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varying voltage dro p is coupled out
through a capacitor (to isolate the de
component of the signal) to the outpu t
port.
With tra nsformer coupling ( Fig. 3) the
active device's op erating current is supp lied
through the primary of a trans former. As
this current varies, the changes induce
corr esponding cur rent in th e transformer
secondary , which is con nected t o the output
port.
Fig. 4. Impedan ce-coupled cir cuit shown here is
cross between transformer and resistanc e cou p­
ling. Ac o utp ut signal se es high impedance at
cho k e, and capacitors couple ou t the resulting
volta ge drop. Tube's dc power, however, is no t
restricted.
c51"C .'
e+
Fig. 3. Transformer-coup/ed circuit shown here is
som et im es used in audio am plifiers, and often
em ployed at rf where tuned tank circuits form
the tran sformer. Trans form er can produce highest
efficien cy of all coupling circ ui ts, but is more
costl y than a resist or and two capacitors.
Resistance cou pling is normally inh erentl
y high-impedance , although certain
special circuits can bring the imp eda nce
level down to low levels. Transfor mer
coupling, on t he other hand, seld om pro ­
du ces ext re mely high impedance levels. It
is used pri marily to produce med ium- to
low-i mp eda nce ports, as for inst anc e in the
output circu it of a vacuum-tube hi-fi unit
whe re the t ube s' current mus t dri ve a 16[2
load. Rf amplifiers often use transfor mer
cou pling because an rf transformer can be
compos ed of tuned circuits for selectivity;
sometimes they use impedance coupling
(Fig. 4) wh ich is similar to resistan ce
cou pling excep t that an rf choke (inductor )
replaces t he resistor.
As you can see, amplifiers operate in
many different ways, even tho ugh th eir
basic principles are all similar. Because of
66
th is, it's been found nec essary to classify
am plifiers into var ious types for study an d
discussion. Many classification schemes are
in use, and some of them are so wid ely
acce pte d that they are part of the FCC
examina tio ns.
How Are Amplifiers Classified?
Just as we have many kinds of amplifiers,
we have many kinds of amplifier
classifications, bec ause amplifiers are classified
into groups. leach of whi ch ha s some
character or propert y in common .
We finally rest ricted our defi nition of
am plifi er, for th is chapte r at least, to " a
vacuum-tube or tra nsistor devic e whi ch
boosts th e po wer of its input signal " ­
and that in itse lf was a classificatio n.
Within this classific at ion, we could describ
e amplifiers according to their output
coupling circuit. This would produce classification
s suc h as " resistance-co upled,"
" transformer-coupled ," and "impedance ­
cou pled," whi ch we were using only a few
paragraphs back .
Anothe r way wou ld be on the basis of
t he frequency range handled by the amplifier
: audio, vide o, rf , i-f, de , etc.
We co uld divide them into voltage amplifiers,
in whic h the po wer boost shows up
as increased volt age with no decrease in
current, and power amp lifiers, in wh ich the
power boost is use d directly.
All of these classificat ions are in wid e
use, bu t none of them attack the problems
73 M A G A ZI NE