Sub-threshold (aka weak inversion) region is normal for diff. pair of OPAMP

For good layout, you should always provide doubel guardring to critical circuits like Differential pair , current mirror & Buffers ( High current carrying transistor).

Part 2 of an article discussing methods of making six basic audio measurements covers how to test for frequency response, THD+N, phase, crosstalk, and signal-to-noise ratio (SNR) using a home theater receiver and an Audio Precision analyzer as an example

a good book about ESD

A table giving the overview of four different filter types,and a master’s thesis for detail