Welcome back to electronics, this is Dr. Robinson.

In this lesson, we are going to look at second-order filter circuits.

In the previous lesson,

you were introduced to second-order transfer functions.

And our objectives, for today's lesson,

are to introduce second-order filter circuits,

circuits that could be used to implement second-order transfer of functions.

And we're going to look at how you design a second-order filter.

Here I've drawn the circuit schematic for

what is known as a Sallen-Key low-pass filter circuit.

Sallen-Key refers to this topology, this interconnection of resistors,

capacitors, and this op-amp.

Now you can identify this portion of the circuit,

as a non inverting op-amp amplifier that you've seen before, where the gain from

the non inverting terminal to the output, is equal to 1 plus R4 over R3.