1. what is the difference between RC circuit and RC with an op-amp ? i said the gain , he said suppose its 1 , he answered that in the RC-op-amp the capacitor will charge lineary , is this correct ??
Really need more information here, as this depends on what you're discussing. For a first order filter, there is very little difference. If it's a linear integrator, then the active integrator will see the capacitor charging linearly, as opposed to the passive integrator.
2. what's the difference between butterworth and cheybychev filters ?
Butterworth filters are an extension to higher orders of the first order RC or RL LPF. The poles of the Butterworth characteristic are equally spaced around a circle. The Chebychev characteristic derives from the Chebychev Polynomials: C(x)= cos(Karccos(x)). The poles are arranged around an ellipse.
3. what happens when there are ripples in a low pass filter circuit in the pass band area ??
This is usually accompanied by sharp skirt selectivity and a nasty, nonlinear phase characteristic. (I.e. Chebychev or Elliptic filters. You wouldn't want to use it where fidelity is the goal.