【求助】由运放组成的有源滤波器（积分器）稳定性问题

hezudao

回复 1# kedaya


    1.输出量会影响输入量，当是是反馈系统，要仿稳定性就从输出到输入这一通路断开。
2.对稳定性当然有影响，具体分析可以用niquist 稳定判据，（相频曲线正穿越和负穿越-180的次数,具体的去看书吧）
3.仿真真设置应该没问题
4.对大环路套小环路的电路，先仿小环路，看有没有问题，再把小环路看作一个模块，仿大环路的稳定性。

you should simulate stability starting from opamp, and then proceed to the top loop. Bode plot(PM) is only valid if T(s) itself is stable. For the top simulation, you observed multiple 180 degree crossing. That's another case where bode plot is not applicable. You can use nyquist plot. It's enclosure of (-1,0) not crossing times that determines stability. And it's quite easy to plot in cadence, btw. Root locus is applied to the closed-loop transfer function not open loop-gain transfer function. you can vary gain of comparator to get root locus. PSS+PSTB could be easiest way to measure stability, which can also discover slewing and nonlinearity caused for larger signal operation(since your circuit is conditional stable system, it's very important to verify)

小麓先森

1.先分析内环，把外环断掉，同时将内环带宽尽量做大，不会影响到外环；
2.在分析外环，此时内环为闭环结构，由于带宽大，故可等效成一个固定增益的buf；
3.两个环叠加分析，在公共处断环，其带宽处的结果基本与外环一致；
4.将瞬态跳变验证最终瞬态下的稳定性