环形振荡器全差分的结构，用两级可以吗？

我确实也见过。把电路的寄生极点搞大，应该是有可能的。不过可能有不起振的风险，而且设计不好，相噪也会很差。

It's OK to use 2-stage differential oscillator to generate IQ phase. 2 stage differential oscillator is actually have 4 Gm cells, so it is more than enough to have 360 phase shift.
However, you need to know the working principle for differential oscillator is somehow different to single end oscillator.
the most common 2 stage differential oscillator is for latch based fully CMOS INV structure.

两级当然可以了
自身的反相180*2，两个极点90*2，total max phase shift 540

三楼说的对。

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