Inductors

Updated November 4, 2005

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Here is an introduction to various types of inductors used at microwave frequencies. This is a companion page to our pages on microwave capacitors and microwave resistors.

Inductor background and definitions

Microwave inductor model

Inductor mathematics (separate page)

Inductive reactance

Solenoid inductors

Spiral inductors (wire)

Toroids

Wirebond inductance rule of thumb

Via hole inductance

Inductor resonances

Wire-wound inductors

Coming: how to make your own

Spiral inductors

Inductor modeling software (if someone steps up to sponsor this topic!)

Inductor vendors

Inductor background and definitions

What is inductance? Inductance is the opposite of capacitance, it is a property that opposes an instantaneous shift in current. Inductance has no effect at DC (an inductor passes direct current), but as frequency increases an ideal inductor starts to look like an open circuit.

The units of inductance are Henries, named after Microwave Hall of Famer Joseph Henry, who was the first curator of the Smithsonian among other achievements. At microwave frequencies, inductors are usually specified in nano-Henries (10^-9 Henries).

Inductors are the problem step-child of microwave circuits. They are harder to model than capacitors, and cut off earlier in frequency. They also have limited current carrying capability, low quality factor (and are lossy), and can radiate. But you'll need them anyway, so learn more about them here.

Wirewound inductors

two types: air core, and other core

Ferrite beads

Spiral inductors

See a formula for wire spiral inductors here:

Microstrip spiral microstrip inductors are commonplace on MMICs, and are offered as discrete components as well. Some day they will get their own Microwaves101 page (as soon a sponsor steps up!)

Most often spiral inductors are rectangular because this is easier to generate and to analyze with CAD software. True circular spiral inductors have better performance at higher frequencies.

Spiral inductors are notoriously lossy, especially for large values. This is because when the inductor is basically a very skinny line made up of many squares, all of which add resistance. Q-factors for spiral inductors can be quite low.

Computing the DC resistance of a spiral inductor is simple, and is often overlooked by designers until they build an amplifier circuit and the part doesn't bias up correctly on the first iteration. First you need to know the sheet resistance of your metalization, in ohms per square, then it is easy to approximate the number of squares to get the resistance. Computing the RF resistance, you may have to consider the skin depth effect.

One word of caution, spiral inductors can radiate. The telltale sign is when you measure them in both directions using a network analyzer, and S11 and S22 magnitude differ greatly.

Distributed inductance (T-line)

More to come!

Vendors for inductors

Silly Rabbit, recommendations are reserved for paying sponsors... you'll have to go find your own inductor vendors for now!

Attention inductor vendors... consider sponsoring this page, it will soon get more hits per month than your company web site does!