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Fully differential Folded Cascode op amp bias with PMOS Inputs

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sherlock176

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Please help me in designing the folded cascode amplifier using PMOS inputs
 

sherlock176

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Yes I consulted textbook Behzad Razavi also. I am designing the circuit for small powered devices in cadence gpdk180. I want to know the optimal assumptions for the circuit.

I assumed the design parameters as
1) Vdd = 1.8V
2) Power = 54uW (I comes out to be 30u)
3) SR = 10V/usec
4) Load cap = 2pF
5) GB = 20MHz
6) ICMR- = 0.8V
7) ICMR+ = 1.6V

Help me assuming the optimium overdrive voltages. I have taken circuit from Behzad Razavi p 302
 

Dominik Przyborowski

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SR=Itail/Cload -> Itail = 20uA
GBW=20 MHz -> gm = 2PI GBW Cload = 250uS -> Itail > 16uA (depends to inversion coefficient of your input mosfets)
 

sherlock176

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folded_nmos.png

here i have designed nmos input folded cascode transistor. problem is that the pmos transistors and last two nmos tx in cascode structure are working in region 2 i.e triode region. also for biasing i have set v3 and v4 as 880mv and 770mv.

Suggest me some changes for making all transistor working in saturation
 

sherlock176

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I have tuned the circuit. I obtained all transistors in saturation region. Now i am faciing one problem.

Gain is coming out to be 52dB and UGB is only 6.7Mhz.
It should be atleast 20MHz and >70dB

How to approach for that?
 

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