State Variable Filters With the advancement in IC technology
A number of manufacturers now offer universal filters having simultaneous low-pass, high-pass, and band-pass output responses. Notchand all-pass functions are also available by combining these output responses in the uncommitted op-amp. Because of its versatility, this filter is called the universal filter. It provides the user with easy control of the gain and Q-factor. It is also called a state-variable filter.
The filters we have discussed so far are relatively simple single op-amp circuits or several single op-amp circuits cascaded. The state-variable filter, however, makes use of three or four op-amps and two feedback paths. Though a bit more complicated, the state variable configuration offers several features not available with the other simpler filters. First, all three filter types (low-pass, band-pass, and high-pass) are available simultaneously. By properly summing these outputs some very interesting responses can be made. Bandpass filters with high Q can be built. The damping and/or critical frequency could be electronically tuned.
This article is from Allicdata Electronics Limited.