Electronic – Find the transfer function of this 3th order filter


I'm working on a project and I need to find the transfer function fhe following filter (as a function of the value of the capacitors and resistors). How can I find that?

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Best Answer

R1 and C1 just form a normal low pass filter, then the opamp R2, R3, C2, and C3 form a low pass Sallen-Key filter. Multiply the results, and you're done :)

A good reference for Sallen-Key designs is this application note, and the Wikipedia page for an RC filter is pretty good. As noted in the comments, the SK will load the RC filter so you will need to calculate the impedance of the SK as well.