Thank you very much, SIDDHARTHA HAZRA.
I am reading up about folded cascode amplifier. Hope you will help again.
DC Shift is clear as the Drain of Q1 will be lower than the input DC level and compensating it by Q2C. But really you wont find an author naming them like that in any book. It might be just for the explanation purpose.
I see it now.
I think the DC shift level and compensating DC shift level are not really main functions of Q1, Q2 and Q1C, Q2C, right?
To me, it seems that transistors has below functions.
Q1, Q2: to amplify input signal
Q1C, Q2C: I don't know any except for compensating DC level shift.
I had a doubt with the Ro equation: the second part Ro4C = gm4C * ro4c * ro3 will this be ro3 or 1/gm4 .... I might be wrong but please do a small signal analysis of a CS casode amplifier where the top MOS is diode connected as in this pic .
I think you meant
ro4 or 1/gm4 not ro3 or 1/gm4.
I think you are right.
I just calculated output impedance of cascode structure Q4 and Q4C and got Rout = ro3 + ro4 + gm4C*ro4c*ro4 which is approximate to gm4C*ro4c*ro4.