Various Types of Inductors and Their Common Applications

In high‑frequency analog circuits and signal processing for applications such as mobile phones, RFID, test equipment, GPS, radar, Wi‑Fi, and satellite radio, inductors are among the most critical components. They typically perform several key functions, including circuit tuning, impedance matching, and implementation of high‑pass and low‑pass filters, and can also serve as RF chokes. Electronic engineers designing with RF inductors have a variety of options to choose from. To simplify this decision‑making process, this article examines the different types of inductor components and their common applications.

　　Applications of RF Inductors

　　Most electronic components contain RF inductors. “To track animals, the glass tubes implanted under the skin of our pets each house an inductor,” says Maria del Mar Villarrubia, a research and development engineer at Plaimer. “Every time you start the car, wireless communication occurs between two inductors—one inside the vehicle and the other in the key.” However, just as these components are ubiquitous, RF inductors also serve very specific purposes. In resonant circuits, they are typically paired with capacitors to select particular frequencies—for example, in oscillator circuits or voltage-controlled oscillators.

　　RF inductors can also be used for impedance‑matching applications to achieve impedance balance in data transmission lines, which is essential for ensuring efficient data transfer between ICs. When employed as RF chokes, the inductor is connected in series within the circuit, functioning as an RF filter. Simply put, an RF choke acts as a low‑pass filter: it attenuates higher frequencies while allowing lower frequencies to pass unimpeded.

　　What is the Q value?

　　When evaluating inductor performance, the Q factor is the most critical metric. The Q factor is a dimensionless parameter that quantifies an inductor’s efficiency by comparing its resonant frequency to its energy‑loss rate. Deryl J. Kimbro, Senior Product Manager at Murata, explains: “The higher the Q factor, the closer the inductor’s performance is to that of an ideal lossless inductor—meaning it exhibits superior selectivity in resonant circuits.” Another advantage of a high Q is lower losses, which translates to reduced energy dissipation. By contrast, a low Q results in a broader bandwidth and diminished resonance amplitude at and near the resonant frequency.

　　Inductance value

　　Aside from the Q factor, the true measure of an inductor is, of course, its inductance value. For audio and power‑supply applications, inductance values typically range in the several henries, whereas higher‑frequency applications usually require much smaller inductors, often in the millihenry or microhenry range. The inductance value depends on several factors, including the coil’s construction, core size and material, and the number of turns in the winding. Inductors come in both fixed‑value and adjustable‑value types.

　　Other specifications

　　The inductance value is a critical parameter. DC resistance, current rating, and self-resonant frequency (SRF) are among the more useful specifications listed in RF inductor datasheets. As del Mar Villarrubia notes, “Depending on the application, each of these characteristics may be a key factor to consider and can influence the selection of other parameters. For example, if the component is to be used in a tire‑pressure monitoring system, maintaining stable inductance over a wide temperature range becomes paramount, and this requirement will, in turn, dictate the choice of magnetic core material.”

　　Rated current

　　When selecting an inductor, the operating current should be kept below the rated current specified in the datasheet. Exceeding the rated current may damage the component.

　　DC Resistance (DCR)

　　Kimbro states that DC resistance (DCR) is closely related to the rated current. With coil resistance as the reference, DCR represents the inductor’s power loss. Increasing the wire diameter reduces DCR while increasing the rated current. A larger wire diameter lowers losses and enhances current-handling capability. Doug Lillie, Product Marketing Manager for Vishay’s Inductor Division, notes: “DCR limits the DC current that a component can carry without overheating or saturating—where the inductance drops sharply.”

　　Self-Resonant Frequency (SRF)

　　Each turn of the winding in an inductor can be regarded as one plate of a capacitor. The combined effect of the capacitance between adjacent turns and between the coil and the core can be represented by a single capacitor connected in parallel with the inductor, known as the distributed capacitance (Cd). The resonant frequency of this parallel configuration is called the self-resonant frequency (SRF). Lillie states: “At this frequency, the inductor appears as a pure resistor with an associated impedance. If the frequency exceeds the self-resonant frequency, the capacitive reactance of the parallel network becomes the dominant factor.”

　　Multilayer Chip Inductor

　　Multilayer chip inductors are fabricated using ceramic materials and integrated manufacturing processes. Ceramic structures deliver excellent high-frequency performance, while multilayer fabrication enables a wide range of inductance values. The inductance range of multilayer devices is broader than that of thin-film or air-core inductors, though it falls short of the inductance span and current ratings achievable with wire-wound components. Thanks to their superior electrical characteristics and, in particular, their low cost, multilayer technologies are becoming increasingly popular.

　　Thin-film inductor

　　Thin-film inductors are manufactured using photolithography, a process that enables the creation of highly precise coil patterns on ceramic substrates, thereby meeting stringent inductance‑tolerance requirements. The ceramic substrate makes these inductors ideal for RF applications. However, thin-film inductors can handle only relatively low current levels, and their inductance values are limited in range.

　　Wire-wound inductor

　　Wire-wound inductors are typically used in low-frequency applications. They are constructed by winding copper wire around a ceramic (alumina) core. Due to their structure and materials, wire-wound inductors exhibit excellent electrical characteristics. The horizontal winding configuration results in tight tolerances and minimal parasitic capacitance, while the copper windings ensure low DC resistance, thereby enhancing the quality factor and rated current capability.

　　Tapered inductor

　　Tapered inductors are designed for broadband and high-frequency applications, and their structure helps broaden the coil’s bandwidth. With a compact physical footprint—typically wound from fine wire—they exhibit low parasitic capacitance. In ultra‑wideband bias‑T networks, tapered inductors simultaneously provide paths for DC bias extraction or injection, thereby isolating the power supply from the active device.

　　Selection of Magnetic Cores

　　High‑frequency components typically employ air‑core or inert (i.e., ceramic) magnetic cores. These offer superior thermal performance compared to ferromagnetic cores, but their inductance values are limited. Mid‑frequency components usually use iron cores. Iron cores do not saturate, yet they cannot achieve the high inductance levels attainable with ferrite cores. Low‑frequency components generally utilize ferrite cores. Ferrite cores should be avoided whenever possible, as they tend to saturate at relatively low DC bias currents and are sensitive to temperature variations (ΔL/ΔT). Manufacturers are also developing and adopting newer ferrite materials, such as amorphous and nanocrystalline alloys.