... and capacitor is non linear device.

Hallo Kobi,

ideal capacitor is called linear, because it always lead to LINEAR differential equation. From the basic capacitance formula C = Q / U it follows with I = dQ / dt: I = C x dU / dt. This equation is linear in I and dU / dt. It wouldn't be, when equation would consist of terms like I^2 or SQRT(dU / dt), example given.

For RC-lowpass filter it follows:

Uin = Ur + Uc = Ur + Uout = Ic x R + Uout = R x C x dUout / dt + Uout

You can see, that this differential equation is also linear in Uin, Uout and dUout / dt.
It wouldn't be linear when differential equation would consist of terms like Uin^2 or SQRT(Uout), example given.


Bye,
Kai