Dc offset in ac signal

[Manuv16589]

Hi,
I just wanted to know how does the dc offset value in ac affects the output in differential amplifier as given in razavi's design of analog Cmos ic circuit page no. 103

 

[BradtheRad]

I don't have Razavi's book, but...

If the DC offset is in one input only, then it has the possibility of throwing the output way off.

If the DC offset is at both inputs, and at an identical level, then it is a common-mode signal and does not appear at the output. This is a method for detecting and amplifying the AC only. Or it's a method to examine a noisy AC waveform in order to spot differences from a clean AC waveform.