When
people refer to "amplifiers," they're usually talking about
stereo components or musical equipment. But this is only a
small representation of the spectrum of audio amplifiers.
There are actually amplifiers all around us. You'll find them
in televisions, computers,
portable CD
players and most other devices that use a speaker to
produce sound.
In this edition of HowStuffWorks,
we'll see what amplifiers do and how they do it. Amplifiers
can be very complex devices, with hundreds of tiny pieces, but
the basic concept behind them is pretty simple. You can get
get a clear picture of how an amplifier works by examining the
most basic components.
Why Amplify? Sound is a fascinating
phenomenon. When something vibrates in the atmosphere, it
moves the air particles around it. Those air particles in turn
move the air particles around them, carrying the pulse of the
vibration through the air. Our ears pick
up these fluctuations in air pressure and translate them into
electrical signals the brain can process.
Electronic sound equipment works the same basic way. It
represents sound as a varying electric current. Broadly
speaking, there are three steps in this sort of sound
reproduction:
Sound waves move a microphone
diaphragm back and forth, and the microphone translates this
movement into an electrical signal. The electrical signal
fluctuates to represent the compressions and
rarefactions of the sound wave.
A recorder encodes this electrical signal as a pattern
in some sort of medium -- as magnetic impulses on tape,
for example, or as grooves in a record.
A player (such as a tape deck) re-interprets this
pattern as an electrical signal and uses this electricity to
move a speaker
cone back and forth. This recreates the air-pressure
fluctuations originally recorded by the microphone.
As you can see, all the major components in this
system are essentially translators: They take the signal in
one form and put it into another. In the end, the sound signal
is translated back into its original form, a physical sound
wave.
In order to register all of the minute pressure
fluctuations in a sound wave, the microphone diaphragm has to
be extremely sensitive. This means it is very thin, and moves
only a short distance. Consequently, the microphone produces a
fairly small electrical current.
This is fine for most of the stages in the process -- it's
strong enough for use in the recorder, for example, and it is
easily transmitted through wires. But the final step in the
process -- pushing the speaker cone back and forth -- is more
difficult. To do this, you need to boost the audio signal so
it has a larger current while preserving the same pattern of
charge fluctuation.
This is the job of the amplifier. It simply produces a more
powerful version of the audio signal. In this next section,
we'll look at the basic elements in this system.
Pump it Up In the last section, we saw that
an amplifier's job is to take a weak audio signal and boost it
to generate a signal that is powerful enough to drive a
speaker. This is an accurate description when you consider the
amplifier as a whole, but the process inside the amplifier is
a little more complex.
In actuality, the amplifier generates a completely new
output signal based on the input signal. You can understand
these signals as two separate circuits. The output
circuit is generated by the amplifier's power
supply, which draws energy from a battery or
power outlet. If the amplifier is powered by household alternating
current, where the flow of charge changes directions, the
power supply will convert it into direct
current, where the charge always flows in the same
direction. The power supply also smoothes out the
current to generate an absolutely even, uninterrupted signal.
The output circuit's load (the work it does) is moving
the speaker cone.
The input circuit is the electrical audio signal
recorded on tape or running in from a microphone. Its load is
modifying the output circuit. It applies a varying
resistance to the output circuit to recreate the voltage
fluctuations of the original audio signal.
The basic concept of an amplifier: A smaller
current is used to modify a larger
current.
In most amplifiers, this load is too much work for the
original audio signal. For this reason, the signal is first
boosted by a pre-amplifier, which sends a stronger
output signal to the power amplifier. The pre-amplifier
works the same basic way as the amplifier: The input circuit
applies varying resistance to an output circuit generated by
the power supply. Some amplifier systems use several
pre-amplifiers to gradually build up to a high-voltage output
signal.
So how does the amplifier do this? If you look inside an
amplifier for an answer, you'll only find a complex mass of
wires and circuitry components. The amplifier needs this
elaborate setup to make sure each part of the audio signal is
represented correctly and accurately. Hi-fidelity output
requires very precise control.
Inside an amplifier, you'll see a mass of
electronic components. The central components are the
large transistors. The transistors generate a lot of
heat, which is dissipated by the heat
sink.
All of the pieces in an amplifier are important, but you
certainly don't need to examine each one to understand how an
amplifier works. There are only a few elements that are
crucial to the amplifier's functioning. In the next section,
we'll see how these elements come together in a very basic
amplifier design.
Electronic Elements The component at the
heart of most amplifiers is the transistor. The main
elements in a transistor are semiconductors,
materials with varying ability to conduct electric current.
Typically, a semiconductor is made of a poor conductor, such
as silicon, that has had impurities (atoms of
another material) added to it. The process of adding
impurities is called doping.
In pure silicon, all of the silicon atoms bond perfectly to
their neighbors, leaving no free electrons to conduct electric
current. In doped silicon, additional atoms change the
balance, either adding free electrons or creating holes
where electrons can go. Electrical charge moves when electrons
move from hole to hole, so either one of these additions will
make the material more conductive. (See How
Semiconductors Work for a full explanation.)
N-type semiconductors are characterized by extra
electrons (which have a negative charge). P-type
semiconductors have an abundance of extra holes (which have a
positive charge).
Let's look at an amplifier built around a basic
bipolar-junction transistor. This sort of transistor
consists of three semiconductor layers -- in this case, a
p-type semiconductor sandwiched between two
n-type semiconductors. This structure is best
represented as a bar, as shown in the diagram below (the
actual design of modern transistors is a little different).
A standard bipolar
transistor
The first n-type layer is called the emitter, the
p-type layer is called the base and the second n-type
layer is called the collector. The output
circuit (the circuit that drives the speaker) is connected
to electrodes at the transistor's emitter and collector. The
input circuit connects to the emitter and the base.
The free electrons in the n-type layers naturally want to
fill the holes in the p-type layer. There are many more free
electrons than holes, so the holes fill up very quickly. This
creates depletion zones at the boundaries between
n-type material and p-type material. In a depletion zone, the
semiconductor material is returned to its original
insulating state -- all the holes are filled, so there
are no free electrons or empty spaces for electrons, and
charge can't flow. When the depletion zones are thick, very
little charge can move from the emitter to the collector, even
though there is a strong voltage difference between the two
electrodes.
You can change this situation by boosting the voltage on
the base electrode. The voltage at this electrode is
directly controlled by the input current. When the
input current is flowing, the base electrode has a relative
positive charge, so it draws electrons toward it from the
emitter. This frees up some of the holes, which shrinks the
depletion zones. As the depletion zones are reduced, charge
can move from the emitter to the collector more easily -- the
transistor becomes more conductive. The size of the depletion
zones, and therefore the conductivity of the transistor, is
determined by the voltage at the base electrode. In this way,
the fluctuating input current at the base electrode varies the
current output at the collector electrode. This output drives
the speaker.
A single transistor like this represents one "stage" of an
amplifier. A typical amplifier will have several boosting
stages, with the final stage driving the speaker.
In a small amplifier -- the amplifier in a speaker phone,
for example -- the final stage might produce only half a watt
of power. In a home stereo amplifier, the final stage might
produce hundreds of watts. The amplifiers used in outdoor
concerts can produce thousands of watts.
The goal of a good amplifier is to cause as little
distortion as possible. The final signal driving the speakers
should mimic the original input signal as closely as possible,
even though it has been boosted several times.
This basic approach can be used to amplify all kinds of
things, not just audio signals. Anything that can be carried
by an electrical current -- radio and
video signals, for example -- can be amplified by similar
means. Audio amplifiers seem to catch people's attention more
than anything else, however. Sound enthusiasts are fascinated
with variations in design that affect power rating,
impedance and fidelity, among other
specifications.
For much more information on amplifiers, check out the
links on the next page.
