insulator. Fortunately, materials
scientists have discovered oxides of
iron and other metals that satisfy
these requirements. They are called
ferrites, since they are based on the
oxide, Fe3O4, called ferrite.
In most cases, some of the iron
(Fe) is replaced by other metals;
notably manganese, zinc, and
You can’t wire your house with
small diameter wire since the wire
resistance is too high; you’ll get a
voltage drop across that resistance.
Too small a diameter wire will
likewise diminish the effectiveness of an inductor, which
ideally should have zero resistance at low frequencies.
The quality of an inductor — known as its Q — is a
measure of how well it acts as a perfect inductor at a
given frequency F:
Q = XL/R = 2pFL/R
where XL is called the inductive reactance (essentially, the
“AC resistance”) which is equal to 2pFL. The Q factor
effectively shows how much better an inductor is at
responding to alternating current than direct current. It
also shows how narrow a band-pass filter you can build
from an inductor and capacitor.
Inductors and capacitors are complementary devices.
Capacitors pass high frequencies while blocking low
frequencies. A capacitor’s ability to block direct current is
one of its most important functions.
An inductor quite differently
passes low frequencies and blocks
the high frequencies.
There’s an electrical circuit called
a gyrator that uses a capacitor to
make a simulated inductor. Gyrators
are built around an amplifier. They
can be implemented using active
components, including transistors,
but it’s most common to use an
operational amplifier (op-amp) as
shown in Figure 5. Gyrators can be
used to simulate huge values of
inductance, but they have a few
Op-amps generally work well
only at low frequencies, so gyrator inductors will only
work at low frequencies. As can be seen in Figure 5, the
simulated inductance has one terminal grounded, so this
limits the types of circuits in which gyrator inductors can
Also, the inductor resistance RL must be reasonably
large since op-amp inputs have limitations. RL must
generally be at least a few hundred ohms for most op-amps, so the inductor Q is limited.
The “skin effect” is the tendency for high frequency
currents to preferentially flow near the surface of wire. In
effect, the middle portion of the wire is wasted conductor at
high frequencies; when you’re winding an inductor, you end
up with a higher resistance for the same number of turns.
An easy way around this is to use Litz wire, which is a
multi-stranded single-conductor wire. Litz wire allows a
parallel combination of smaller wires that mitigates the
July/August 2018 47
FIGURE 6. A resonant parallel combination of an inductor and
capacitor driven by a signal generator and monitored by an
FIGURE 5. An op-amp gyrator circuit. RC forms a phase-shifting
circuit, and the simulated inductance is proportional to this phase
shift. The resistor RL must generally be at least a few hundred ohms
for most op-amps, so the inductor Q is limited.