How to set an opamp to operate the same common mode input?

[cjupiter]

common mode input

In a folded cascode opamp there is a tail current at the differential pair (assuming an NMOS differential pair.). I have simmulated this tail current using a current source that is available in spice. According to my frequency response everything is fine, however....

My problem is that when I offset my input voltage to the desired common mode level, (in my case around 1.3V) my output voltage seems to sink below Vss which is ground. How do i set my opamp to operate about the same input common mode level.

I should also mention that the voltage at the differential input tail is negative. Could this be the problem?

 

[djalli]

common mode input

It is too blury picture or maybe I am trying to visualize it , an actual picture (circuit schematic) would help.

 

[cjupiter]

Re: common mode input

There are three figures in the attachment.

opamperror1.gif: This is the output i get when i offset my input (black curve)

opamperror2.gif: This is the output i get when i dont offset my input.

opampschematic: This is my folded cascode schematic.

 

[Hughes]

common mode input

The problem arises from the tail current source. It has such a large current value (1mA!) that forces the Vgs of the input pairs to a very large value (larger than 1.3V). Thus, the voltage at the differential input tail goes to negative.

The output stage, however, is biased by current mirrors. I guess the current is much less than 1mA if the input stage is removed from the circuit. Since the tail current source is ideal, it will always draw exactly 1mA current from the output stage. If M1 and M2 can't sources such a large current, the lower part of the output stage (M3~M10) will draw currents from vss to M5 and M6 ( and hence to I1). In this case, the output voltage becomes negative.

If you replace I1 to a mosfet current sink, the current will decrease when the input common voltage is too low. So, in this case, we don't see a negative output voltage.