Because looking into the source of at low enough frequencies you see big resistance and gain is defined by ro1||ro5. When CL reduces the impedance at the output, then looking into n2 you start seeing 1/gm2 and you get the behavior shown on the plots.
Because looking into the source of at low enough frequencies you see big resistance and gain is defined by ro1||ro5. When CL reduces the impedance at the output, then looking into n2 you start seeing 1/gm2 and you get the behavior shown on the plots.
The gain bandwidth relation just relates two frequencies on a + or - 20db/dec response. It doesn't care if you call one frequency a zero or a pole or just pick two frequencies on the slope.