tenso
Advanced Member level 4
so I was asked this question during an analog design interview and I got it wrong. The picture of the diff. pair is linked above. VG1 is at Vdd/2 while VG2 is swept from 0 to VDD. The question was to plot Vout and to talk about the operating modes of the M1 and M2.
So my answer would be that in the beginning when VG2 is zero, M2 would be in cutoff while M1 would in saturation. At this point Vout is at VDD. As VG2 is swept towards VDD, at VDD/2 both M1 and M2 are in saturation and current across them is equally half the tail current. I am not sure what Vout is at this point. I am guessing around VDD - |VSG3,4|
When VG2 is at VDD, M1 is in cutoff with M2 being triode, Vout goes to a very low value.
so as VG2 is swept from 0 to VDD, Vout decreases from VDD to a low value. M1 goes from saturation to cutoff while M2 goes from cutoff to saturation and then to triode. My question how do I determine how low Vout is pulled to when Vg2 is at VDD?
From simulation it seems to depend on the drain of M5? If the minimum value across it is Vov of 5, is there any relationship between this and VG1, VG2?
So my answer would be that in the beginning when VG2 is zero, M2 would be in cutoff while M1 would in saturation. At this point Vout is at VDD. As VG2 is swept towards VDD, at VDD/2 both M1 and M2 are in saturation and current across them is equally half the tail current. I am not sure what Vout is at this point. I am guessing around VDD - |VSG3,4|
When VG2 is at VDD, M1 is in cutoff with M2 being triode, Vout goes to a very low value.
so as VG2 is swept from 0 to VDD, Vout decreases from VDD to a low value. M1 goes from saturation to cutoff while M2 goes from cutoff to saturation and then to triode. My question how do I determine how low Vout is pulled to when Vg2 is at VDD?
From simulation it seems to depend on the drain of M5? If the minimum value across it is Vov of 5, is there any relationship between this and VG1, VG2?