ic_engr
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Salma,
What is your supply voltage. What is the VT of the NM0 & NM8.? You definitely need to raise the DC levels on the diff pair (NM0 & NM8). Remember, the lower the DC on the diff pair, the lower VDS will be on the current source NM4 in question.
More importantly, looking at the model names (n_12_hs130e), it seems you are using high speed devices, which will definitely suffer with poor output impedance thus excerbating your problem.
Good luck.
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