Let's consider an op-amp. In every book I've read till now, it is said that if we have:
- negative feedback
- finite output
then we have:
Vout = A * (v2 - v1) --> (v2 - v1) = Vout / A
If A is very very large, then v1 and v2 tend to be equal (although they are not equal)
Now the question: who says that at the very beginning (that is, when the circuit switches on), the difference between v2 and v1 is so small that we are allowed to use the linear region shown in image b (thus justifying the previous calculation)? What about if at the beginning (v2-v1) is not so small?