opamp output common mode varying with input CM?

[Usman Hai]

Hi fellows,
I designed an gain boosted telescopic opamp with ideal CMFB. Now problem is when my input CM changes even around ±100mV output CM changes a lot (900mV to 600mV). And it is also not fixed for any range of input CM, which seems to be weird.
I am attaching the schematic of my opamp. I also used SC CMFB as well and getting the same response.

So my specific questions are:
1) Is it possible for the output CM to change with input CM even in the presence of the ideal CMFB loop?
2) How to deal with this problem

Regards,

 

[surianova]

Hi fellows,
I designed an gain boosted telescopic opamp with ideal CMFB. Now problem is when my input CM changes even around ±100mV output CM changes a lot (900mV to 600mV). And it is also not fixed for any range of input CM, which seems to be weird.
I am attaching the schematic of my opamp. I also used SC CMFB as well and getting the same response.

So my specific questions are:
1) Is it possible for the output CM to change with input CM even in the presence of the ideal CMFB loop?
2) How to deal with this problem

Regards,

i think u don't have reference voltage to your cmfb. That's why the output change with input

 

[drDOC]

That's weird. I think you have to study your CMFB better, this behavior should not happen.

You also need common mode control on your gain boost amps.

Another possibility is that your input common mode range is very limited, and that you exceed your input common mode range. Your input common mode range is from Vgs+Vsat to Vref-Vgs where Vref is the biasing of the inputs of the lower gain boost amplifier. That seems very tight biasing to me.

 

[zhaoshibin]

Hi drDOC
in your reply,I have noted that you mention the upper limitation of the Input CM range is vref-vgs(maybe vsg),Is "vref" the biasing volt of the PMOS nail current source of the lower boosted amplifier? if really,I think the upper limitation is vref+vth-vgs.Can you agree with it?