Inductor Basics: Principles, Impedance, and Applications

An inductor is a coil of wire, often wrapped around a core of either air or a magnetic material like iron. Inductors resist changes in current flow, a property known as inductance, denoted by the symbol ‘L’.

An inductor allows direct current (DC) to pass through it unimpeded, but it blocks alternating current (AC) signals. This behavior is the opposite of that of a capacitor. The unit of inductance is the Henry (H), though millihenry (mH) and microhenry (µH) are also commonly used.

Inductors with magnetic cores exhibit significantly higher inductance values compared to air-core inductors. Moreover, the greater the number of turns of wire in the coil, the higher the inductance. Inductors come in various types, constructed with iron cores, air cores, iron dust, and ferrite cores, depending on their construction and intended use.

Inductor Impedance

Inductive reactance, denoted as XL, is calculated as follows:

XL = (2 *pi * f * L)

Where:

• L is the inductance in Henrys (H)
• f is the frequency in Hertz (Hz)

This inductive reactance translates to a positive imaginary impedance, specifically impedance at +90 degrees. Inductive reactance (XL) represents the opposition to AC within an inductor.

Inductor impedance = 0 + i*XL

According to this equation, the inductive impedance increases with either an increase in inductance (L) or an increase in the AC frequency (f).

High Frequency Inductor

High-frequency inductors are specifically designed for radio frequency (RF) applications. They provide higher reactance at elevated operating frequencies, effectively blocking such higher radio frequencies. These inductors are often employed in RF circuits to separate DC and RF circuit paths.

This arrangement allows DC to pass to RF components for biasing purposes. Furthermore, it prevents high frequencies from the RF circuit from reaching and potentially damaging the power supply.

In the circuit diagram above, the “RFC” (Radio Frequency Choke, i.e., inductor) passes VCC (supply voltage) to the MMIC (Monolithic Microwave Integrated Circuit) for biasing while simultaneously blocking the RF signal from propagating towards VCC.

Inductor Manufacturers

Numerous manufacturers produce inductors, including Vishay, Coilcraft, Coil Electronics Co. Ltd, Murata, and Symcom. Renco Electronics, for instance, manufactures shielded and unshielded inductors ranging from 0.1 µH to 180 mH. Renco inductors are available in 14 families with diverse shapes, including molded, vacuum impregnated, encapsulated types in both radial and surface mount configurations.