Universal Active Filters – With the advance of integrated circuit technology, integrated circuits with improved capabilities are appearing in ever increasing numbers. Innovative design methods and fabrication procedures have not only helped to produce a large variety of new integrated circuits but also improved the old ones.
Often the use of specialised ICs produces a simpler and more accurate circuit, such as Datel’s universal filter FLT-U2 which has simultaneous low pass, high pass and band pass output responses. Notch and all pass functions are also available by combining these output responses in the uncommitted opamp. Because of its versatility, this filter is called a universal filter. The universal filter is sometimes also called a state variable filter.
The various filter networks discussed in this chapter are used in many circuits, but in critical applications specially designed filter ICs are preferred. Besides being more accurate, specially designed IC filters are simpler, easier to use and more flexible. Detal’ s FLT-U2 is a typical example of such a specialised IC filter.
Datel’ s FLT-U2 is a universal filter which uses the state variable active filter principle to implement second order low pass, high pass and band pass outputs functions. These output functions are simultaneously available at the outputs of the three committed opamps (Pin nos 3, 13 and 5), as shown in the FLT-U2 block diagram of Fig. 15.23(a).
A fourth uncommitted opamp can be used as a gain stage buffer amplifier, or to raise the order of the low pass, high pass or band pass functions. The uncommitted opamp can also be used to realise the notch and all pass functions.
Frequency tuning is accomplished by using two external resistors and Q tuning by using a third resistor. In addition, any of the filter types, such as Butterworth, Chebyschev or Bessel, may be designed by the proper selection of external components by using the FLT-U2.
Figure 15.23(b) shows an FLT-U2 as a second order low pass, high pass and band pass filter.