hello im studying the differential amplifier from Sedra Smith and i have problem with the ac mode when the signal is differential (vid/2 and -vid/2) I know that in Nmos transistor the current flows form drain to source so I dont understand why when the imput is -vid/2 the current flows from source to drain. the operation is different in ac? This is that i dont understand


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In AC analysis, DC or common mode (CM) = 0 because you ignore it for analysis purposes since you normalize everything around the bias point. The common mode or DC is still there in reality and the AC is riding on top of the CM voltage. So it's not actually flowing backwards; It's still flowing forwards but in a smaller amount, as long as |-Vd/2| < Vcm.

So in AC analysis you're looking at deviations/changes from the bias value, values that are relative to the bias, not the absolute values.

  • \$\begingroup\$ I understand that the DC operation is still there, and that -Vd/2 is smaller than vd/2 but not negative because is riding on top of the Dc voltage. But why the circuit show the current from source to drain in Q2 transistor, if in nmos transistor the current goes from drain to source? \$\endgroup\$ – Ariel Feb 22 at 13:57
  • \$\begingroup\$ The current source is AC too, not in the sense of 60Hz sine wave, but in the sense it rides on top of the DC bias current as well. \$\endgroup\$ – DKNguyen Feb 22 at 14:46

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