A Beginner's Guide to Filter Topologies

　　represent zeros and the pn terms in the denominator represent poles. Looking at the equation， the first

　　thing that becomes clear is that filters that can be written in this form are biquads. This means SallenKey filters， state-variable variable filters， multiple feedback filters and other types are all biquads. There

　　also is a “biquad” topology to help further confuse things. Thus， the real filter names are biquad SallenKey， biquad state variable， and biquad （which will all be explained a little later）

　　Using low pass filters as our example， a low pass filter can be written in a general equation form as：

　　H（s） = K/（as² + bs + 1）， where a = R1R2C1C2 and b = R1C1 + R2C1

　　This can be simplified by making R1 = R2 and C1 = C2， resulting in：

　　H（s） = K/（R²C²s² + 2RCs + 1）

　　The block diagram of a low-pass 2nd order Sallen-Key filter is shown in Figure 1. This filter is also

　　referred to as a positive feedback filter since the output feeds back into the positive terminal of the op

　　amp. This topology is popular because it requires only a single op amp， thus making it relatively

　　inexpensive.