What is a filter? A filter is a device that passes electric signals at certain frequencies or frequency ranges while preventing the passage of others. — Webster. Filter circuits are used in a wide variety of applications. In the field of telecommunication, band-pass filters are used in the audio frequency range (0 kHz to 20 kHz) for modems and speech processing. High-frequency band-pass filters (several hundred MHz) are used for channel selection in telephone central offices. | Chapter 16 Active Filter Design Techniques Literature Number SLOA088 Excerpted from Op Amps for Everyone Literature Number SLOD006A Texas Instruments Chapter 16 Active Filter Design Techniques Thomas Kugelstadt Introduction What is a filter A filter is a device that passes electric signals at certain frequencies or frequency ranges while preventing the passage of others. Webster. Filter circuits are used in a wide variety of applications. In the field of telecommunication band-pass filters are used in the audio frequency range 0 kHz to 20 kHz for modems and speech processing. High-frequency band-pass filters several hundred MHz are used for channel selection in telephone central offices. Data acquisition systems usually require anti-aliasing low-pass filters as well as low-pass noise filters in their preceding signal conditioning stages. System power supplies often use band-rejection filters to suppress the 60-Hz line frequency and high frequency transients. In addition there are filters that do not filter any frequencies of a complex input signal but just add a linear phase shift to each frequency component thus contributing to a constant time delay. These are called all-pass filters. At high frequencies 1 MHz all of these filters usually consist of passive components such as inductors L resistors R and capacitors C . They are then called LRC filters. In the lower frequency range 1 Hz to 1 MHz however the inductor value becomes very large and the inductor itself gets quite bulky making economical production difficult. In these cases active filters become important. Active filters are circuits that use an operational amplifier op amp as the active device in combination with some resistors and capacitors to provide an LRC-like filter performance at low frequencies Figure 16-1 . L R VIN-----------------------V0UT C Figure 16-1. Second-Order Passive Low-Pass and Second-Order Active Low-Pass 16-1 Fundamentals of Low-Pass Filters This chapter covers active filters.
