like below figure shown. I am not sure why there is a factor "2" and "4", as
below red circle indicate:
The 2 comes from the difference at the Vop and Von. The Vop is (RF/RG+1) while
the Von is –(RF/RG+1), the difference of the two is 2(RF/RG+1).The 4 comes from
squaring the term, where the noise gain is 2 = (RF/RG+1).