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What Is An Amplifier?
Several acres of paper could be (a nd
pro bab ly have been) covered with words in
unsuccessful attempts to provide detail ed
answers to th e qu estion " What is an amplifier?"
It gets compli cat ed because an amplifier
need not contain eit her tubes or
tra nsistor s (although most of th ose you're
likely to mee t in amate ur radio and particularly
in the FCC exa ms do) , and for
that matter need not even be an electronic
device !
F or insta nce, on e df the simp lest imaginable
amp lifiers of mechanical force is a
lever, or pry-bar. Our am plifiers are, in
some ways , merely the electronic equivalent
of levers.
In electronics, th ough, the range is a bit
more limite d, and we can define an amplifier
in gene ral as a circuit which increases
the power level of an electrica l signal. Most
such amp lifiers have at least two " ports" or
sets of term inals, one for " input" and th e
ot her for "output" - and the nce com es
the possibly puz zling ti tle of th is installment,
because som e technicians with a high
disregard for engineeringese like to refe r to
these ports as the " goes int o" and «goes
out of," slurring th e words to " gazinta"
and "gazouta."
As we've defined it here, then , an
amplifie r is a device with an inp ut an d an
output port (Fig. I) , and any signa l fed into
the input will appear at higher power level
at th e output. This definition isn 't really
tight enough to get through engineering
courses with, but it 's good eno ugh fo r
everyday use.
I N~ AM PL ~-
Fig. 1. A ny amplifier can be considered to be a
black box with two ports, one for inp ut and th e
other f or outp ut . Amplitude of signal appli ed to
input is changed by amplifier as signal passes
through to o utp ut.
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It leads, naturally eno ugh, to another
question , though. If an amplifier boosts the
power of a signal , th en what, pray tell , is a
"signal " ? As we use the term th ro ughout
this study course, a signal is a sequence of
electrical power, levels which, by th eir
variation , carry so me sort of information.
This information may be the mere fact that
the signal is present (as in a powe r signa l},
or it may be as complex as a composite
video/a ud io TV broadcast signal.
The reason we define signals as "seque
nces of pow er levels" rather than in
te rms of "voltage " or " curre nt" variations
is tha t voltage and current have little
significan ce of the ir own when we speak of
signal s - on ly th e combination of volta ge
and cur rent (o r power) is meaningful. For
insta nce, a simple trans former can doub le
the voltage present in a circ uit, but it
doesn't change the power level much, and
the cha nge it does introduce is loss rather
than gain. Therefore a transfor mer is lI o t
an am plifier.
The only types of amplifiers we 'll be
FEBRU A R Y 1971
63