opamp voltage gain descreases sharply as supply voltage varies

[anhnha]

I am design an opamp with supply voltage varying from 2 to 3.3V. I need to get high gain (more than 90dB). The problem is that my opamp only has a large gain (100dB) at supply voltage 2V. As supply voltage increases, the gain decreases sharply. It is only 80dB at 2.1V, 76dB at 2.2V, ..., 52dB at 3.3V.
Could you tell me how to solve this problem?

 

[FvM]

How do you measure gain? Is the OP biased correctly?

 

[anhnha]

I run ac analysis and then plot ac gain & phase margin in spectre of Cadence.
The gain above is DC gain.
All transistors are biased to operate in saturation at supply voltage 2V.

 

[FvM]

You have to check for suitable bias at all supply voltages. Usually the bias will be achieved by DC feedback in simulation. That's easier as in a real circuit because you can use ideal capacitors or inductors as AC block. If the respective circuit is over your head, adjust the input DC bias manually until you achieve a suitable DC output level, e.g. Vdd/2.

 

[kenambo]

hi,
i hope your design is based on some pdk..

MOSes are characterised differently for various voltages.. so you first check the Mos variations according to supply..

Because pdks have different moses which operate on different supply voltages.. for example 2.5Moses, 3.3 Moses, 1.8Moses ..etc..

these different supply voltage variation also cause some problem for your design. so choose the device accordingly..

and check the DC conditions for every operating ranges..

thanks