与虚轴的交点：\(G(j2) = -\dfrac{k}{2}\)

\(N(A) = \dfrac{4}{\pi A} \Rightarrow -\dfrac{1}{N(A)} = -\dfrac{\pi A}{4}\)

有交点且从不稳定区到稳定区，有自振

(2)

以e(t)作为横坐标,e(t)作为纵坐标