"M2 is completely off, M1 does not inject noise into its tank because it is heavily degenerated by the tail current source"
书里也写了,X为低电压 M2关闭。此时Y为高,此时M2打开,M2和最下面的电流源形成了一个,cascode 电路,cascode电路的噪声由下面的mos管子决定,cascode管子(M2)噪声被电流源regulate了。cascode电路的噪声分析你可以去看Razavi的那本基础cmos analog design的书