Hello, I'm studying the MOS/BJT differential pair on Microelectronics by Sedra-Smith. The book refers to voltage signals with the following notations: = DC voltage = AC voltage = generic time-varying voltage When it comes to differential pairs, it uses a quite confusing notation for differential inputs: they are always called , like they could be only AC signals, but I'm not positive on this restriction. I happened to find some exercises on the MOS differential pair based on Sedra-Smith's figures (link) and they seem to confirm that may be both a DC and an AC signal. Is this definitely correct? If it is, I have a second question: what would be the point of applying a DC differential input to a differential pair? I mean, is there any application in the real life in which a DC voltage needs to be amplified? Thanks in advance for your help.