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M17 , M18 are the source-coupled input transistors,and M12 is a bias (tail current source);
M47 and M48 are the cascode transistors, and M49 & M50 are biases;M13 through M16 constitue a cascode current load.
To simulate the circuit, you must be sure that all transistors be correctly biased and that the offset voltage Vos is in the linear area of the transfer curve. if not , you shold solve the problem and give the circuit a appropriate dc operational point.
because the op may have very hign gain, so you should be careful when doing ac analysis.