Help_Fully Differential Miller Ota _Closed Loop DC Sweep

[onrkya]

I am designing a fully differential miller ota in 0.18u technology.
At first stage I observed the open loop gain but I could not observe the
closed loop dc sweep. My CM range is 0.6 - 1.2 V .

I searched the internet but there is everything but the simulation techniques.
Can you please explain it clearly?

Thanks

 

[ljy4468]

If you use spectre,
you can break the loop & place 'iprobe' in analoglib.
Then run stability analyze(.stb)
It shows closed loop gain & phase curve.

 

[yschuang]

The following link is probably helpful.
https://www.designers-guide.org/Forum/YaBB.pl?num=1155668476

 

[onrkya]

I have attached the circuit. The tech is 0.18 Tsmc
The point is I have designed the circuit such that my input range is to be btw 0.6 V and 1.2 V . While I sweep one input from 0.6 to 1.2 and the other one 1.2 to 0.6V using Voltage controlled voltage source at the input, I see a gain of around 80dB with a little offset.

Also while both inputs are at 0.9V level all transistors work in saturation.
I ve sized every transistor choosing the appropritate compansation capacitor which is the fundamental method to size the Miller Ota. My load caps are 6pF and my desired GBW is around 100MHz. 37uA passes thru each diff input transistors and 375uA passes thru each side branches.

I have placed resistors to the end of the circuit with 0.9V Common Mode Voltage to the middle of the resistors and also placed the Control Voltage . So that the circuit is expected to work as if there is a CMFB circuit . The resistors are big which are 10k not to load the circuit .

When I closed the loop , the input of my circuit does not follow the output.
The circuit performance also depends on the resistors that are placed in the closed loop configuration.



Can you tell me what may be the problem?

I have also tried by lowering the open loop gain but it didnt work.