Designing High Speed Active Filt

　　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.