Filtering is a fundamental part of many circuits and has wide-ranging applications, including audio processing, radio reception, and power circuit conditioning. A basic understanding of capacitors as a filtering component begins with understanding the types of filters and what they do.
In general, filters are used to remove unwanted parts of a signal. They may be used, for instance, to block undesirable frequencies emitted near a radio receiver (i.e. to reduce radiated radio frequency interference). Because signal processing filters respond to frequency, there are different types of filters, determined by the frequency they affect.
The Different Types of Signal Processing Filters
Low-Pass Filters
As their name suggests, low-pass filters block high frequencies while allowing low frequencies through the circuit.
Low-Pass Filter: For more information on calculating component specification values, see here.
High-Pass Filters
High-pass filters perform the opposite function, permitting high frequencies and blocking low ones.
High-Pass Filter: For more information on calculating component specification values, see here
Band-Pass Filters
Band-pass filters respond to a particular bounded frequency range and allow only frequencies in that range to pass through the circuit
Band-Pass Filter: For more information on calculating component specification values, see here.
Band-Stop Filters
Band-stop filters do the opposite of band-pass filters by blocking a bounded range of frequencies and allowing all other frequencies through
Band-Stop Filter: For more information on calculating component specification values, see here.
Beyond their functional differences, filters can be divided into active and passive types: active filters require an external power source while passive filters do not.
Filter Capacitor Applications
Capacitors in Circuit Filtering
Capacitors are the circuit component that blocks low frequencies. However, they are not limited to use in high-pass filters only. Depending on the configuration of the circuit, capacitors can also be used in the formation of low-pass filters (e.g. a capacitor with a resistor can form either a high-pass or a low-pass filter, depending on the arrangement of the parts).
Capacitors can also be used as part of a band-pass or band-stop system to determine the lower boundary of the signal that gets passed by the circuit (the lower threshold of the band-pass system, for example). Variable capacitors (those with a capacitance that can be altered either mechanically or electronically) are often used as the tuning component of a radio receiver.
Capacitors Used in Noise-filtering Applications
Capacitors consist of two main parts: an insulating layer called the dielectric, sandwiched between two electrical conductors called “plates.” Capacitors used in noise-filtering applications can be broken down into three main types, according to the material used for the dielectric:
- Ceramic: characterized by long life and high voltage, but low capacitance. These are an often-used all-around choice.
- Plastic film: a thin sheet of plastic for the dielectric may be composed of any number of plastics (mylar, polypropylene, etc.). Film capacitors are relatively large and expensive, but offer high voltage and exceptionally long life, with metallized film capacitors offering the best durability. Because of the short current path inherent to film capacitors, ohmic losses (ESR) are very low. These are the best choice for high-end audio signal path applications.
- Electrolytic: a chemical insulator is applied directly to one of the plates, allowing the plates to be much closer together, thereby increasing capacitance. As a result, these capacitors offer high capacitance at low cost, and are ideal for power filtering. However, they have a shorter life than the other types and are prone to leaking current across the plates.
Applications of Capacitor Circuit Filters
Capacitors can be used to great effect, both to impose limits on the systems in which they are employed and also to sort information in the signal. For instance:
- Home Audio: A capacitor might be employed in a home stereo receiver to block lower frequencies on a signal sent to the tweeter cone in the speaker set. This ensures that the tweeter only has to reproduce the higher-frequency sounds for which it is designed.
- Live Audio/Music: Band-stop filters are frequently used to eliminate or reduce feedback from amplification equipment by impeding a narrow frequency band (sometimes referred to as a “notch”) and thereby preventing the development of the feedback loop and resulting sound. High-pass filters are also frequently employed to clean up audio signals from non-bass instruments: unless a channel is intended to carry very low frequencies (such as the bass guitar or piano), it is unlikely that the very low frequencies contain useful sound, so a high-pass filter between the signal source (like a microphone) and the board can enable a higher sound quality.
- Recording: Low-pass filters are used to eliminate or reduce the sibilance (or “hiss”) that is often present in microphone recordings. Anti-aliasing filters are another type of low-pass filter, used in analog-to-digital conversion to condition the analog signal and ensure that it meets the requirements of the sampling theorem.
- Electronics/Power Circuits: Capacitors can be employed in band-stop circuits to protect delicate electronics. Some devices are sensitive to harmonics in the power supply, and require “conditioning” of the power supply to remove these specific harmonic frequencies. In these cases, a band-stop circuit is used to allow the fundamental power waveform to pass but not the higher-frequency range containing the harmonics. Despite being overall a “low-pass” system (filtering out the higher-frequency harmonics), this band-stop filter still employs capacitors to define the upper limit of the blocked frequency band.
Capacitors can be found in every sort of electronics we use, from guitar amps to your car’s starter to the surge protector your computer is plugged into right now. Understanding the various types and how they each work is critical knowledge for any electrical engineer.
