Principles of Analog Electronics

339

Amplifiers

10.1 DEFINITIONS AND CLASSIFICATIONS

In electronics, amplication concerns the production, by means of an appropriate

circuit, of an output signal that has the same form as the input signal but with an

increased level of voltage and/or current and/or power (Figure 10.1a andb).

The input signal is provided by the signal source, and we need to amplify it

when the source available is not powerful enough to pilot a load. If we consider an

audio system, the AC signal source can be a CD or DVD player, the microphone in a

telephone handset, the antenna of a radio receiver, etc. The related voltage signal

can range between a few μV and a few mV, and the current can range between μA

and mA values, so a very low level of power, not enough to pilot many loads (for

example, a system of speakers needs a power that can reach even hundreds or

thousands of watts).

Amplication concerns a huge number of signal sources very different from

each other, including many we really do not imagine as, for example, a fuel-level

sensor in an automobile gas tank or a smoke detector in a re alarm system.

10

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Principles of Analog Electronics

340

Curiosities

In electronics, the possibility to amplify certain electric signals is fundamental,

but the concept of amplication is obviously not exclusive to it. Let us think

about our sense of hearing: a physical event that perturbs the air (the clas-

sic stone in the pond) produces around it a phenomenon of air rarefaction/

compression (sound) that, propagating, reaches our ear. The acoustic wave

causes our eardrum (the tympanic membrane) to vibrate and the vibration

is sent to the three little ossicles (the auditory ossicles: malleus, incus, and

stapes, Figure 10.2) contained within the middle ear space, which behave

practically as ampliers. In fact, the stapes forwards the acoustic waves to the

internal ear where the waves arrive amplied about 180 times. Therefore, we

perceive sounds with greater amplitude than they actually have!

In the internal ear, sensory cells are predisposed

to transform sounds into electric signals that arrive

in the brain, which interprets such signals as “sound

sensations.”

Amplication is fundamental in radio equipment.

Motorola was one of the rst companies to make

radios using transistors. Originally, the transistors

were made with germanium as the semiconductor,

but it could happen that if a car was parked in the

really hot sun, the radio no longer worked. This was

because the Ge had become intrinsic (no longer

extrinsic) with temperature. So Motorola replaced

Ge with Si semiconductors.

Through ampliers we can control a great variation of an output signal by means of

a small variation of an input signal. There are four main types of ampliers, which as

distinguished by the input and output signals, as in the scheme shown in Table10.1.

Malleus

Incus

Stapes

FIGURE 10.2 Malleus, incus,

and stapes in inner ear.

AC

signal sourc

e

DC

power supply

Load

Amplifier

(a)

(b)

FIGURE 10.1 (a) Logical scheme and (b) circuit symbol of an amplier.

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