Design review: Fully differential telescopic Operational amplifier.

[imrankhanPNU]

Hello,
I used Behzad Razavi's book to design a fully differential telescopic op-amp [screenshot attached]. Then I follow the same steps to design my proposed opamp, but I am not getting the desired result. Can someone point out where I'm doing mistakes?
For your reference, I have attached hand calculation and simulation screenshots. Thanks
My design specifications are: Supply voltage: Vdd=5V, Output voltage swing: 5V, Power ≤ 5mW, Gain: 500, UnCox= 151uA/V2, UpCox: 26uA/V2, Vthn=0.84V, Vthp: |1V| .

 

[Dominik Przyborowski]

Feedback. Opamp is working as linear circuit only in negative feedback configuration.

 

[nitishn5]

Firstly, A fully differential telescopic amplifier would require a Common Mode Feedback (CMFB) to define the output common mode.
Without that, a small mismatch i the PMOS current sources on Top and the NMOS tail current at the bottom will cause the whole output to saturate in one direction. In your case it seems that the PMOS is slightly stronger than the NMOS causing the outputs to reach 5V.

Secondly, operating an opamp in open loop is prone to difficulties. Everything will change depending on what your input common mode voltage is (Which you have as 1.2V, I think). Connect your opamp in negative feedback closed loop as Dominik mentioned.. Then you can back calculate your open loop gain and such.

Thirdly, your input has a differential swing of 5V. If you have a gain of 500, are you expecting to see an output swing of 2500V?
Reduce your input swing to 10mV or so.