Filters (Low-pass, High-pass, Band-pass, Band-stop)

Understanding Filters

A filter is a crucial element in electronics that allows certain frequencies to pass while blocking or attenuating other frequencies.
Filters are primarily utilised to eliminate unwanted interference, improve signal quality, extract necessary signals, and to create sound effects.
The four main types of filters are low-pass, high-pass, band-pass, and band-stop filters.
Each filter is defined by its frequency response, or in other words how it reacts to different frequencies.

Low-Pass Filters

A low-pass filter allows signals with a frequency lower than a certain cutoff frequency to pass through while attenuating frequencies higher than the cutoff frequency.
This type of filter is extensively used in audio processing to eliminate high-pitch noises while allowing low-frequency tones to pass.
The degree to which frequencies are attenuated can be described by the roll-off rate or slope which is often quantified in decibels per octave.

High-Pass Filters

A high-pass filter operates oppositely to a low-pass filter. It allows signals with a frequency higher than a certain cutoff frequency to pass while attenuating lower frequencies.
High-pass filters are often used in audio amplifiers to block direct current (DC) components that may harm the transistor amplifier stages.

Band-pass Filters

A band-pass filter allows signals within a specific frequency range to pass while attenuating signals outside this range. This type of filter has two cutoff frequencies, a low cutoff frequency and a high cutoff frequency, defining the band of frequencies that can pass.
Band-pass filters are utilised in various applications including wireless transceivers and audio processing equipment where it is required only to allow a chosen range of frequencies.

Band-stop Filters

A band-stop filter, also known as a reject filter, operates as an inverse to the band-pass filter. It attenuates a range of frequencies, allowing signals both below and above this range to pass.
The notched frequency range is between two frequency points, the stop-band edge frequencies. This type of filter is used to eliminate unwanted frequencies or noise.

Designing Filters

Electrical filters can be created utilising resistors, capacitors, inductors or even active components like op-amps.
RC filters use resistors and capacitors. They are simple to build, but their performance is population density
RL filters use resistors and inductors. Inductors can be bulky and difficult to integrate onto a chip, so RL filters are less commonly used than RC filters.
For more complex filtering tasks, active filters using op-amps can provide better control over the filter characteristics.
Whichever component set is used, the principles of filter design revolve around controlling the reactance of the components to control the circuit’s frequency response.
Designing filters might necessitate familiarity with complex impedance, resonance, Bode plots, and other advanced topics in AC analysis.
Filter order generally reveals the number of reactive components in the filter. A first-order filter has one reactive component, a second-order filter has two, and so on. The order of a filter also corresponds to the roll-off rate or slope, a first-order filter has a roll-off rate of 20 dB/decade, a second-order filter has 40 dB/decade, and so forth.
Active filters differ from passive filters in that they require an external power source. Active filters can involve multiple stages, where a stage references to a single reactive component (generally a capacitor) in the filter design. Therefore, a two-stage filter is also referred to as a second-order filter, a three-stage filter as a third-order filter, and so on.
Various types of responses can be shaped using filters, such as Butterworth (maximally flat magnitude), Chebyshev (equal ripple magnitude), and Bessel (maximally flat delay) responses. Each includes specific features suitable for various applications.
The quality factor (Q) of a filter characterizes the bandwidth and sharpness of the filter frequency response. A high-Q filter has a narrow passband and steep roll-off, while a low-Q filter has a wide passband and gradual roll-off.