Jim cage
Full Member level 2
Hi,
Suppose you have an CMOS differential amplifier which has 2 exactly matched transistors.assume the transistors acts acording to the square law model.
since this is differential amplifier, the second order terms are in phase and thus cancell each other and you dont have 2nd order distortion as expected.
my question is, why there is 3rd order distortion? since a single mos transistor has only square terms (and thus second order nonlinearity) where is the 3rd comming from ? ( Assume also that you are not in triode region of the transistor).
suppose you enter a cos(wt) as the differential input, you can divide it for +0.5cos(wt) for one gate transistor and -0.5cos(wt) for the second gate transistor.
each transistor according to the square law model will exhibit the square of the 0.5cos(wt) according to :
Ids=k(W/L)(Vgs+Vi-Vt)^2 =k(W/L)(Vgs+0.5cos(wt)-Vt)^2
so you dont see here a 3rd order nonlinearity (you wont encounter the cos(3wt) for instance...)
Does anybody knows what is the reason? I am right? wrong?
Thanks,
Jimmy
Suppose you have an CMOS differential amplifier which has 2 exactly matched transistors.assume the transistors acts acording to the square law model.
since this is differential amplifier, the second order terms are in phase and thus cancell each other and you dont have 2nd order distortion as expected.
my question is, why there is 3rd order distortion? since a single mos transistor has only square terms (and thus second order nonlinearity) where is the 3rd comming from ? ( Assume also that you are not in triode region of the transistor).
suppose you enter a cos(wt) as the differential input, you can divide it for +0.5cos(wt) for one gate transistor and -0.5cos(wt) for the second gate transistor.
each transistor according to the square law model will exhibit the square of the 0.5cos(wt) according to :
Ids=k(W/L)(Vgs+Vi-Vt)^2 =k(W/L)(Vgs+0.5cos(wt)-Vt)^2
so you dont see here a 3rd order nonlinearity (you wont encounter the cos(3wt) for instance...)
Does anybody knows what is the reason? I am right? wrong?
Thanks,
Jimmy