Active Filter Design Techniques
Chapter 16
Active Filter Design Techniques
Literature Number SLOA088
Excerpted from
Op Amps for Everyone
Literature Number: SLOD006A
16-1
Active Filter Design Techniques
Thomas Kugelstadt
16.1 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
C
VIN VOUT VIN
VOUT
R1
C1
C2
R2
Figure 16–1. Second-Order Passive Low-Pass and Second-Order Active Low-Pass
Chapter 16
Fundamentals
Active Filter Design Techniques
Literature Number SLOA088
Excerpted from
Op Amps for Everyone
Literature Number: SLOD006A
16-1
Active Filter Design Techniques
Thomas Kugelstadt
16.1 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
C
VIN VOUT VIN
VOUT
R1
C1
C2
R2
Figure 16–1. Second-Order Passive Low-Pass and Second-Order Active Low-Pass
Chapter 16
Fundamentals
References: D.Johnson and J.Hilburn, Rapid Practical Designs of Active Filters, John Wiley & Sons, 1975. U.Tietze and Ch.Schenk, Halbleiterschaltungstechnik, Springer–Verlag, 1980. H.Berlin, Design of Active Filters with Experiments, Howard W.Sams & Co., 1979. M.Van Falkenburg, Analog Filter Design, Oxford University Press, 1982. S.Franko, Design with Operational Amplifiers and Analog Integrated Circuits, McGraw–Hill, 1988 16-64