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Query on folded cascode ota with switched capacitor cmfb (Read 4814 times)

RaDex

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Query on folded cascode ota with switched capacitor cmfb
Sep 07^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

circuit_003.png

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harpoon Senior Member Offline Posts: 112 Hampshire	Re: Query on folded cascode ota with switched capacitor cmfb Reply #1 - 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 ?
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RaDex Junior Member Offline Posts: 10	Re: Query on folded cascode ota with switched capacitor cmfb Reply #2 - 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.
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harpoon Senior Member Offline Posts: 112 Hampshire	Re: Query on folded cascode ota with switched capacitor cmfb Reply #3 - 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 ?
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RaDex Junior Member Offline Posts: 10	Re: Query on folded cascode ota with switched capacitor cmfb Reply #4 - 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
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harpoon Senior Member Offline Posts: 112 Hampshire	Re: Query on folded cascode ota with switched capacitor cmfb Reply #5 - 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 ?
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