两级运放的增益怎么分配？

以前有人说根据噪声第一级的增益大会比较好，可以减小第二级到输入口的等效噪声。
我猜测还有原因是之所以采用两级，是为了让第一级主要负责增益，第二级为了大摆幅。
问题是这样说还是很含糊啊，如果我一直增大第一级的增益，这个上限到底在哪里?

Increasing the gain of the first stage will reduce the overall noise figure for sure.  Typically, the input devices are dominant noise contributors.  Large current/area will lead to small thermal/flicker noise.  However, large devices also have huge cap.  Large parasitic cap of input devices leads to smaller dominant pole and smaller phase margin.  You will see much slower settling time if the first stage is too large (or the gain is too large).
Engineering is all about trade-offs.  :-)

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