Principles of Analog Electronics

93

Two-Terminal Components

4.1 DEFINITIONS

An electrical component (or device) is an element that allows a ow of charges

with a specic voltage–current relationship. It is provided with terminals, two or

more, that connect leads or metallic pads, as access and exit points for the elec-

tric current, across which the voltage is applied. The simplest electrical compo-

nent has only two terminals, so only one current and one voltage is available.

The voltage–current relationship is known as a transfer function (or constitutive

equation). Two terminals of the same component are also known as one-port, so

the two-terminal component is a one-port device (Figure 4.1a).

The symbol of a two-terminal component is given in Figure 4.2 where the sche-

matization of the applied voltage and the current owing is highlighted.

When a component is made of a single device, it is said to be elementary, but a

two-terminal component can also be made by a set of electrical devices. Examples

of elementary components are resistors, capacitors, and inductors, dened later.

In the case of three terminals, we talk about a three-terminal electrical compo-

nent (Figure 4.1b), when the terminals are generally N, the component is referred

as N-terminals (or an N-pole/N-terminal device). The terminals are also referred to

as pins.

An electrical component can be passive or active.

4

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Principles of Analog Electronics

94

It is active if it contains at least one elementary

active component. An active component furnishes

energy to other components. The active components

are typically voltage and current sources or generators.

The passive component contains no source that

could add energy to the signal, but provides an

energy transformation or dissipation.

Observation

There are some misleading denitions of active component. In fact, they

include semiconductor devices, which are unable to furnish energy alone.

These semiconductor devices, such as the transistors (treated in Chapter 8,

or tunnel diodes) are sometimes referred as active because these are able to

amplify a signal, i.e. to increase its strength. But this amplication is possible

only thanks to synergetic effects with external power supplied by other com-

ponents. So, recalling a different denition, passive refers to transform or to

dissipate the energy and active to drive it.

A component is said to be linear when its voltage versus current relationship

can be described by a linear function. Please remember that a function y = f(x) is

linear if and only if f(a

1

x

1

+ a

2

x

2

) = a

1

y

1

+ a

2

y

2

for any two inputs x

1

and x

2

, with

a

1

and a

2

constant values.

When electrical components (whether two-terminal or more generally, N-terminal)

are connected to allow the ow of current, their set denes an electrical circuit.

(a) (b) (c)

FIGURE 4.1 Examples of (a) two-terminal,

(b) three-terminal,and (c) N-terminal components.

v

i

+–

FIGURE 4.2 Rrepresentation

of a generic two-terminal

electric component.

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