Filters are frequently thought of as passive networks consisting of resistors, capacitors, and inductors. Therefore the energy out of a passive filter is always less than the energy applied to the input. Attenuation and insertion losses limit the effectiveness of passive filters and make some applications impractical. However, resistors and capacitors can be combined with operational amplifiers to form active filters.
In an active filter the amplifier may add energy to the system, resulting in both filtering and some power gain. Other advantages of active filters include low output impedance, cascaded stages without gain loss, and the capability of generating filtering functions having relatively high Q at low frequencies without inductors. In low frequency applications, the inductors required for passive filters are generally cumbersome as well as difficult and costly to build. On the other hand, only a few easily used components are needed for active filters in these applications.
Depending on the circuit type; low-pass, high-pass, band-pass, or band-reject filters can be designed with a roll off characteristic from 6 to 50 dB or greater per octave. An active filter offers several advantages over a passive (LC) filter.
- No insertion loss. The op amp can provide gain it’ needed.
- Active filter components are more economical than inductors.
- Active filters are easily tuned and adjusted over a wide range without altering the desired response.
- Isolation, Active filters have good isolation due to their high input impedance and low output impedance, assuring minimal interaction between the filter and its load.