Dear Jrom,
I reply you quite fast. I hope these reply are clear enough, if not I will edit the text to clarify. Also, if any of this explanations are not ok at all don't hesitate to correct me.
Thank you in advance.
- I added that resistor to reduce the Vgs of M9 and M6, which seemed to reduce the output offset in transient simulation. However, with the adjusted bias voltages, a resistance of 0 ohm gives better results. I'm not sure why.
1- Because of the source connected to V7 and the gate connected to V3, M8 and M2 are generating a fixed Ids current.
2- On the other hand, M0 and R9 are working like a current source. If M0 operates alone (like with the source connected to ground), the Vgs value will be fully controlled by V11 (Because Vgs would be directly Vgs). The effect of R9 on this source is, when you increase Vg of M0, the raise of the Ids current produces an increase in the voltage difference in R9, increasing the Vs voltage in the source node of M0, and reducing the absolute increment of Vgs. As a resume: As big is R9 smaller is the current that you will generate applying a V7 voltage.
3- Combining 1 and 2 and applying Kirchhoff to node which the source of M3 is you can see that you are injecting the current produced by 1, and subtracting half of the current produced by 2. The rest of the current will flow as Ids of M3. As bigger is this current bigger is going to be the DC voltage at the output node.
So, if you reduce R9, you increase 2, you subtract more current from 1 and 3 is smaller as it is also the output DC voltage.
- If I add that NMOS to keep symmetry, will the output of my opamp still be single-ended?
If you include this transistor you would have a CMOS differential amplifier, whose inputs would be the gates of M9 and M6, and their differential outputs the drain of M3 and M1 (the differential gain would be the same in magnitude if you keep the symmetry including the suggested transistor).
The main drawback of this differential output is that you will need to DC bias the M1-drain-node output. And if you are not looking for a differential output it is not necessary to complex the design in this way.
I think that I didn’t give you a good advice on this…