# differential amplifier can suppress even order harmonics?

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#### katrin

##### Full Member level 1
I often read that the fully differential amplifier can suppress even order harmonics,
but I don't really understand why.
Could someone explain it?

thanks

#### MSSN

##### Full Member level 3
Re: differential amplifier can suppress even order harmonics

For any amplifier:
Vo=a0+a1Vi+a2*(Vi^2)+a3*(Vi^3)+a4*(Vi^4)+a5*(Vi^5)+............
where a2,a4,a6,....... are the even harmonics coefficients
while a3,a5,a6,........ are the odd harmonics coefficients
Since the sign of the input will not appear in the even harmonics terms as it is always raised to the power of an even number, thus at the two outputs of the differential amplifier the even harmonics are the same(assuming perfect matching) and thus when the two output are subtracted(differential output) the even harmonics on both sides will cancel each other.

dinos7 and sabbasi

Points: 2

Points: 2

### mvj

Points: 2

#### bobchentan

##### Junior Member level 3
it is cancalling at the output because differential output is Vp-Vn

But if the circuit has any mismatch between the two sides of the differential pair, the even harmonics will show up at the output.

#### iamxo

##### Full Member level 4
Re: differential amplifier can suppress even order harmonics

so, exactly speaking, because of device mismatch, fully differential amplifier cann't completely suppress even order harmonics, just reduce them to a low level.

### katrin

Points: 2

#### nassim_el85

##### Member level 4
"For any amplifier:
Vo=a0+a1Vi+a2*(Vi^2)+a3*(Vi^3)+a4*(Vi^4)+a5*(Vi^5)+............
where a2,a4,a6,....... are the even harmonics coefficients
while a3,a5,a6,........ are the odd harmonics coefficients "

I really don't know are they true or not.

"Since the sign of the input will not appear in the even harmonics terms as it is always raised to the power of an even number, thus at the two outputs of the differential amplifier the even harmonics are the same(assuming perfect matching) and thus when the two output are subtracted(differential output) the even harmonics on both sides will cancel each other. "

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