# Fully-differential MFB comparing to single-ended one

#### Junus2012

##### Advanced Member level 4
Hello

In the image shown below the circuits of the fully differential and single ended MFB filter,

Assume all passive components of the two circuit are identical, will both have the same fc and Q ? or I should multiply the capacitor C2 with factor two for the differential MFB to make the MFB fullly identical to single ended one

Thank you

#### FvM

##### Super Moderator
Staff member
Simple symmetry considerations answer your question. The fully differential circuit behaves identical. But the dual C2 topology isn't the usual differential circuit, you normally have a single capacitor between both R1/R3 nodes, half the single ended value.

### Junus2012

points: 2

#### Junus2012

##### Advanced Member level 4
Simple symmetry considerations answer your question. The fully differential circuit behaves identical. But the dual C2 topology isn't the usual differential circuit, you normally have a single capacitor between both R1/R3 nodes, half the single ended value.
Thank you FvM for your help,

I understood from your answer that to make the single ended circuit identical to the fully differential one (with equal charasterictics of fc, Q) then the circuits will looks like the image below

I for sure believe in your answer, but the circuit of fully diff MFB made me confused because they put the complete C2 in the differential form not the half one as shown below

Thank you once again for your help

#### sutapanaki

##### Advanced Member level 4
It is all relative. If you call C2 the differential capacitor, then when you work with the single-ended circuit you will have to use 2xC2. I think this is what they show with the dashed caps of 2xC2 to each input in your figure..

Junus2012

### Junus2012

points: 2

#### Junus2012

##### Advanced Member level 4
Thank you Suta and FvM

you both are right and I prooved it by calculations