I know that the circuits which employ negative feedback are stable, whereas those which employ positive feedback are not stable.
How can I prove that a voltage buffer (ideal op-amp) which uses positive feedback is not stable?
I know only the dual case ( \$ A=+\infty \$ because the op-amp is ideal):
$$v_u=A \, v_{in}=A (v^+ - v^-) = A \, v_i - A \, v_u$$
$$v_u = \frac{A}{1+A} v_i = v_i$$
If I consider:
$$v_u=A \, v_{in}=A (v^+ - v^-) = A \, v_u - A \, v_i$$
$$v_u = -\frac{A}{1-A} v_i = v_i$$
The last result should be wrong.
Thank you so much for your time.