If the DC common-mode input is fixed at 0.5V you could use either P or N input differential pair, so long as you have enough headroom for the current source.... this depends on your technology (e.g BJT wouldn;t work). Assuming CMOS, low thresholds help.
You can also use parallel N and P input pairs, this is especially required if your common-mode range input is wide. Various techniques exist to keep the gm constant over the common-mode input range required.
A good reference these parallel N & P inputs is:
Shouli Yan and Edgar Sanchez-Sinencio, “Low Voltage Analog Circuit Design Techniques: A Tutorial,” IEICE Transactions on Analog Integrated Circuits and Systems, vol. E00-A, no. 2, February, 2000.

cheers

aw

I believe Folded Cascode is NOT practical for 1.0V operation due to headroom limitations. I need a DC OpenLoop Gain of atleast 70dB.

ic_engr

Hi,
    Quote:

Use Low-Voltage Cascode Biasing & Go for two stage design to achieve  70 dB Gain.

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Mayank.

70dB gain will reflect to a maximum input of less than 1 mV for 1 Volt supply(even neglecting the overdrive voltages). How can u get 70dB gain for 600mV peak to peak input with 1V supply ??

Of course ! It wasn't mentioned how much closed loop gain is required ! Anyways A rail to rail opamp with both PMOS and NMOS input will be the solution 4 this problem. I guess max of the guys have suggested d same ! other could be Quasi-floating gate techniques by Ramirez !! I wonder if there are any other solution 4 it ! And don't even think of cascoding ! nothing more than a 5 transistor structure !