let's consider a generic differentiator circuit, i.e. a circuit whose transfer function is \$\text{constant}\cdot j\omega\$. For instance, you may consider this circuit with Op-Amp:
I was reading this paragraph about it ("Pulse And Digital Circuits", Prakash Rao):
Which is the precise reason for its unstability? In general, I know that a linear system is stable if and only if its transfer function has only poles with negative real part. In this case I do not see any poles, and so?
Moreover, I have seen that usually negative feedback (as that present in this circuit) improves the stability: is, despite this fact, this circuit unstable?