Why moving one of the nondominant pole towards orgin does not improve the PM

[pz5921087]

Why moving one of the nondominant pole towards orgin does not improve the phase margin?? Its the question in razavi's book(design of analog CMOS integrated circuit), I cant figure it out, can anybody explain it in detail? Its in the book P358.

Thanks in advance

 

[erikl]

The phase shifts of all poles are always added. So if you shift a nondominant pole towards the origin, this means the essential part of the phase shift of this pole becomes noticeable at a corresponding lower frequency, decreasing the phase margin of the gain curve.