Dear friends,
Thank you very much for your contribution,
You made the non-linearity of the feedback elements very clear to me.
Coming to the op-amp part, as Dominik mentioned, this requires high gain, in my opinion, I prefer to say high loop gain.
The other quantity I more interested in is the overdrive voltage of the differential pair of the amplifier. In gm-C filter, the amplifier integrates the signal in an open loop, no negative feedback mechanism, the differential input signal will then appear as it is between the op-amp inputs, so in order to increase the differential range, one try to maximize the overdrive voltage of the differential pair by using longer transistors, high biasing current, using degenerative resistors, and or using different linearization techniques. Indeed I have some work in this direction.
But if we talk about closed-loop op-amp amplfier, and assume for now ideal feedback element (to only blame the opamp), and also assume high gain amplifier, now with the negative feedback principle, vin+=Vin-, hence we don't need to maximize the differential input overdrive voltage (unless for different purposes). The only difference that appears at the inputs is as Mrs. Dana said, at the time of slewing, however, designing op-amp with sufficient slew rate will avoid this distortion.
I found this paper where the authors increased the overdrive voltage of the input stage toward increasing the linearity of the op-amp used to construct Sallen key filter (stated in the second part of the introduction)
and here I cut this part for your further discussion please