gm-c

[RockeyMurad]

why is it that for filters based on two integrators loops one of the integrators must be loosy ?

 

[FvM]

Consider that a useful second order filter respectively second order building block of higher order filters has complex poles. Losses implement the real part of pole frequency.

 

[LvW]

why is it that for filters based on two integrators loops one of the integrators must be loosy ?

Two (ideal) integrators in closed-loop with negative feedback form an oscillator (phase shift within the loop: -90-90-180=-360 deg, equivalent to 0 deg).
For a filter, phase shift must be reduced within the "critical" frequency range.