whats the effect feed back resistor in sallen key low pass filter.....

 

It creates the second-order function that such a filter requires.

 

Isn't the feedback resistor in the high-pass configuration?

 

You can have a feedback resistor in the non-unity gain configuration for the low pass case. In that case the feedback resistor forms part of a two resistor non-inverting gain configuration.

 

My response was incorrect. t_n_k is correct. In the normal Sallen Key filter low-pass filter, the feedback resistor (if used) is just to determine the gain of the op amp.