Simple differential amplifier with active load for ADC comparator
Usually there is a bit of positive feedback, to prevent oscillation that will trash the power rails, and cause many comparators (those not well overdriven into the rails) to
also oscillate.

If the Rnoise is 10,000 ohms (a 12 nanovolt/rtHz noise density) with 10GHz noise bandwidth, the noise of that comparator will be 

12nV * sqrt(10GHz)

or

1.2e-8 * 10^+5  == 1.2e-3 = 1.2 milliVolts rms

=================================

how to oscillate? there is feedback using the common VDD rail.

phaseshift and voltage gain ---- oscillate