#### crystalbrite

##### Newbie level 3

Hello,

To summarise, I'm currently designing a folded cascode amplifier and the following gain equation doesn't hold true for larger ICMR (input common mode range) values and I can't figure out why.

All devices are in saturation.

Gain = gm2/((gds8*gds10/gm8)+(gds6*(gds4+gds2)/gm6))

Design attached.

I'm trying to increase the input common mode range while maintaining gain.

I've designed the overdrive on the devices to be ~150mV.

I'm using the following equation to calculate gain using the extracted operating values of gm and gds:

Gain = gm2/((gds8*gds10/gm8)+(gds6*(gds4+gds2)/gm6))

I'm using this equation to try and optimise the gain.

This equation holds true for the ICMR<3V but doesn't match the ac analysis gain for ICMR>3V.

Below shows a plot of the opamp gain as the ICMR voltage is swept from 0.6V - 4.9V.

VDD = 5.5V

VSS = 0V

The gain from the ac analysis and the gain calculated using the above equation with gm and gds values extracted using a .op analysis are plotted.

It can be seen that the gain equation matches the ac analysis very well below 3V but above 3V the gain from the ac analysis starts drops off.

I've also run a transient analysis with a small ac input signal and it matches the ac analysis.

I've also checked the operating regions of all transistors and all transistors are in saturation for ICMR = 4.9V with some headroom to spare.

Does anyone know why the gain equation doesn't hold true for the higher ICMR values?

Gain = gm2/((gds8*gds10/gm8)+(gds6*(gds4+gds2)/gm6))

Thanks in advance!

To summarise, I'm currently designing a folded cascode amplifier and the following gain equation doesn't hold true for larger ICMR (input common mode range) values and I can't figure out why.

All devices are in saturation.

Gain = gm2/((gds8*gds10/gm8)+(gds6*(gds4+gds2)/gm6))

Design attached.

I'm trying to increase the input common mode range while maintaining gain.

I've designed the overdrive on the devices to be ~150mV.

I'm using the following equation to calculate gain using the extracted operating values of gm and gds:

Gain = gm2/((gds8*gds10/gm8)+(gds6*(gds4+gds2)/gm6))

I'm using this equation to try and optimise the gain.

This equation holds true for the ICMR<3V but doesn't match the ac analysis gain for ICMR>3V.

Below shows a plot of the opamp gain as the ICMR voltage is swept from 0.6V - 4.9V.

VDD = 5.5V

VSS = 0V

The gain from the ac analysis and the gain calculated using the above equation with gm and gds values extracted using a .op analysis are plotted.

It can be seen that the gain equation matches the ac analysis very well below 3V but above 3V the gain from the ac analysis starts drops off.

I've also run a transient analysis with a small ac input signal and it matches the ac analysis.

I've also checked the operating regions of all transistors and all transistors are in saturation for ICMR = 4.9V with some headroom to spare.

Does anyone know why the gain equation doesn't hold true for the higher ICMR values?

Gain = gm2/((gds8*gds10/gm8)+(gds6*(gds4+gds2)/gm6))

Thanks in advance!

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