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Differential pair question (Read 122 times)
tenso
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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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bluejay
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Re: Differential pair question
Reply #1 - Jun 24th, 2022, 7:40am
 
Hi,

You are that M1 will be saturation throughout, whereas M2 is in cutoff when VG2 is 0, in saturation when VG2 is VDD/2 and in triode when VG2 is VDD.

But the output will not be exactly zero for VG2=VDD. It will be a small value close to the overdrive of M2 because some current is still flowing through this branch.
Also when both input voltages are VDD/2, ideally Vout should be VDD/2 for single ended configuration as in this picture, and Vout is zero for a fully differential output, as for a fully differential structure the output Vout = Vout1-Vout2.

So in the given picture Vout = VDD-Vov4, when VG1=VG2 = VDD/2,    where Vov4 = VGS4 - Vth4 = overdrive voltage of M4.

Attaching a simulated plot of the same. Vin and Vo are shown with currents of both branches. In the plot M0 corresponds to M1 in the picture with VG=VDD/2, and M6 from the plot is the same as M2 of picture, with VG2 being swept from 0 to VDD as indicated by the ramp.
Note the VDD here is 1V.
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