Need comparator work with 20Mhz, power < 1mW

[tatbocmui]

I have a problem to design ADC 4 bit flash, especially, the comparator. I use two stage comparator (I show it below):

My problem is that it does not work with when Vin (Vp, Vn) is too small maybe < 0.3V, the output is always 0. Higher it work good. Need some tips to redesign this comparator, or new comparator.

 

[Calika]

To resolve smaller inputs you need to increase the gain of the structure. You can achieve this by increasing the current (within your power limit), or increasing input pair size.
This may or may not get you to meet your specifications.

If this doesn't get you the desired solution, you can change the topology to a latched comparator. A strong arm latch should be able to solve your problem quite easily since you already have a clock available in your system. It has no static power dissipation and the positive feedback in the latch greatly increases the speed and resolution of the comparator.

You can refer to these slides for the functioning and design considerations for latched comparators.
https://www.ece.tamu.edu/~spalermo/ecen689/lecture13_ee689_rx_circuits.pdf