Designing High Speed Active Filters
Butterworth Response, Bessel Response · 1997
Filters built from resistors (R), inductors (L) and capacitors (C) are known as RLC or passive filters and are the dominant type of filter for high frequency applications. The performance of these filters is typically limited by the inductors which are large, expensive and far from ideal in an electrical sense. By using amplifiers and feedback, the same filter characteristics can be achieved without the use of inductors, these filters are known as “active filters”. An active filter will perform well only to the extent that the amplifiers in it behave in an ideal sense, so traditionally active filters have been limited to applications at frequencies below 1MHZ. With the advent of low cost, very high speed operational amplifiers, it is now feasible to realize active filters with frequency ranges in the tens of MHZ. With some of the newer high speed amplifiers such as the CLC449 ‐ a 1.2GHz bandwidth amplifier, filters with corner frequencies above 100MHz have been built. This application note will provide a recipe to realize active filters using low cost, high speed operational amplifiers. The filter topology that is used is the Sallen-Key Filter which uses the operational amplifier as a fixed gain block. This filter topology can be used with current feedback or voltage feedback amplifiers, and you will be shown how to design low pass, high pass and band pass filters using this filter topology. This application note discusses a printed circuit board which will allow you to prototype active filters of up to 8 pole complexity. This is not a complete treatment of the subject of active filters, there are several good books published on that topic, some of which are listed in the bibliography. This is a simple method of getting a circuit that will work for most applications. The topics covered are: