very high, so these factors can be ignored at low frequencies. At radio frequencies,
however, these residuals become noticeable, and the capacitor functions as a complex
RLC circuit. Most of these effects can be greatly minimized by keeping the capacitor
leads very short. This problem is mostly eliminated by using the newer chip capacitors,
which have no leads as such.
Capacitance is generally added to a circuit by a capacitor of a specii c value, but
capacitance can occur between any two conductors separated by an insulator. For example,
there is capacitance between the parallel wires in a cable, between a wire and a
metal chassis, and between parallel adjacent copper patterns on a printed-circuit board.
These are known as stray, or distributed, capacitances. Stray capacitances are typically
small, but they cannot be ignored, especially at the high frequencies used in communication.
Stray and distributed capacitances can signii cantly affect the performance of a
circuit.
Inductors. An inductor, also called a coil or choke, is simply a winding of multiple
turns of wire. When current is passed through a coil, a magnetic i eld is produced around
the coil. If the applied voltage and current are varying, the magnetic i eld alternately
expands and collapses. This causes a voltage to be self-induced into the coil winding,
which has the effect of opposing current changes in the coil. This effect is known as
inductance.
The basic unit of inductance is the henry (H). Inductance is directly affected by the
physical characteristics of the coil, including the number of turns of wire in the inductor,
the spacing of the turns, the length of the coil, the diameter of the coil, and the
type of magnetic core material. Practical inductance values are in the millihenry
(mH 5 1023 H), microhenry (μH 5 1026 H), and nanohenry (nH 5 1029 H) regions.
Fig. 2-9 shows several different types of inductor coils.
● Fig. 2-9(a) is an inductor made of a heavy, self-supporting wire coil.
● In Fig. 2-9(b) the inductor is formed of a copper spiral that is etched right onto the
board itself.
● In Fig. 2-9(c) the coil is wound on an insulating form containing a powdered iron
or ferrite core in the center, to increase its inductance.
● Fig. 2-9(d) shows another common type of inductor, one using turns of wire on a
toroidal or doughnut-shaped form.
● Fig. 2-9(e) shows an inductor made by placing a small ferrite bead over a wire; the
bead effectively increases the wire’s small inductance.
● Fig. 2-9( f ) shows a chip inductor. It is typically no more than 1⁄8 to 1⁄4 in long.
A coil is contained within the body, and the unit is soldered to the circuit board
with the end connections. These devices look exactly like chip resistors and
capacitors.
In a dc circuit, an inductor will have little or no effect. Only the ohmic resistance
of the wire affects current l ow. However, when the current changes, such as during the
time the power is turned off or on, the coil will oppose these changes in current.
When an inductor is used in an ac circuit, this opposition becomes continuous and
constant and is known as inductive reactance. Inductive reactance XL is expressed in
ohms and is calculated by using the expression
XL 5 2fL
For example, the inductive reactance of a 40-μH coil at 18 MHz is
XL 5 6.28(18 3 106)(40 3 1026) 5 4522 V
In addition to the resistance of the wire in an inductor, there is stray capacitance
between the turns of the coil. See Fig. 2-10(a). The overall effect is as if a small capacitor
were connected in parallel with the coil, as shown in Fig. 2-10(b). This is the equivalent
circuit of an inductor at high frequencies. At low frequencies, capacitance may be