When I add two stages of first order passive low pass filter to form a second order filter, The loading effect appears because the two stages have mutual relationship.

I would like to know How to drive the transfer function of the second order low pass filter taking in consideration the loading effect.

I think the following equation is for

**Ideal**filters only:

Is there a link to website or a pdf that tells me how to get the transfer function of practical (real) filters, Please?

Thank you very much,