this circuit FAILED on SS corner. I don't know why

[allennlowaton]

Hello EDA felows,

I had made a corner simulation of this circuit and I found out that it failed on the SS corner.
I can't explain why.
Do I need a start-up circuit for this?

Below is the circuit diagram.


Below also are the pertinent node voltages.


The DC output voltages of OPA in all corners are the same.

Regards,

 

[leo_o2]

Pls also show voltage for N2

---------- Post added at 02:29 ---------- Previous post was at 02:26 ----------

and VB1 for all corners.

 

[allennlowaton]

the voltages at n2 on all corners are constant at 0.6V...

---------- Post added at 02:35 ---------- Previous post was at 02:30 ----------

the voltages at VB1:
TT - 788mV
SS - 900mV
SF - 1V
FS - 1V
FF - 800mV

 

[leo_o2]

I guess in SS corner VT for NMOS is too high. And Mn2 also have body effect that will increase VT further.

 

[allennlowaton]

I did a simulation using an ideal OP for the OPB and it works well on all corners.
Now, I don't know where to start the debugging.

 

[leo_o2]

I still suspect the VB1.
Could you try this?

 

[allennlowaton]

thank you and hello leo...
this is the resulting graph of your suggestion.(N1 and N3 node voltages are shown)


N1 and N3 should be a constant value, not a ramp.

 

[allennlowaton]

thanks leo_o2..you're right..the problem is with VB1.
I increased it a bit and the circuit is now working perfectly on all corners.

 

[leo_o2]

If my schematic doesn't work well, you should have a big increase for W/L of Mn2 and Mn3.

thank you and hello leo...
this is the resulting graph of your suggestion.(N1 and N3 node voltages are shown)


N1 and N3 should be a constant value, not a ramp.

 

[allennlowaton]

I just used the VB2 from the bias (VB2>VB1).In fact, I decreased the size of Mn2 (by 10 times) so that most the drop of Mn2 is much more higher than that of the Mn3. As much as possible I want a very low voltage of MN3 which also happens to be the node voltage of N4.

From this experimentation, I found out that the + terminal of the OP copied from the - terminal. I always believe before, that - terminal is the one who do copying.

 

[leo_o2]

Understand your requirement now. It will be OK.

---------- Post added at 03:09 ---------- Previous post was at 02:49 ----------

Another way for your requirement:

 

[hejinjie]

mark mark

 

[allennlowaton]

Understand your requirement now. It will be OK.

---------- Post added at 03:09 ---------- Previous post was at 02:49 ----------

Another way for your requirement:

so nice of you leo_o2...thank you very much...