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Aug 18th, 2019, 8:58am
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Differential pair question (Read 110 times)
Junior Member

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Differential pair question
May 05th, 2019, 11:27pm
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.

When VG2 is at VDD, M1 is in cutoff with M2 being triode, Vout goes to zero and all the current flows through M2.

so as VG2 is swept from 0 to VDD, Vout decreases from VDD to zero. M1 goes from saturation to cutoff while M2 goes from cutoff to saturation and then to triode. Is this right?

Also what happens to Vout when both inputs are at VDD/2. I know that in fully differential outputs, the difference between both outputs is zero. Does Vout become zero at this point?
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