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Capacitive feedback OTA Design Issues in Loop gain

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niranjan1984

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HI
I have designed a capacitive feedback OTA ( OTA is folded cascode single stage OTA with ideal CMFB). When I simulate the loop gain of OTA (using iprobe and stb analysis in cadence) I get a 'High Pass' response at low frequencies.
I am not sure why this is happening. Please help to solve this issue.

Thanks
 

Just want elaborate on this. When I did loop gain simulation at low frequency (till 100Hz ) gain was increasing and after that it settled down and started decreasing at pole frequency. This in all I am getting a band pass waveform. Also in the step response it settles down at 10nsec but after that it starts decreasing and picks up again,giving a undershoot. This phenomenon keeps on repeating. I am not sure why this is happning.
Please help.

Thanks
 

Could you post a quick schematic maybe? Is the open loop gain of your amplifier alone allright?
 

Hi
Fig1 shows the test bench used for loop gain simulation
Fig2 shows the loop gain and fig3 shows the step response. I have following questions

1. Why loop gain curve is not flat at the beginning? Theory shows that at low frequency loop gain equals to T0=β.Gm.Ro and hence should be flat

2. Why is the step response showing these humps? Is this because of loop gain issue? or something else

I am not getting this please help.

Thanks

 

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