I'm studying the differential amplifier on different books (Razavi and Sedra-Smith).
I've understood how this circuit works, but I have some questions. When authors analyze this circuit (2 identical mosfets or two identical BJTs with a current source below them, as shown in figure) for small signals, they always assume differential inputs, that is: v1 = -v2
Question: why is this assumption required? When this circuit is used as the first stage of an operational amplifier in a negative feedback fashion, who says that the inverting and non-inverting terminals will have v1 = -v2 ? Negative feedback says that the input differential input of the op-amp is very very small (and for a very very small differential input collector currents of the differential amplifier are linear), I don't understand then why v1 should be equal to -v2. Small signal analyses should (in my opinion) consider the case in which v1-v2 is very very small, but not necessarily v1=-v2