 11^th, 2013, 4:24am

its the open loop gain of the ota with switched capacitor cmfb.
I have done PSS followed by PAC analysis for simulation of open loop gain.

Title: Re: Query on folded cascode ota with switched capacitor cmfb
Post by harpoon on Sep 11^th, 2013, 6:17am

Hi RaDex,

Just a few more questions ...
a) ideal CMFB : Is this a Continuous time or a SC version ?
b) have you checked the DC or PSS node voltages ? Are they the same/similar between the 2 sims ?

Title: Re: Query on folded cascode ota with switched capacitor cmfb
Post by RaDex on Sep 12^th, 2013, 3:13am

ideal cmfb is made up of balun and an amplifier in negative feedback
can be said as ct cmfb

Title: Re: Query on folded cascode ota with switched capacitor cmfb
Post by harpoon on Sep 12^th, 2013, 7:34am

Hi Radex,

OK ... sounds like your SC-CMFB is broken or something. Did you run a transient sim to see if your OTA is working as it should ?