question for DC gain of the fully differential OTA

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noiseless

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I found one weird thing in the simulation of the DC gain of the fully differential OTA. When I use continuous-time CMFB circuit, the DC gain can be 80dB, but when I use switched capacitor CMFB, the DC gain is 60dB. Anyone knows what is the solution?
 

post you schematic
 

By the way, when I use switched capacitor, I use SpectreRF to do the simulation. Is that will affect DC gain?
 

I just don't see any reason to simulate the DC gain of fully differential OTA with SC CMFB using spectreRF: you may have some improper settings there which screwed up the OTA DC gain. You can do some tricks to get the DC gain of the SC CMFB without using spectreRF.
 

willyboy19,

I don't understand. If I use the SC CMFB, there is clock there. I have to use spectreRF to do the PAC simulation to get DC gain.
 

in my opinion,
DC gain such as 80dB is only meaningful at the correct biasing voltages.
if u find something make it different from ur original value, u should first check ur biasing conditions of the main amp., it should be different.
usually SC CMFB is slower than continuous time CMFB.
in continuous time CMFB, u get the right bias voltages at almost instant, but it is not in SC CMFB, it need more time to settle to the correct Vcom, and other bias voltages.
 

I don't think the DC gain will reduce in SC-CMFB. Could you make sure your DC bias for SC-CMFB is in its settle point ?

Yibin,
 

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