Active Filters Information

Active filters are electronic filers that use active components such as amplifiers. Their output is not attenuated with respect to the input voltage. The amplifier—normally an operational amplifier—provides a feedback mechanism from output to input. This, in turn, provides stability for any signal frequency, and allows for a wider choice of frequency responses and time domain behavior.

Theoretically, any active filter design could be replicated as a passive filter. However, an important advantage in using active filters is that the feedback mechanism provided by the amplifier allows the construction of filters with imaginary poles using only capacitors and resistors. Without feedback, a filter with imaginary poles would need capacitors and inductors. An inductor-free filter is ideal because inductors are prone to pick up unwanted signals due to stray magnetic fields; they are also bulky and expensive. With feedback, smaller capacitor values can be used as well. An active filter often scales down all component values, allowing the production of smaller and less expensive filters.

Active electronic filter schematic

An active low-pass filter schematic, showing the use of resistors, capacitors, and an amplifier.

However, active filters also have disadvantages when compared to passive designs:

  • Capacitor values: Capacitors can be plagued with the same value and spacing issues that affect inductors in passive filters, although to a lesser degree. Standard capacitor values are often widely spaced, making off-the-shelf solutions nearly impossible for applications requiring high accuracy. For designs intended for precise applications, space-constrained environments, or high values, capacitors can be prohibitively expensive.
  • Power: Active filters require a power supply to drive amplifier components.
  • Noise: Active filters inject noise into the system, although this can be remedied through low-noise amplifiers.
  • Frequency range: Active filters have limited bandwidth and are often impractical at high frequencies.

For in-depth information about electronic filter theory and design, please visit the Electronic Filters Specification Guide.

Types

Filters are grouped and specified according to the type of frequencies they suppress or attenuate. The four common filter classifications are listed below.

  • Low-pass filters attenuate or suppress signals above a particular cutoff frequency. For example, a low-pass filter (LPF) with a cutoff frequency of 50 Hz can eliminate noise with a frequency of 70 Hz.
  • High-pass filters suppress or attenuate signals with frequencies lower than a particular frequency, also called the cutoff or critical frequency. For example, a high-pass filter (HPF) with a cutoff frequency of 100 Hz can be used to suppress the unwanted DC voltage in amplifier systems, if desired.
  • Band-pass filters are found in TV and radio circuits. They attenuate or suppress signals with frequencies outside a band of frequencies.
  • Band-reject, or notch filters attenuate or suppress signals with a range of frequencies, for example 50-150 Hz. 

Response Characteristics

The frequency response of any filter can be designed by properly selecting the circuit components. Filter characteristics are defined by the shape of the frequency response curve, and filters can be classified as:

  • Bessel filters have a passband that maximizes the group delay at zero frequency, causing a constant group delay in the passband. Group delay is a measurement of the time it takes for a signal to move between two points in a network. A constant group delay in a filter's passband implies that for signals within the passband have an identical time delay. This fact is important in many applications, especially audio, video, and radar applications.
  • Butterworth filters are designed so that the frequency response is flat in the passband.
  • Chebyshev filters feature a very steep roll-off, but have ripples in the passband.
  • Elliptic filters exhibit equalized ripple in both the passband and the stopband.
  • Gaussian filters produce no overshoot in response to an input step. They optimize rise and fall times.
  • Legendre filters are designed to produce the maximum roll-off rate for a given order and a flat passband frequency response.

References and Resources

Circuits Today—Active and Passive Filters

National Semiconductor—A Basic Introduction to Filters: Active, Passive, and Switched-Capacitor

Texas Instruments—Active Filter Design Techniques

 



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