PEARL Archive of Vacuum Tube Technology - Volume 1: Power Amplifiers

94 Pages·2012·16.1 MB·English

Web: http://www.pearl-hifi.com 86008, 2106 33 Ave. SW, Calgary, AB; CAN T2T 1Z6
E-mail: [email protected] Ph: +.1.403.244.4434 Fx: +.1.403.245.4456
Inc.
❦
Perkins Electro-Acoustic Research Lab, Inc. Engineering and Intuition Serving the Soul of Music
Please note that the links in the PEARL logotype above are “live”
and can be used to direct your web browser to our site or to
open an e-mail message window addressed to ourselves.
To view our item listings on eBay, click here.
To see the feedback we have left for our customers, click here.
This document has been prepared as a public service . Any and all trademarks
and logotypes used herein are the property of their owners.
It is our intent to provide this document in accordance with the stipulations with
respect to “fair use” as delineated in Copyrights - Chapter 1: Subject Matter and
Scope of Copyright; Sec. 107. Limitations on exclusive rights: Fair Use.
Public access to copy of this document is provided on the website of Cornell Law School
at http://www4.law.cornell.edu/uscode/17/107.html and is here reproduced below:
Sec. 107. - Limitations on exclusive rights: Fair Use
Notwithstanding the provisions of sections 106 and 106A, the fair use of a copyrighted work, includ-
ing such use by reproduction in copies or phono records or by any other means specified by that section,
for purposes such as criticism, comment, news reporting, teaching (including multiple copies for class-
room use), scholarship, or research, is not an infringement of copyright. In determining whether the use
made of a work in any particular case is a fair use the factors to be considered shall include:
1 - the purpose and character of the use, including whether such use is of a
commercial nature or is for nonprofit educational purposes;
2 - the nature of the copyrighted work;
3 - the amount and substantiality of the portion used in relation to the copy-
righted work as a whole; and
4 - the effect of the use upon the potential market for or value of the copy-
righted work.
The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made
upon consideration of all the above factors
♦ PDFCover Page ♦
♦ Verso Filler Page ♦
Puzzled About Amplifiers?
NORMAN H. CROWHURST
Is a triode amplifier better than a pentode amplifier, or vice versa? The author
shows you that either type of tube can be incorporated into an excellent amplifier
if the design is correct-and then he fells you about some of the design problems.
IF YOU FOLLOW the r~ports of certain f>t.yond just the silllple specifications Let's take 8n example. A 25-watt am
CODsumer testing organizations, you usually qnoted, turns up a numhfor of plifier is capable of handling a peak
may be puzzled why it is a certain reasons for this difference. power of 50 watts. The average power
rnanufadurer's product can be rated as In the first place, the transfer charac in a program signal may not be more
"best buy'" <Jne year, while the following teristic of pentodes eontains much than 2 to 5 watts. Bnt such average
year the same product is rejected as "un 11 igher order harmonics than the triode program material may well include a
acceptable, not worthy of further test." type charae1eristie whieh, single-ended, pt"ak here or there that runs up to what
The answer to that question does not ap produces predoruinaDUy second har should be 60 watts, 10 watts beyond the
pt>ar to be a technical one so we won't mornc and, in push-pull, produces a rela maximum handling capacity of the am
attempt it here-. However, there are dif tively small· amount of third and not plifier. This. is what. causes the. trouble.
ferences in amplifier performance which much abo,e this . .! ~ntode-type output, Earh sueh peak momentarily o'\'"erloads
make a '\'"ery similar question-t.hat is in rontrast, produN"S third and fifth har and blo('ks the amplifier so that, for a
techniral--quite pertinent. What r£>ally monics and sometimt"S ewn seventh in fraction of a second thereafter, it will
is the ~t type of amplifier circuit '! quite sizable proportion. True the feed not even handle one watt and then it
Thi:; question repeatedly occurs ill back redures these dramatically, but the comes baek into action distorting the 2-
,arions guises, :';0 it is not untimely to amount of feedbaek neC('ssary to do a tc. 5-watt le,-el that follows the momen-
review it. One still hears from people real cleaning np job on the harmonie tary peak. .
who have triode amplifiers and who tell and intermodulatioD distortion can also Obviously such an amplifier will not
ns that their neighbors and fri£>nds produC'e other troublf>S. These oeeur appear to gi'\'"e as llIuch good, clean out
testify the "good old amplifier" still under a \ariety of ('ircunIstnnces. put as one that handle-s say 15 watts and
gin's performance comparable with the then clips for a moment. Even though
bt>st modern one. Blocking the corresponding peak may still run up
Others who hear reports like this want One of them is what hnppens at o,er to what would be 60 watts, this just gets
to know just what the score is. If the load. )Ieasurt.>ments DlPrely tell how pure lost and the following 2- to 5-watt level
"good old triode" was the best kind of &1n amplifier is up to a ct'rtain point. is amplified without further di5tortion.
!'utput stage for an amplifier, why is it They do not say what hapP(llIs when a rsing the latter amplifier, the level
that manufaeturers uni,ersally adoptfld ~harp peak momeDtarily drin's th(l am eould probably be turned up so it runs
pentodes or tetrodes in '\'"arious types of plifier beyond this poi lit, ns can often at from 6 to 15 watts instead of 2 to 5
output tircuits f happen with pr~lll material. The watts, an incN'a~e in level of about 5
\,ben we examine the difference be a\"erage -"good old triode" amplifier db, which is quite noticeably louder,
tween the two types of tube we find that merely lopped off the high peak and and yet will still sound clean as com
the pentode is more efficient alld conse cllrried on working. )10re re(,f'nt work pared with the 25-watt amplifier work
quently, for a specific amount of dissi with transistor eirl'uits, which achieve ing at an average output of 2 to 5
pation, or dollars' worth of tubt's, it is the same rf>Sults by somewhat different watts. This eomparison is illustrated at
possible to deliver a larger output methods, has shown that such peak ('lip Fig. 1.
power. Also, having a higher gain than ping can beeome quite dra~tic before it A re we to ('ollclude then that, in spite
the older t.riode tubes, it is easier to ap i:; appreeiably andible. of the bf'tter fignres a pentode will give,
ply a greater amount of feerlhaek and But many ampliliel'" employing a it does not rf'ally produce better results
thereby redul'e distortion to a lower lurge amount of feedbllt·k. particularly than the triode , Not at all. A pentode
figure. Conseq nently a Ulodern amplifier, those using peDtode output tubes, do properly used can produce quite good
using the same aollars' worth of com more than dip off tht' pt'Hk. \\:"hen such results and still notain the advantages.
punents, can quote a higher power out a high peak comes through, it throws a pparent in the figures, of better effi
put with lower distortion than the "gooe} the amplifier ont of balance in such a t'iency and impro\'ea gain which 8.1so
old triode" amplifier. way as to block the signal that immedi enable the distortion to be satisfactory
One would imagine that such statistics, ately follows it. This produces a notice reduced.
which are measurable objectively, should ahle interruption or hrt>nking up in the The twin-eoupl('d amplifierl described
I)f' more reliable than the opinion of a program. As tht" amplifier comes back in these pagt>S in November, 1957, is an
number of people who listen to modem into action, after the blocking, it dis example of t.his. A great many readers
amplifiers against the older triode torts be<.'ause the tube that wns blocked ha..-e written in saying that they ba,e
counterpart and who still aver that the does not suddenly eome bark to its cor compared this amplifier with others,
triode lOlU1ds as though it does a better rfoct operating condition. Thus the effect using much larger nominal output:;, and
job. flo ~e\'er, a serious in'\'"estigation of of the sudden peak is t.o block the ampli
the te('hn~cal performance of amplifiers, sfioerrt aonf dl 'tnallnlogwlt> di te ftfot> Cct.O Tlllhe isb iasc kg enweirtha llay 1 Lows Bourget. "Stereo· monaural com·
panion amplifier for the Preamp with Pres·
• ~16-18 .JOth At't'., Bayside 61, N. Y. giv(ln the name "break up." cnre," ACDIo, Nov., 1957.
that the twin~up]ed gives 8uperior school of thought rerommends that the appearance of signals having excessive
performance, both as to apparent undis- earliest stages have quite a nice margin amplitnde.
-torted output and general cleanness. (which is easier to do), while the drive For example, suppose, at m.a.ximum
And yet the t.in~()lIpled circuit uses and output stages should run into over output, the input is really 1 volt but is
the output tubes strictly as pentodes load pretty well at the same point. Often held down at the grid of the first stage
with a variety of unity coupling. This it has been recommended that the out to an effective 0.1 volt because there is
well illustrates that pentodes can be op put stage should overload before all 0.9 volts of feedback. Then the onset of
erated in such • way as to achieve the earlier stages, ~cnuse this means that clipping results in a signal that suddenly
benefits of their improved efficiency and only one stage is responsible for pro looks like the 1 volt it really is because
give performante that is quite aceept- ducing distortion instead of many stages the 0.9 volts signal gets chopped off
8 ble to critir.al listent'rs. running into distortion conditions at the short. This results in a high peak being
snme time. amplified by the .early stages of the am
Th. ae.t Circuit Each of these recommendations may plifier until sollie stage fails to handle
What then is the best of the modern have its point, e.onsidering the amplifier it. An increase of actual input from 1
output circuits' This is a question quite without feedback. But when feedback is volt to 1.1 volts in a waveform at the
often asked and one to which there is no applied, as it is on all modern amplifiers, first grid that suddenly shoots up from
direct reply. It depends on how well the situation is considerably altered. 0.1 volt to 0.2 volts, and proportionately
each type of output circuit is designed Feedback theory is usually confined through successive stages. (Fig. 2)
or used. to the condition where the amplifier is What happens due to this sudden peak
For quite a while, there seemed to be assumed to have all its gain. Unfortu then depends on further details in the
a belief among amplifier designers that nately, as soon as clipping occurs the amplifier design. If this sudden peak
optimum perfonnance is achieved if all amplifier does not have all its gain. This produces overload at a point where there
the stages reach o.erload point at about does not necessarily introduce any in if'; direct coupling, say between an ampli
the same level of amplification. Another stability, but it {'an result in the sudden fier and phase-splitter stage, the ampli-
110 ~Ii 110
so
i
, •
20 i 120
co
100
~
80 ; •I eo
,--. i •
60 :
so ~ WATT I
11 NOMiNAl rEAK •
40 I ~ • 3l WATT
30 L 25 WATT " (NOMINAl f'EA1O
20 r (V-TED AVBAGE) -IV.. rItIf! 1(5l AWnADn nIV nI 20
.. AVElAGf)
10 , -::.r. .. _ • 1o
• I - ~&.t-- - - 5 WATT ..~.~.1 ~ .I,I .I. I\.I
\ ,; ,"'- • _,.. AVElAGE , '''I'!1
2I I • ;• • r ~I I
0 :I \:I \'1-.\j,li r /'\ -:'i .', V,,-, ~ (1 o
21 ·•· 4j -----• .-1 I• .. j "'. ;.. u- , I,f tI,. v • 2I
4 4
5 WATT '-.0. ,-----~- U
10 •~ "II AVElAG£ -, l'J 1~ WATT I o
(lATED AVElAGE) I
~; 25 WATT ". V 20
~ (tATED AVEl!-GE) (NOM~i NWAAlT TP fAIO 30
40 ~ 40
6so0 . -.-- ---- , -- 6500
J
...
80
co Jc o
20 -- - .... J 20
J lA2DS WOAVTETl LAOMAPD'l IFPIlEOIP £WlITTEHS ,. "" GO1O5 DW OATVTB ALO.WA\D.IF "IBo rWaITTHE S
so I
80 1e o
Fig. 1. Comparison of the same transient waveform amplified by (left) a 2S-watt amplifier with bad overload characteristic,
and <right) a 15-watt amplifier with good overload characteristic. In each easel the solid line represents the actual waveform,
while the dashed line shows the correct waveform where the amplifier departs from it. The lS-watt amplifier is handling the
same waveform at three times the power level.
25
AUDIO • NOVEMBER, 1959
fir.rt- 18 pentOde,--the second is triode.
Connecting them to a tapping results in
Ultra-Linear. This achieves practically
the efficiency ot a pentode while main
taining the linearity or Iow~rder dis
tortion of a triode.
This would seem to he ideal. The diffi
culty is that, to 'Work perfectly, the
transformer must maintain the correl't
tapping, both in voltage and phasE', at
all audio frequencies_ This is not too
difficult for the Tow-frequency end but,
at the high-frequency end, stray leak
age inductances between different parts
of the winding, along with winding
intercapacitances, can really play havoc
mtra ..
with an Linear circuit resulting
in some quite weird wavefonns at some
specific frequencies.
The solution to this is to have a cor
rectly designed Ultra-Linear trans
former that avoids any spurious devia
Fig. 2. Wh~t hap~ns in any feedback amplifier when clipping occurs. This shows
tion from correct tapping up to a
the waveforms associated with the input stage. Dashed line is the waveform at
frequency beyond the audio range and
maximum undistorted signal level. Solid line at a level 10 per cent above this.
also beyond the cutoff of the transformer
fier will not be disturbed by it. As soon output stage. These are almost invari as a primary-to-secondary transformer.
aE; the peak dis.ap~.ars- the amplifier re. ably resistance!c apacitance coupled and This.. a not impossible,.. but . onlJ rela
verts to its normal operating condition consequently, immediately following the tively few transformers manufactured
and carries on amplifying normally. excessive peak, the output stage is mo under the name of Ultra-Linear achieve
But if this peak reaches its limit of mentarily over-biased so as to produce this objective.
amplification at a stage that is resist crossover distortion if not completE' The McIntosh 'Version of the unity
ance/capacitan~ eoupled, a grid lllay blocking for the moment. coupled circuit relies on the famous
be driven a long way posith"e, causing bifilar-wound output ttansformer.2 The
a negative charge to appear on the grid The Curel fact that the high-voltage and ground-
side of the coupling capacitor after. the . voltage primaries are wound with the
There are two ways of obviating this.
peak disappears. This biases that stage wire actually side-by-side achieves a very
One is to use a cathode follower, direct
momentarily back beyond cutoff and intimate coupling between the winding
coupled to the output stage with ap
causes blocking . ...\S the stage drifts back connected to cathode and that connected
propriate negative supply to enable the
into its normal operating condition some to screen of the same tubes. To try and
cathode follower to have an even more
distortion is erident before the amplifier achieve this version of the unity-coupled
negative return than the necessary bias
resumes proper operation. circuit without a bifilar-wound trans
voltage for the output stage (Fig. 3).
Another place where the trouble can former would be asking for trouble ..
The other is much simpler and almost
occur, if the amplirler is "held up" right Of course, it is also necessary to use
as effective. It consists of interposing
through to the grids of the output stage, the various refinements developed with
what at one time would have been called
is that the sudden removel of amplifica that circuit for avoiding the other kind
a grid-stopper resistor between each
tion due to clipping results in an ex of blocking we discussed earlier. In the
coupling capacitor and the output tube
cessive postive drive at the grids of the case of the McIntosh circuit the output
grids.· It does not serve the one-time
tubes are driven by cathode-follower
function of stopping parasitic oscilla
direct-coupled stages.
tion in the grid circuit, but does prevent
the large grid-current flow that momen 2 Norman H. Crowburst, "Realistic engi
CATt'C::>E OUTPUT neering philosophy," At:'DIO, Oct., 1959 ..
tarily occurs during the high peak condi
FOlLOWERS STA(>f
tion and thus avoids the radical over
bias condition after the peak (Fig. 4) .. GIlD CURRENT
These are some general measures to lIMITUS
obviate the sudden overload troubles
that beset high-feedback pentode-typ(~
amplifiers. But what about some of the
other types of circuits, Ultra-Linear,
unity-coupled, Circlotron, single-ended
push-pull, and so on' "Which of these 81AS
would you recommend as best '" is a
not uncommon question. Here again it
is not so much a question of choosing
the best circuit as seeing that the one
..
you do choose is correctly used.
In the case of mtra-Linear the choice
of the tube operation is virtually one be
Fig. 4. A simpler method of at least re
tween pentode and triode. The tappings
Fig. 3. One way to obviate output. . st~ge ducing the effect i. the use of grid-cur
blocking is to use direct-coupled cathode on the transformer primary "split the rent .. limiter resistors, shown here in the
followers between drive and output, as difference" between connecting the autput-st~ge grids. They can also be used
shown here. screen to B + or direetly ·to plates. The at any stage thot couse. blocking.
An alternative approach is the twin
coupled amplifier referred to previousgr.
This uses completely separate trana
formers but employs large capa~itance
from screen to c.!\thode to achieve tight
coupling at the higher frl~ueneies. The
double transformer achieves sufficienUy
close coupling for the low frequencies,
which is not difficult.
The Circlotron circuit utilizes a com
paratively ordinary output transformer
and only needs one of them. Its disad
van~o-e is the fact that it needs two
high-voltage suppliers. While this may,
not be any more expensive (probably is
less expensive, in fact) than using two
output transformers or a more expensive
single one, it does have the <fu.advantage
that the high-voltage suppliers are vir
tually attached to the plates and cath
odes of the output stage. This means
that capacitances-in- the. supply circuit
and the power transformer are effec
tively in the audio circuit of the ampli
fier which can introduce complications
in that direction.
So each circuit has its critical factors
and it is only by taking careful account
of the various critical facton in each
circuit, watching out for the possibilities
of blocking, or other spurious conditions
-the things that can happen due to the
difference between practical program
material and the kind of signals used
for measuring amplifier performance-
that a satisfactory amplifier can be pro
duced. Using any basic circuit as a
starting point it is possible to design
out the various bugs that spoil amplifier
performance.
So, rather than saying that anyone
particular circuit is basically the best
circuit, it is better to look a little closer
and see how well the circuit has been
designed as regards avoiding some of the
spurious things that can happen in am
plifiers. JE
Amplifiers
EDGAR, M. VILLCHUR*
An analysis of the fundamental nature of amplification, and a
description of the working principles of pneumatic, mechanical, car
bon, vacuum-tube, transistor, magnetic, and dielectric amplifiers.
AC OM MOS-SE:-<SE DEFI:-<ITION of the out of nothing. The trick is that the in Sound generators like the human voi<;e
word "amplifierHis "a device that put stimulus borrows and directs power mechanism, or the phonograph pick-up
makes things bigger." But in tech from an independent second source diaphragm following the record groove,
nical language the term has a much (such as the electric company's gen simply didn't have enough driving
more restricted meaning; the device re erators), and shapes this independent power for the work they were called
ferred to becomes an amplifier only power to its own form_ upon to perform, even with the care
when the things that are made bigger The need for amplifiers arises when fully designed horns that increased
consist of energy-patterns. The nature we are dealing with impulses which their radiating efficiency. The solution
of amplification can probably be better must remain, in a very definite time was to injfct outside energy into the
understood by considering first the oper pattern if they are to be useful. One of systems and to use the original stimuli
ation of another energy transmission the earliest amplifying devices was the as controlling rather than driving
device that is not an amplifier-an in pipe organ, whose player was able to forces, which is to say, to amplify.
strument that is. called, in mechanics, a control, with relative1)C. light pressures
machine. of his fingers, the steady flow of air
Early Amplifiers
The machine receives input power, produced by sweating bellows-oper
shapes' it for the required task, and re ators. Amplifiers in the more generally In 1876 Edison patented a device
leases it, less the inevitable losses from accepted sense, however, were invented which he called an aerophone. It was a
friction, in its new form. \Vere it not when nineteenth century technology be pneumatic public-address amplifier. il
for these losses the amount of energy came concerned with the transmission lustrated in Fig. 1. in which the
released would be e..xactly equal to that and reproduction of vibratory power: speaker's voice controlled the instan
recei\·ed. Although the Indian hunter first sound, and then radio waves. taneous flow of compressed air by means
was able to bring down buffalo with bow . Sound consists of successive and al of a sound-actuated valve. The air was
and arrow, his arrow was driven by less ternating cOTQpressions and rarefac thus released in vibratory bursts and
energy than had been put into flexing tions radiated by an oscillating source. puffs similiar to those that came from
the bow. His machine was able to store The telephone and the phonograph the speaker's mouth, except that they
and concentrate the power that it re therefore depended for their operation were more powerful. and the speech,
ceived when the string was drawn back, on acoustical, mechnical, or electrical still intelligible, was louder. Edison en
so that the shaft sped with lethal veIoc forces whicn continually reversed their visioned broadcasting in stentorian
itv. Without the machine the hunter's directions, and which carried the trans tones over distances of several miles.
strength would have been totallv in- mitted intelligence in the time sequence Such a system has actually been used
effective. . and pattern of these oscillations. The in ports, but it found its main appli
The mechanical lever. the acoustical problem that faced engineers was to cation in the designs of two British in
horn. and the electrical transformer extend telephonic communication over ventors who applied it to the phono
are other eKamples of transmission de longer distances, to make phonograph graph. Short developed. and Parsons
vices whose useful output energy, while reproduction louder than was possible iurther improved the auxefophone,
re-formed in such a wav as to be most with the original, limited power. The whose pneumatic valve was attached
suitable for the appli~ation at hand, first approach, successful up to a point, directly to a phonograph reproducing
must always be somewhat less than the was to increase the efficiency of the stylus. Although pneumatic phonographs
input energy. The word "machine" ap passive transducer elements. But the produced a constant background his
plies to mechanical devices only; the best acoustical arid electrical passive sing noise due to escaping air, they were
term whi-ch includes all instruments of transducers that could be designed to fairly popular in Europe, and in the
this nature. whatever type of energy harness effectiYely the sources ot this early nineteen hundreds the French
is transmitted, is passi~'e transducer ciicillatory energy proved inadequate. Pathe company experimented with them
(from traalfCere, to lead across).
An amplifier is also an energy tnns
mission ckvi~e, and hence a transducer,
but it is an active one. It does that which Fig. 1. Edison's aero
would be- impossible without a sort of phone, or pneumatic
engineering sleight-of-hand-it provides amplifier, provided a
a transmission channel whose output, sound transmission
seemingly the same in identity to the channel into which
additional e n erg y
received stimulus, contains more energy
was injected in the
than its input. The difference is that
form of compressed
between a pulley and a powered capstan. air. Inset shows how
It is ohvious that the useful output the sound·aehlated
energy of an amplifier cannot be greater .al.e throttled a
than the total energy supplied, any more steady flow of air,
than it is possible for such a condition to create an instan
to e..xist in the case of a passive trans taneous variation in
tlow that imitated
ducer, or energy will have been created.
the original sound
.,ibrations.
• Wood-.nock. N. Y.
sums, and rolted cigarettes were the
wonders of applied science. was passing.
Electronics was taking o\'"er. and the
amplification of sound Wa3 destined to
include an intermediary step. the tem
porary transformation of mechanical
vibratory energy into electrical energy
possessing the same characteristics in
time.
Electrical ampli fication may be
achieved (and still is. in some tele
phone circuits) by carbon amplifiers,
which extend the principle of the carbon
microphone. The carbon gr:mules
through which current is directed act
as a variable electrical gate, whose
resistance to current flow is controlled
by the pressure of a cnaphragm_ Changes
of pressure, such as would be created
by stimulating the diaphragm with
sound, create corresponding changes in
the amount of current drawn from the
Fig. 2. Th~ Patlte phonograph of 1905 used source of electric power, and the elec
Q ~ompressed-air amplifier. trical source releases energy greater in
magnitude than that possessed by the
wi~ a vi~w towards develop,ing talking input stimulus.
motion plctures. (See Fig. 2.)
A~other type of device, the me
The Vacuum-Tube
chamcal or friction amplifier, found Fig. 3. The stylus of Columbia's cylinder 9raph
more favor in the United States; It was The device which really-opened up-t~ opi1one was coupled to the reproducing dia
phragm through a lever-type shonk, a string,
used in certain models of Columbia's field of ampli fication was the \'"acuum
and a friction shoe that picked up extra energy
cylinder '''graphophone,'' as shown in tube. Fleming had made an electronic
from the rotating drum.
[ig. 3. The reproducing stylus of these valve that contained two electrodes
ll'~struments, instead of being coupled sealed in an evacuated glass charpber,
dIrectly to, its diaphragm as in standard a cathode emitter and an anode collector. ImItative vanation in the relatively
acous,tlcal phonographs, was attached to \Vhen the cathode was heated a cloud of heavy output current flow, as may be
the dIaphragm via a string and friction electrons was given off, and if the device seen in Fig. 4. This output power could
shoe that passed over a rotating drum. was then connected in series with a bat follow the input characteristics more
\Vhen the stylus tightened up on the tery, in s~ch a way that the anode was closely than had been possible with any
string, fric:tion between the shoe and positively charged relative to the other device designed previously. The
the drum was increased and force cathode, the electrons were attracted to limits imposed by mechanical systems
picked. up from the drudt augmented and entered the anode. Since electrons their intractability when subjected to
the dlspla-cement of the diaphragm. in motion constitute electrical current forced vibration in modes foreign to
\Vhen the: record groove forced the the circuit was completed through this natural resonances, the uneven restraint
stylus in tbe opposite direction, so as to one-way path. of elastic suspensions, and the fact that
loosen up the string, the diaphragm The stream of electrons flowing in supposedly rigid parts become flexible
{.)fl
returned tCh its original position due to the empty space between cathode and \vhen subjected to vibration at high
spring tension. In this way the vibra anode provided an especially favorable frequencies-all disappeared, and de
tory path o-i the diaphragm was extended area for sensitive control of the current velopment workers found themselves
by, the emergy of the independently drawn from the battery. The opportun operating in a dream-world of virtually
drIven drum, and sound output was in ity was seized by de Forest, who intro massless units, where incredibly swift
creased. duced a control element into the valve oscillation could be controlled and am
Both of the above designs were re by inserting a "grid"-an open network plified without having to reckon the
ferred ~o. a,t the time as relay systems. of fine wire--across the electronic price of inertia, elasticity or gravity.
The on~rQ.l stimulus was thought of stream. De Forest's grid was a sieve An early application of vacuum-tube
as touchIng; off latent power, like a relay mechanically, but if it was charged
amplifiers was to the generators and
runner passing the baton to his suc negatively relative to the cathode it
receiver of radio waves. Like sound,
cessor. These systems were the fore tended to repel electrons (which are
runners of our present-day electronic , also negatively charged) and to retard electromagnetic radio energy is oscil
latory, although at frequencies which
amplifiers. but they were themselves current flow. A weak input "signa!"'
doomed to :a short life. The golden age voltage applied between grid and cath may be millions of times higher than
of mechani€s, when the diabolical iron ode, varying according to' a given fre those of acoustical vibrations. The ele
fingers that set printing type, tabulated quency and wave form, produced' an ment analogous to the phonograph horn
is the antenna. acting as a passive trans
ducer to the "atmosphere"-and, as in
the case of the horn, more efficient
Fig. ... Amplification antennas were not enough. With trans
.1 c~M . ~zW I obimyf pQu alsw e ~lki'Os Ceaulceuhcmit-ertiyucebadel am iftetewr owuattptsu t toam hpulinfiderde,d sh oowf evkeilro, wfraottms
~o> . u~:> circuit_ The input and receiver sensitivity raised to the
electrical stimulus point where a few millionths of a vol1
T'ME has alternating p0- at the antenna created usable re~eption
larity, "hil. the out wireless global communication becaml
put is in the form
possible. Other applications followe<
of pulsating one-way
current_ Th. cath quickly. The recording and reproductiOI
ode heating element of sound, the detection and measure
is DOt shown. ment of very small quantities of light
sound, pressure. or voltage, the myriai
ceptor, and vacancies are available for
electronic current flow in the form of
"hole" conduction (an effective migra
tion of the unfilled spot from one atom
to another. a phenomenon which has
been aptly compared to the motion of
an air hubble in water). These two
modes of contluction occur in opposite
Fig. 5. The junction
transistor is tin., directions and are called. respectively,
compared to the II-type for negative, and p·I),l'c for posi
'~I u b - miniature" tive. Hole conduction has a positi·. . e
tube, the smallest designation bec.luse the migration of
type made. These are holes has the same experimental effect
approltimately full
as the transfer of positive charges.
size. (CourtesyGen-
eral Electric Co.) The development of semi-conductor
devices has followed the same course as
that of the_ __ vacuum:-tube from .. two-
s
terminal systems providing :L one-\vay
electronic path, to three-terminal sys.
terns in which the electronic flow is made
subject to control from an area astride
the path. Serni-conductors were used
as rectifiers of alternating current long
before the word transistor was coined.
tasks performed by calculating machines, atom. The attachment, originally weak A potential applied in one direction
and the sensitive control and regulation because of the relative distance from across the junction of a p-type and an
of massive machinery became part of the nucleus, disappears with the close II-type substance will encounter rela
the electronic field. atomic spacing typical of these mate- tively low resistance to current flow,
But with poetic injustice, after the----rials, and the outer electrons -aTe free-· bur relativdy- higlr resistuIce---if the
vacuum-tube has served as the vehicle to rove. These free electrons are able to polarity and hence the direction of cur·
for the modem science of electronics, respond to the force of an electric poten rent flow is reversed. This is because
it is being prepared for the scrap-heap, tial applied across the conductor, and the electrons and holes travel towards
at least in certain applications. The form an electronic wind blowing across each other for one polarity, facilitating
vacuum-tube has several disadvantages, the relatively stationary atoms them transfer across the junction, and away
foremost among which is its unre1ia- selves towards the positive terminal, from each other for the opposite polar
biJity. Besides having too short a normal constituting the flow of current. Current ity. The rectifying action may also be
life, the possibility of failure at any time does not flow to any appreciable extent described from the point of view of
after installation must always be taken in non-conductors because the atoms of
energy-level states; for one polarity,
into consideration by design engineers. insulators hold on grimly to their outer
electrons belonging to energy levels
The unreliability of the vacuum-tube is shell electrons, which are more numer
capable of releasing electrons are driven
such an accepted fact-of-life that instead ous, closer to the nucleus and much
towards atoms containing energy levels
of being wired permanently into the more difficult to dislodge.
capable of receiving added electrons,
circuit, like other components of elec- To impart motion to an electron is to
tronic apparatus, it is plugged into a give it added kinetic energy. Quantum while for the other polarity the opposite
tube socket to facilitate periodic replace- requirements dictate that the electrons effect occurs.
mente In addition to this unreliability must fill certain discrete energy levels, A p-type substance sandwiched be
the vacuum-tube requires a separate that is, that they cannot possess a ran tween two n-type substances, or vice
power supply to heat its filament (divert- dom amount of energy, and that each versa, creates the basic design of one
ing and wasting most of the energy taken energy level can only accommodate a type of transistor amplifier. The con
from the independent source), it must given number of electrons. Therefore ducting properties of one of the junc
be given a warm-up period prior to serv- the energy of an electron can only be tions for "wrong-way" current may be
ice, and it is too bulky in some applica- increased or decreased by an amount
controlled by creating either hole or
tions. The feature which redeems all of which brings it into a new step level in
electron carriers in the sandwiched ele
these disadvantages is the superb control which a vacancy exists. The quantum
ment (by means of a current through
which may be exerted over the captive levels of the atoms of a conductor have
the other junction )-to put it another
electron stream. vacancies. permitting electronic transfer
way, by causing a shift in the electron
Without abandoning the last feature, from one level to another. The energy
new ways in which electrons can be levels of the atoms of insulators, on the energy level states responsible for con
made to submit to instantaneous regu- other band, are all filled. so that the duction. The pattern of variation of a
lation at high frequencies are being in- system is locked. small controlling current shapes the in
vestigated. The transistor, a revolution- The energy level states of semi-con stantaneous resistance of the unit, and
ary experimental device a few years ago, ductors (substances such as germanium, large currents may then be forced to
can aready be ordered by the part num- selenium, silicon, and the oxides of cop follow the same pattern in time.
ber at radio dealers, and development per and barium) form a special case. The transistor requires no warm up
work is also being performed on mag- The locked system is upset by the pres period, is smaller (see Fig. 5), cheaper
netic, dielectric, and other types of am- ence of minute impurities, whose outer in operating cost, and is potentially so
plifiers. electronic orbits contain electrons in a
much more reliable than the vacuum
number either greater than or less than
tube that it may be wired permanently
the amount normal to the pure substance,
into the circuit rather than plugged
From the electrical point of view and which introduce energy levels capa
into a socket. Transistor hearing aids.
materials may be classified according ble of releasing or accepting electrons.
to their resistance to the passage of cur Where the number Of outer electrons for example, which are already produced
rent, as conductors, insulators, and is greater than normal, excess electrons . commercially, are smaller than their
semi-conductors. In an atom of a good are available for current flow in the vacuum-tube counterparts, consume only
electrical conductor the outermost elec form of an electronic wind, and the sub a small fraction of electrical power for
tronic shell is held so loosely that its stance is called a donor. Where the the same amplification (they have no
electron inhabitants are not associated number of outer electrons is Jess than A battery) and may ultimately be ex
exclusively with any particuar parent normal, dl~ subsWlce ~I ealled an ac- pected to require less service. The tran-
sistor has been developed to a point and the material of the core. None of
where it can duplicate many, although these can be ~ltfrui.~ Q: high fre
not all, of the vacuum-tube functions. quencies, but there is another, more
One application of the transistor is easily controllable characteristic that
illustrated in Fig. 6. can influence the coil's field strength and
a.c. impedance-the magnetic condition
of the core. The core will not continue
Malnetic Amplifiers
to a.ccept added magnetization indefi
The electrical amplifiers that have nitely; there is a natural limit to its
been here described provide circuit paths capabilities. As the current is incre:lsed
whose resistance to current flow is var the core begins to satrcrate, which means
ied by an input signal. Such a path may that a further increase of current flow
also be produced by an electro-magnetic through the coil will produce less than
rather than a resistive unit, which is the corresponding increase in magnetic
called a saturable reactor. field strength. 1 The degree of this sat
The impedance of an electrical coil uration may be controlled, electrically,
to alternating current is far more than by the input signal.
would be expected from the inherent A separate winding-oiltlie same·core,
resistance of the wire. Each time that through which the controlling input cur
the current increases, drops to zero, and rent flows, will cause the degree of sat
then increases in the opposite direction uration to increase and decrease accord
a magnetic field around the coil builds ing to the instantaneous polarity and
up, collapses, and builds up again with value of the input signal. A larger cur
reversed polarity. This pulsating mag- rent flowing in the output winding,
netic field cuts the wires transversely drawn from an a.c. source of power, will
I
each time that it builds up and each then vary in step with the varying im
time that it collapses, inducing current pedance.
of such instantaneous direction as to If the input current must do all of
oppose and reduce the original flow. the saturating the power gain will be
This is the descriptive analysis ot in.; --low, -as an appreciable crmount of energy Fig. 6.. With the -transistor reducing s.,oce re
ductive reactance. In the magnetic am is required to saturate the core. A third quirements of tubes and batteries, an electronic
plifier the input signal controls the in winding is therefore assigned the major megaphone can contai. microphone, amplifief',
tensity to which the self-induced field burden of saturation. This winding may batteries and speaker in one independent unit.
can build up, and hence it controls the carry direct current from a separate (C~rtesy General Electric Co.)
electrical impedance of the coil. electrical supply, or it may carry recti
Among the factors that determine the fied current from the output circuit. is of a type" called reacti\·e, which does
intensity of the field are the number of In the latter" case the third winding in not itself absorb energy. (The resistive
turns in the coil, the size of the core, troduces "positive feedback," because barrier to current flow introduced by
the effect of a small. input current is vacuum-tubes and transistors wastes
re-introduced into the circuit in such a energy in heat.) ~Iagnetic ampli fiers
way as to intensify the effect on the are at present advantageously applied
output. Small input currents can then in circuits which must control appre
ACPOWER control very much larger output cur ciable amounts of power at relatively
-~-t;---a+- ----oo ~ rents, and power gains of the order of low frequencies-adjustable-speed mo
INPUT ~ 100,000 times are obtainable. tors, winding reels, automatic pilots,
In practice it is found necessary for voltage and frequency regulators, and
the independent energy source of the other automatic control apparatus. A
magnetic amplifier to supply pulsating magnetic amplifier used in servo work
direct current rather than alternating is illustrated in Fig. 8.
current, as shown in Fig. 7, so that the
saturation effect of the current in the Dielectric Amplifiers
output winding can never oppose that In the search for new, more compact,
of the input winding. Pure direct cur and simple amplifier de\"ices research
rent in the output circuit, however, such is being pursued in yet aoother direction,
as is used with vacuum-tubes and tran that of the capacitor or d:electric ampli
sistors, will not work. Direct current fier. The principles of operation are
would remain uninfluenced by the quite similar to those 0; the magnetic
changes in core saturation; the impe amplifier, in that a circuit element with
dance of the coil to d.c. is entirely a variable a.c. impedance is connected in
matter of the resistance of the wire series with an a.c. source of power. The
conductor. Thus the power that is var element is not a coil, however, but a
. ied by the input signal is itself a steadily capacitor, a system of parallel plates
- oscillating quantity, but it is a. relatively separated by an insulating material or·
· :-s simple matter to separate and extract the dielectric.
amplified impulses from the alternations If a battery is connected across a
of the power source. For this purpose the capacitor there will be no steady-state
frequency assigned to the power supply current flow. Electrons move from the
is made much higher than the highest negative terminal and charge one side
frequency input that is to be amplified. of the capacitor by surieiting its plates
Magnetic amplifiers are very reliable, with negative charges ;. at the same time
have the ability to withstand severe electrons move from the opposite plates
Fig. 7. Th. top diagram ShoWl-the essentials shock, and require no warm-up period. of the capacitor into the positive battery
of a magnetic amplifaer circuit. Current i. the
They are also exceptionally efficient. terminal, and leave these plates posi
input winding colltrols magnetic saturation of
because most of the impedance which tively charged by reason of their lack
the core, which in turn controls the impedance
they introduce into the output circuit of the normal number of negative
of the output winding to the flow of altet'nating
current. The botto. diagrolll includes rectifi charges. The process continues for a
cation of the a.c. power to pulsatinl d.c., and 1 A familiar example of this phenomenoa short time, until the storage "capaci
11M of aa additional "positiT. feedback" wind- is the decrease of mductance in a choke tance" of the device for electric charge
iDg to inc,... power sensitiyity. when the current rating is exceeded. is reached. at which point the short-