about Folded-cascode opamp

[yuguo]

Hi, I am a beginner in analog design, and I am trying to build a folded-cascode opamp,
While I found in the simulation results that the linearity is poor, there is 10% difference of the gain between the maximum input and input ten times lower.
Is there any suggestion on how to improve?
I have checked the biasing, seems there is no problem.

Another question is that, how to calculate DC gain?
I am using spectre, is it ok if I load the circuit with an infinite resistor?

P.S. The supply voltage is 1.1V and output swing is 0.5Vp-p, with four transistors cascoded.

Thank you very much for your help in advance

 

[bloodemon]

I would say 10% difference is OK for the analog design. You can not achieve perfect linearity.

When calculating the DC gain, you can load the Op-Amp with a capacitor, say 5pF.

 

[yuguo]

I would say 10% difference is OK for the analog design. You can not achieve perfect linearity.

When calculating the DC gain, you can load the Op-Amp with a capacitor, say 5pF.

Thank you, bloodemon

But the problem is that, if I use a capacitor for loading, the nodes of output don't have right biasing
My mentor told me to define the output resistor with normal resistence value in DC, but infinite otherwise. While seems Spectre doesn't have this ability?

For linearity, I designed LNAs before, they do have good linearity. Maybe opamps are different...

 

[lastdance]

how did u sim. the circuit?

u can't put ur opamp in open loop, which seems to me as if u were doing that.

LNA is different, it's low gain compared with opamp, and u do bias the input and output using resistors, don't u?

so in opamp, have u done something to bias it correctly? esp the output

 

[ambreesh]

First I'm unable to understand how could your DC bias point change with a Capacitor.
And we are talking DC here a capacitor is open circuit for DC.
Something wrong at simulation end.

 

[ashishvora]

You need to bias the amplifier properly. Here is what you can do. Connect a huge 100M resistor from the ouptut to the -ve input and and huge cap say 10F from -ve input to ground. Connect a appropriate CM voltage to the +ve input. What will now happen is ur other input will also come at the same input that you apllied. Now you can connect a AC signal to either input and Do the AC Analysis.

 

[hood]

It's OK!