History of the Transistor.doc

8/10/2019 History of the Transistor.doc

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History of the Transistor

by Tony van Roon (VA3AVR)

Dr. John Bardeen(left), Dr. Walter Brattain(right), and Dr. William Shockley(center)discovered the transistor effect and developed the first device in December, 1!", #hilethe three #ere members of the technical staff at Bell $aboratories in %&rray 'ill, J.hey #ere a#arded the obel *ri+e in physics in 1-.

The PN Junction:

What the Bell $abs scientists discovered #as that silicon #as comprised of t#o distinctregions differentiated by the #ay in #hich they favored c&rrent flo#. he area that

favored positive c&rrent flo# they named * and the area that favored negative c&rrentflo# they named . %ore importantly, they determined the imp&rities that ca&sed thesetendencies in the * and regions and co&ld reprod&ce them at #ill. With thediscovery of the */ 0&nction and the ability to control its properties, the f&ndamentalgro&nd #ork #as laid for the invention of the transistor. his Bell $abs discovery #asinstr&mental in the development of all semicond&ctor devices to come.

The First Silicon Transistor:

t #as late afternoon in 1! at a conference for the nstit&te of 2adio 3ngineers. %anypeople giving talks had complained abo&t the c&rrent germani&m transistors//they had abad habit of not #orking at high temperat&res. Silicon, since it4s right above germani&m

on the periodic table and has similar properties, might make a better gadget. B&t, theysaid, no one sho&ld e5pect a silicon transistor for years.

hen 6ordon eal of e5as nstr&ments stood &p to give his talk. 'e p&lled three smallob0ects o&t of his pocket and anno&nced7 8ontrary to #hat my colleag&es have told yo&abo&t the bleak prospects for silicon transistors, happen to have a fe# of them here inmy pocket.

hat moment catap&lted from a small start/&p electronics company into a ma0orplayer. hey #ere the first company to prod&ce silicon transistors // and conse9&ently thefirst company to prod&ce a tr&ly consistent mass/prod&ced transistor.

Scientists kne# abo&t the problems #ith germani&m transistors. 6ermani&m #orked, b&tit had its mood s#ings. When the germani&m heated &p//a nat&ral o&tcome of being partof an electrical circ&it//the transistor #o&ld have too many free electrons. Since atransistor only #orks beca&se it has a specific, limited amo&nt of electrons r&nningaro&nd, high heat co&ld stop a transistor from #orking altogether.

While still #orking at Bell $abs in 1:, eal began gro#ing silicon crystals to see if


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they might #ork better. B&t 0&st as it had taken years to prod&ce p&re eno&ghgermani&m, it took several years to prod&ce p&re eno&gh silicon. By the time hes&cceeded, eal #as #orking at e5as nstr&ments. $&ring someone as kno#ledgeableabo&t crystals as eal a#ay from Bell proved to be one of the most important things ever did.

;n


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;nce they4d gotten slight amplification #ith that tiny drop of #ater, Bardeen and Brattainfig&red they #ere on the road to something #orth#hile. sing different materials anddifferent set&ps and different electrolytes in place of the #ater, the t#o men tried to get aneven bigger increase in c&rrent. hen on December C, Bardeen s&ggested they replace thesilicon #ith germani&m. hey got a c&rrent 0&mp, all right//an amplification of some ??:

times//b&t in the e5act opposite direction they4d e5pected. nstead of moving the electronsalong, the electrolyte #as getting the holes moving. B&t amplification is amplification //it #as a start.

Brattain %akes a %istake

nfort&nately this giant 0&mp in amplification only #orked for certain types of c&rrent //ones #ith very lo# fre9&encies. hat #o&ldn4t #ork for a phone line, #hich has to handleall the comple5 fre9&encies of a person4s voice. So the ne5t step #as to get it to #ork atall kinds of fre9&encies.

Bardeen and Brattain tho&ght it might be the li9&id #hich #as the problem. So theyreplaced it #ith germani&m dio5ide // #hich is essentially a little bit of germani&m r&st.6ibney prepared a special slab of germani&m #ith a shimmering green o5ide layer onone side. ;n December 1>, Brattain began to insert the point contacts.

Nothin" ha%%ene&:

n fact the device #orked as if there #as no o5ide layer at all.


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cond&ct a fe# tests, it #as official // this tiny bit of germani&m, plastic and gold #as thefirst #orking solid state amplifier.

///////////////Shocley nvents the Junction Transistor

Jan&ary and ebr&ary, 1!C

< Solitary e# Eear4s 3ve

William Shockley spent e# Eear4s 3ve alone in a hotel in 8hicago. 'e #as there for a*hysical Society meeting, b&t he #as most e5cited abo&t having some time to himself toconcentrate on his #ork. here may have been a party going on do#nstairs, b&t Shockley#anted nothing to do #ith it. 'e had more important things to think abo&t. 'e spent thatnight and the ne5t t#o days #orking on some of his ideas for a ne# transistor/one that#o&ld improve on Bardeen and Brattain4s ideas.

Scratching page after page into his notebook, one of Shockley4s ideas #as to b&ild asemicond&ctor sand#ich. hree layers of semicond&ctors all piled together, he tho&ght,0&st might #ork like a vac&&m t&be/#ith the middle layer t&rning c&rrent on and off at#ill.


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point/contact transistor and he didn4t #ant to risk that happening again.

he 3&reka %oment

hen, on ebr&ary 1C, Shockley learned it co&ld #ork. #o members of the gro&p,

Joseph Becker and John Shive, #ere #orking on a separate e5periment. heir res&ltsco&ld only be e5plained if the electrons did in fact travel right thro&gh the b&lk of asemicond&ctor. When they presented their findings to the gro&p, Shockley kne# he hadthe proof he needed. 'e 0&mped &p and for the first time shared his concept of a sand#ichtransistor to the rest of his team.

Bardeen and Brattain #ere st&nned that they hadn4t been filled in before no#. t #as clearthat Shockley had been keeping this secret for #eeks. t added still more space to theever/#idening gap that #as gro#ing bet#een them.///////////////

Tellin" the +ilitaryJ&ne >?, 1!

hey had no #ay of kno#ing all that the transistor co&ld do, b&t the administrators atBell $abs still kne# they #ere on to something big. hey #ere abo&t to hold a h&ge pressconference to anno&nce #hat they4d invented // b&t before telling the p&blic they had tocheck #ith the military.


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While one lab at Bell #as trying to improve those first type/< transistors, WilliamShockley #as #orking on a #hole different design that #o&ld event&ally get rid of theseproblems.

3arly in 1!C, Shockley conceived of a transistor that looked like a sand#ich, #ith t#o

layers of one type of semicond&ctor s&rro&nding a second kind. his #as a completelydifferent set&p #hich didn4t have the shaky #ires that made the point/contact transistorsso hard to control.

Not Just on the Surface

< #orking sand#ich transistor #o&ld re9&ire that electricity travel straight across acrystal instead of aro&nd the s&rface. B&t Bardeen4s theory abo&t ho# the point/contacttransistor #orked said that electricity co&ld only travel aro&nd the o&tside of asemicond&ctor crystal. n ebr&ary of 1!C, some tentative res&lts in the Shockley labs&ggested this might not be tr&e. So the first thing Shockley had to do #as determine 0&st

#hat #as going on.

8aref&l e5periments led by a physicist in the gro&p, 2ichard 'aynes, helped. 'aynes p&telectrodes on both sides of a thin germani&m crystal and took very sensitivemeas&rements of the si+e and speed of the c&rrent. 3lectricity definitely flo#ed straightthro&gh the crystal. hat meant Shockley4s vision of a ne# kind of transistor #astheoretically possible.

!ro,in" rystals

B&t 'aynes also discovered that the layer in the middle of the sand#ich had to be verythin and very p&re.

he man #ho paved the #ay for gro#ing the best crystals #as 6ordon eal. 'e didn4t#ork in Shockley4s gro&p, b&t he kept tabs on #hat #as going on. 'e4d even been askedto provide crystals for the Solid State team &pon occasion. eal tho&ght transistors sho&ldbe b&ilt from a single crystal/as opposed to c&tting a sliver from a larger ingot of manycrystals. he bo&ndaries bet#een all the little crystals ca&sed r&ts that scattered thec&rrent, and eal had heard of a #ay to b&ild a large single crystal #hich #o&ldn4t haveall those crags. he method #as to take a tiny seed crystal and dip it into the meltedgermani&m. his #as then p&lled o&t ever so slo#ly, as a crystal formed like an iciclebelo# the seed.

eal kne# ho# to do it, b&t no one #as interested. < n&mber of instit&tions at the time,Bell incl&ded, had a bad habit of not tr&sting techni9&es that hadn4t been devised at home.Shockley didn4t think these single crystals #ere necessary at all. Jack %orton, head of thetransistor/prod&ction gro&p, said eal sho&ld go ahead #ith the research, b&t didn4t thro#m&ch s&pport his #ay.

$&ckily, eal did contin&e the research, #orking #ith engineer John $ittle. hree months


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later, in %arch of 1!, Shockley had to admit he4d been #rong. 8&rrent flo#ing acrosseal4s semicond&ctors co&ld last &p to one h&ndred times longer than it had in the old c&tcrystals.

!ro,in" -ven 'etter rystals

ice crystals are all #ell and good, b&t a sand#ich transistor needed a sand#ich crystal.he o&ter layers had to be a semicond&ctor #ith either too many electrons (kno#n as /type) or too fe# (kno#n as */type), #hile the inner layer #as the opposite. nderShockley4s prodding, eal and %organ Sparks began adding imp&rities to the melt #hilethey p&lled the crystal o&t of the melt.


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n J&ly of 11, Bell held another press conference // this time anno&ncing the inventionof a #orking and efficient 0&nction transistor.///////////////

Sharin" the Technolo"y: 'ell Hosts Transistor Sy*%osia

11/1>

Bell $abs had an important reali+ation7 development of the transistor #as going to movea lot more 9&ickly if they opened &p the field to other companies. So in September 11,Bell $abs hosted a symposi&m to spread the gospel abo&t #hat the transistor co&ld do.

,:::. #enty/si5 companies, from both the S andabroad, signed &p for the privilege. he companies #ere both big, s&ch as B% and

6eneral 3lectric, and small, s&ch as then/&nkno#ns like e5as nstr&ments.

;ver one h&ndred registrants from the select companies ret&rned for the ransistorechnology Symposi&m in . or eight days Bell $abs #orked the attendeesday and night // b&t at the end, they #ere e9&ipped to go off and b&ild transistors forthemselves.

Bell took all the information from the meeting and bo&nd it into a t#o vol&me book setcalled ransistor echnology. he book became fondly kno#n as %other Bell4s8ookbook.////////////////

$illia* Shocley +oves to alifornia

1-

William Shockley had gone as far as he #as going to go at Bell $abs. 'e had #atched thepeople &nderneath him get promoted above him // and #ith good reason. oo many top9&ality scientists hadn4t been able to #ork #ith him . < geni&s he may have been, b&t agood manager he #as not.

Shockley decided he needed a big change. he first thing to go #as the car // he traded inthe fancy %6 for a Jag&ar convertible. e5t7 the 0ob. 'e spent a semester at 8altech andthen a year #orking for the Weapons Systems 3val&ation 6ro&p in Washington D8., b&tnothing completely satisfied him. 3ager to be able to r&n things his o#n #ay, he finallydecided to strike o&t on his o#n // get some f&nding and start his o#n company.

n


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Shockley #as l&red to the *alo


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devices // abandoning silicon, abandoning all of today4s man&fact&ring methods. S&chtransistors are kno#n, not s&rprisingly, as single electron transistors, and they4d beconsidered on or off depending on #hether they #ere holding an electron.(ransistors at this level #o&ld be solely &sed as s#itches for binary coding, not asamplifiers.) n fact, s&ch a tiny device might make &se of the 9&ant&m #eirdness of the

&ltra/small. he electron co&ld be coded to have three positions // instead of simply onor off it co&ld also have some#here bet#een on and off. his #o&ld open &p doors forentirely ne# kinds of comp&ters. :1:. With nearly a billion transistors on ntel4s latest processor that #o&ldmean fo&r times as many transistors on a chip are theoretically possible. 8hips like this#o&ld allo# comp&ters to be m&ch smarter than they c&rrently are.

///////////////H///////////////

hat #as 9&ite a bit of interesting history of the transistor@ $et4s move on to the tid bidsof the Bipolar J&nction ransistor (BJ for short), mostly the common .N3/01type inthis e5citing /part t&torial.

o%yri"ht an& re&its

&torial, photos, dra#ings, schematics, etc., copyright I>::-, ony van 2oon, &nlessother#ise indicated.he content of this article is a collection of e5cerpts fo&nd on the #eb and re/posted #ithpermission.

*age copyright I>::- / ony van 2oon, =


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Transistors TutorialPart 1:

"Bipolar Basics"

"We look at the tiny devices that have reshaped the world of electronics."


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m&ch the same as a common * 0&nction diode.he c&rrent that flo#s (composed of electrons for * &nits and holes, in the case of** transistors) is mainly from the emitter to the base rather than vice versa. hat is#here the emitter derives its name//it emits or in0ects c&rrent carriers in the other regionsof the device.

he third region of a transistor, the collector, is lightly doped, m&ch the same as the base,e5cept #ith the opposite type of doping imp&rity, so it (like the base region) has relativelyfe# free carriers available to cond&ct c&rrent in the normal #ay. he collector/base0&nction is normally reverse biased, so a depletion layer forms, spreading o&t on eitherside of the 0&nction. he depletion layer effectively removes the carriers that #o&ldother#ise balance o&t the charges on the fi5ed imp&rity atoms of the crystals, setting &p apotential barrier to match the applied reverse voltage.

o the normal ma0ority carriers in the base and emitter, that potential barrier is a big #allthat m&st be overcome before they can pass to the other side. So 0&st as in the case of anormal diode, virt&ally no c&rrent flo#s across the collector/base 0&nction #hen left to itso#n devices. 'o#ever, the 0&nction is not left to its o#n devices.2emember that the base region is deliberately made very thin and lightly doped, #hile


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the emitter is made m&ch more heavily doped. Beca&se of that, applying a for#ard bias tothe emitter/base 0&nction ca&ses vast ma0ority carriers to be in0ected into th the base, andstraight into the reverse/biased collector/base 0&nction. hose carriers are act&allyminority carriers in the base region, beca&se that region is of opposite semicond&ctor typeto the emitter. o those ma0ority/t&rned/minority carriers, the collector/base 0&nction

depletion region is not a barrier at all b&t an inviting, accelerating filedA so as soon as theyreach the depletion layer, they are immediately s#ept into the collector region.or#ard biasing the emitter/base 0&nction ca&ses t#o things to happen that might seems&rprising at first7 ;nly a relatively small c&rrent act&ally flo#s bet#een the emitter andthe base. m&ch smaller than #o&ld flo# in a normal * diode despite the for#ard biasapplied to the 0&nction bet#een them. < m&ch larger c&rrent instead flo#s directlybet#een the emitter and the collector regions, in this case, despite the fact that thecollector/base 0&nction is reversed biased.hat effect is ill&strated in ig. 1


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doped (for a good carrier in0ection) and can be relatively smallsince the emitter/base 0&nction does not need to dissipate m&chpo#er (heat). n contrast, the collector is lightly doped (for a#ide depletion area) and its 0&nction is m&ch larger since, beingreversed biased, it m&st dissipate m&ch more po#er.

8onnections to the emitter and base regions are made by #ay ofal&min&m electrodes deposited on the s&rface. hin #ires arebonded to the electrodes for connection to the main device leads.he lo#/resistance s&bstrate itself is &sed to connect to thecollector region.hat is the basic constr&ction &sed for most modern bipolartransistors, #hether they are discrete &nits or part of an

8 containing tho&sands of transistors. he main difference is si+e, altho&gh, in an 8, thecollector region of the transistor #ill generally be in an epita5ial layer gro#n on theopposite kind of s&bstrate, and separated by diff&sed #alls (of the opposite type material)to separate the transistors from each other.

n an 8, the active part of an individ&al transistor might only be a co&ple micrometerss9&are, #hile a very large transistor (&sed to s#itch h&ndreds of amperes) might be on asingle #afer of 1: mm or more in diameter. ypical small/to/medi&m po#er, discretetransistors &sed in cons&mer and hobby electronics are gro#n on chips meas&ring from 1/to abo&t ?/mm s9&are//the rest of the component is protective packaging.

Transistor %eration:

2efer to ig. >, a ** version of the ill&stration sho#n in ig. 1


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non/linear change in those 0&nction capacitances.

here is also a base/emitter resistance (2bethat m&st beconsidered. n practical transistors, emitter resistance is on

the order of a fe# ohms, #hile the collector resistance is many

h&ndreds or even tho&sands of times larger. he 0&nction capacitancein combination #ith the base/emitter resistance determine the &sef&l&pper/fre9&ency limit of a transistor by establishing an 28 time

constant.Beca&se the collector is reversed biased, the collector/to/base resistance is high. ;n theother hand, the emitter and collector c&rrents are s&bstantially e9&al, so the po#er in thecollector circ&it is larger than the po#er in the emitter circ&it.(* >2, so the po#ers are proportional to the respective resistances, if the c&rrents arethe same.)

n practical transistors, emitter resistance is on the order of a fe# ohms, #hile the

collector resistance is many h&ndreds or tho&sands of times larger, so po#er gains of >:to !:dB, or even more, are possible.ig&re ?sho#s the schematic symbols for both the * and ** version of the bipolartransistor. he first t#o letters of the designators (* or **) indicate the polarities ofthe voltages applied to the collector and emitter in normal operation. or e5ample, in a** &nit, the emitter is made more positive #ith respect to the collector and the base, andthe collector is made more negative #ith respect to the base.


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Beca&se a vast array of amplifier circ&its in &se in modern electronics, amplifier circ&itsare often s&bdivided by application//


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be tens of tho&sands of ohms, depending on the signal4s so&rce impedance. he common/emitter amplifier has a lo#er c&toff fre9&ency than does the common/base type, b&t givesthe highest po#er gain of the three config&rations. ote that the o&tp&t signal is 1C:M o&t/of/phase #ith (or the opposite of) the inp&t (base/c&rrent) signal, so the feedback thatflo#s thro&gh the small emitter resistance is negative (degenerative), keeping the circ&it

stable. he common/emitter amplifier is one of the most often seen config&rations for thebipolar transistor.he common/collector amplifier (also referred to as an emitter follo#er), see ig. !8, hasa high inp&t impedance and a lo# o&tp&t impedance.

he impedance is appro5imately7

he fact that the inp&t resistance is directly related to the load resistance is a disadvantageof this type of amplifier if the load is one #hose resistance or impedance varies #ithfre9&ency.

he c&rrent transfer ratio of this type of circ&it is7

and the c&toff fre9&ency is the same as in the common/emitter amplifier circ&it. heo&tp&t and inp&t c&rrents of this type of circ&it are in phase.


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A*%lifier lassifications:


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(radio fre9&ency) applications.

8lass/< operation is s&itable for voltage amplifiers. n a voltageamplifier, the emphasis is on the magnit&de of the o&tp&t voltage.ig&re -sho#s a single/ended class/< amplifier. S&ch an amplifier

might be &sed in a preamplifier stage, #here inp&t signals aretypically small, and a faithf&l reprod&ction of the inp&t &sing asingle transistor is needed. hat config&ration allo#s a small inp&tc&rrent to control c&rrent dra#n from a po#er so&rce, and th&sprod&ce a stronger replica of a #eaker original signal.

n 8lass/B operation, the transistor is biased at c&toff (see ig. B),so that o&tp&t c&rrent flo#s d&ring only half of the inp&t cycle. t is &sed #here highefficiency and lo# distortion are re9&ired//for instance, in po#er/o&tp&t config&rations.When the 8lass/B amplifier is &sed for a&dio applications, t#o s&ch amplifiers connectedin the p&sh/p&ll config&ration are re9&ired, so that c&rrent can flo# alternately thro&gh

the t#o amplifiers. n other #ords, on amplifier is t&rned on, #hile the other is t&rned off.

;n the other hand, #hen the 8lass/B amplifier is &sed in 2 applications, it can beconfig&red for single/ended operation. Since, in the absence of an inp&t signal its c&rrento&tp&t is negligible, it is &sed #here high efficiency (-:/":N) and lo# distortion arere9&ired, #hich is very important in high/po#er amplifiers.8lass//?N) and distortion characteristics that lie bet#eenthose of 8lass/< and 8lass/B amplifiers. 8lass/


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Transistors Tutorial

Part 2:

"Bipolar Transistors"

"The bipolar junction transistor is still one of the cornerstone's of modern

solid-state electronics. Learn (or review the basics of this important active

device.

he Bipolar J&nction ransistor (BJ) triggered the revol&tion in modern solid/stateelectronics in the 1-:4s.


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ig&re 1/asho#s the cross section of an * BJ. ts base andemitter terminals are metal depositions on top of the silicon #afer,and its collector is the metali+ed lo#er s&rface of the #afer. ig&re>/asho#s the cross section of a ** BJ. t is similar to the *BJ, e5cept that the / and */type materials have changed places.

ig&re 1/band >/bare the schematic symbols for the * and **transistors, respectively. otice that they are the same e5cept for thedirection of the arro#head #ithin the symbol at the emitter terminal.his difference #ill be e5plained shortly.he term bipolarmeans that the BJ4s operation depends on themovement of t#o different carriers7 electronsand holes. n *BJ4s the electron is the majority carrierand the hole is the

minoritycarrier. his sit&ation is reversed in the ** BJ.By contrast, all filed/effect transistors (J34s and %;S34s) depend &pon themovement of only one carrier, either electrons or hoes, depending on #hether they are /channel or */channel devices, so they are technically unipolar devices. (or more on this,

see -lectronics No,, ?:! */type and the >?:- **/type. Both are packaged in small, three/pinplastic cylindrical ;/> packages #ith flat faces.

Brief definitions for the parameters in able 1 are7ower dissipationis the ma5im&m meanpo#er that the BJ can dissipate #itho&t ane5ternal heatsink, at normal room temperat&re, >M8. is the#ain-bandwidth product, the fre9&ency at #hich the common/emitterfor#ard c&rrent gain is &nity. =8B;is collect/base voltage (emitter open), the ma5im&m voltage that can beimpressed across collect and emitter #hen the base is open. =83;is the collector/emitter voltage (base open), the ma5im&m voltage that can beimpressed across collector and emitter #hen the base is open.


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8(ma5)is the ma5im&m meanc&rrent that sho&ld be allo#ed to flo# thro&gh thecollector terminal of the BJ.


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h3is the D8 for#ard/c&rrent gain, the ratio ofD8 collector c&rrent to D8 base c&rrent for atransistor in a common/emitter config&ration.

he gain/band#idth prod&ct, the fre9&ency at #hich

common emitter for#ard c&rrent gain is &nity, appliesin the follo#ing #ay7 if a transistor in a voltagefeedback circ&it has a voltage gain of O 1::, itsband#idth #ill be one h&ndredth of gain band#idthval&e. 'o#ever if the voltage gain is red&ced to O 1:,the band#idth #ill increase to that val&e divided by1:.

Transistor haracteristics:

< kno#ledge of thestaticand dynamiccharacteristics of BJ4s #ill be &sef&l in obtainingthe optim&m performance from the device. $taticcharacteristics are val&es obtained#hen the device is in a test circ&it and operated &nder D8 conditions #ith the

meas&rements made by an ohmmeter.ig&re !/asho#s the static e9&ivalent circ&it of an * BJ, and ig&re ?/bsho#s thestatic e9&ivalent of a ** BJ. 3ach device can be considered as e9&ivalent to a pair ofreverse/biased +ener diodes in series bet#een the collector and emitter terminals, #ith thebase terminal connected to the common point bet#een to the t#o +eners.35amination of igs. 1/aand >/asho#s that each BJ is really t#o diodes7 the emitterand base form one * diode #ith an emitter/base 0&nction, and the base and collectorform a second * diode #ith a base/collector 0&nction. When these diodes are properlybiased, they reach an avalanche or +ener breakdo#n point.n most small/signal BJ4s, the base/to/emitter 0&nction has a typical +ener val&e of to1: volts, #hile the base/to/collector 0&nction has a typical +ener val&e of >: to 1:: volts.

h&s, if the base/to/emitter 0&nction of the BJ is for#ard biased, it e5hibits thecharacteristics of a +ener diode. he for#ard/biased 0&nction in a silicon BJ blocksvirt&ally all c&rrent &ntil the bias voltage rises to abo&t -:: millivolts.Beyond that val&e the c&rrent #ill increase rapidly. When for#ard biased by a fi5edc&rrent, the for#ard voltage of the 0&nction has temperat&re coefficient of abo&t />millivolts per degree 8. When the transistor is config&red as an emitter open/circ&ited,the base/to/collector 0&nction e5hibits similar characteristics of those 0&st described//e5cept for a greater +ener val&e.f the transistor is config&red #ith its base open/circ&ited, the collector/to/emitter pathacts like a +ener diode in series #ith an ordinary diode.

6yna*ic haracteristics:he dynamic characteristics of a BJ can be better &nderstood by e5amining the typicalcommon/emitter collector characteristicsfor a small/signal silicon * transistor sho#nin ig. . Direct c&rrent collector c&rrent cis plotted on the %a5is, and D8 collector/emitter voltage =ceois plotted along the&a5is.

< family of c&rves for different val&es of D8 base c&rrent bis dra#n of ig. . Basec&rrent is plotted beca&se the BJ is a current-operateddevice.


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&nderstood by e5amining the typical common/emittercollector characteristicsfor a small/signal silicon *transistor sho#n in ig. . Direct c&rrent collectorc&rrent cis plotted on the %a5is, and D8 collector/emitter voltage =ceois plotted along the&a5is.

< family of c&rves for different val&es of D8 basec&rrent bis dra#n of ig. . Base c&rrent is plottedbeca&se the BJ is a current-operateddevice.


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When the collector/to/emitter voltage e5ceeds a fe# h&ndred millivolts, the collectorc&rrent val&e is almost directly proportional to the base c&rrent val&e. t is only slightlyaffected by the act&al collector voltage val&e. h&s, the transistor can perform as aconstant/c&rrent generator by feeding a fi5ed bias c&rrent into the base.he transistor can also perform as a linear amplifier by s&perimposing the inp&t signal on

a nominal inp&t bias c&rrent. (his #ill be disc&ssed in more detail later.)

ircuit A%%lications:

3ven a simple small/signal BJ has many applications related to its ability to amplify ors#itch. Some of the most important and practical circ&it designs are described her. Withfe# e5ceptions, all of the circ&its are based on the >?:! * transistor. (With certainminor component val&e changes, other * transistor can be s&bstit&ted.) he circ&itscan also be made #ith a ** transistor s&ch as the >?:-, if the polarities are altered.

6io&es an& S,itches:

t #as e5plained earlier that both the base/emitter and base/collector 0&nctions of a silicon

BJ can be considered e9&ivalent to a +ener diode.


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7inear A*%lifiers:

< BJ can f&nction as a linear c&rrent or voltage amplifier if as&itable bias c&rrent is fed into its base, and the o&tp&t signal isapplied bet#een a s&itable pair of terminals. < transistor amplifier

can be config&red for any of three operating modes7 common-emitter(ig. ), common-base(ig. 1:), and common-collector(ig.11). 3ach of these modes offers a &ni9&e set of characteristics.n the common/emitter circ&it of ig. , load resistor 2lis connectedbet#een the collector and the positive s&pply, and a bias c&rrent isfed into the base thro&gh 2b. he val&e of 2b#as selected so that thecollector takes on a 9&iescent val&e of abo&t half the s&pply voltage(to provide ma5im&m &ndistorted signal s#ings).he inp&t signal in the form of a sine #ave is applied bet#een thebase and the emitter thro&gh 81. he circ&it inverts the phase of the

inp&t signal, #hich appears as an o&tp&t bet#een the collector and emitter. his circ&it is

characteri+ed by a medi&m/val&e inp&t impedance and a high overall voltage gain.he inp&t impedance of this amplifier is bet#een :: and >::: ohms, and the loadimpedance e9&als 2l. =oltage gain is the change in collector voltage divided by thechange in base voltage (from 1:: to abo&t 1:::). 8&rrent gain is the change in collectorc&rrent divided by the change in base c&rrent of 'fe.n the common/base linear amplifier circ&it of ig&re 1:, the base is biased thro&gh 2band


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inp&t signal to the base. ig&re 1!sho#s ho# this partic&lar circ&it is str&ct&red.he emitter/follo#er circ&its of igs. 1>to 1! cansourceor feed relatively high c&rrentsinto an e5ternal load thro&gh the emitter of the transistor. 'o#ever, those circ&its cannotsinkor absorb high c&rrents that are fed to the emitter from an e5ternal voltage so&rcebeca&se the emitter is reverse/biased &nder this condition. and diodes D1 and D>.ransistor P1 can provided large so&rce c&rrents, and P> can absorb large sink c&rrents.


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Phase S%litters:

ransistor linear amplifiers can be &sed in active filters or oscillators by connectings&itable feedback net#orks bet#een their inp&ts and o&tp&ts. *hase splitting is another&sef&l linear amplifier application. t provides a pair of o&tp&t signals from a single inp&tsignal7 one is in phase #ith the inp&t phase, and the other is inverted or 1C:M o&t of phase.ig. 1-and 1" sho# these alternative circ&its.n the circ&it sho#n in ig. 1, the BJ is connected as a common/emitter amplifier #ithnearly 1::N negative feedback applied thro&gh emitter resistor 2!. t has the same val&eas collector resistor 2?. his config&ration provides a &nity/gain inverted #aveform ato&tp&t 1 and a &nity/gain non/inverted #aveform at o&tp&t >.he phase/splitter circ&it sho#n in ig. 1-is kno#n as a lon#-tailed pairbeca&se the t#oBJ4s share common/emitter feedback resistor 2".


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+ultivibrators:

ig&res 1" to >:sho# BJ4s in the fo&r different kinds of m&ltivibrator circ&it7 bistable!astable! monostable! and $chmitt tri##er.he bistable m&ltivibrator is a simple electronic circ&it that has t#o stable states. t ismore often kno#n as theflip-flop, b&t is also called a binary multivibrator, or an)ccles-*ordan circuit. he circ&it is s#itched from one state to the other by a p&lse or other

e5ternal signal. t maintains its state to the other by a p&lse or other e5ternal signal. tmaintains its state indefinitely &nless another inp&t signal is received.ig&re 1"is a simple, man&ally/triggered, cross/co&pled bistable m&ltivibrator. he basebias of each transistor is obtained from the collector of the other transistor. h&s onetransistor a&tomatically t&rns +,,#hen the other t&rns +, and this cycle can becontin&ed in definitely as long as it is po#ered.he o&tp&t of the m&ltivibrator in ig. 1"can be driven lo# by t&rning off transistor P>#ith s#itch S>. he circ&it remains locked or stable in this state &ntil transistor P1 is


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t&rned off #ith s#itch S1. if transistor P1 is t&rned off #ith s#itch S1. t then ret&rns to tits originalstate.he p&lse d&ration time of the monostable m&ltivibrator can be determined from thee9&ation7 T 4 089/ RWhere7 is in microseconds, 2 is in ohms, and 8 is in microfarads.%onostable m&ltivibrators are &sed as p&lse generators and #eep generators for cathode/ray t&bes.


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ig&re 1is the schematic for an astable multivibratoror free/r&nning, s9&are/#aveoscillator. he transistors are in a common/emitter config&ration so that the o&tp&t of oneis fed directly to the inp&t of the other. #o resistance/capacitor net#orks, 2? and 81,and 2> and 8>, determine the oscillation fre9&ency.he o&tp&t of each transistor is 1C:M o&t of phase #ith the inp&t. . t is again inverted at the collector of P> and therefore ret&rns to the base of P1 inits original phase. his prod&ces positive feedback, res&lting in s&stained oscillation.he astable m&ltivibrator is fre9&ently &sed as an a&dio oscillator, b&t is not &s&ally &sedin radio/fre9&ency circ&its beca&se its o&tp&t is rich in harmonics.ig&re >:is a schematic for a $chmitt Tri##er, a form of bistable m&ltivibrator circ&it. tprod&ces rectang&lar #aves, regardless of the inp&t #aveform. he circ&it is #idely &sedto convert sine #aves to s9&are #aves #here these is a re9&irement for a train of p&lses#ith constant amplit&de.he Schmitt trigger circ&it remains off &ntil the rising inp&t #aveform crosses the presetthreshold trigger/voltage level set by the val&e of resistors 21 and 2>. When transistor

P1 is s#itched 4on4, transistor P> is 4off4 and, the Schmitt trigger4s o&tp&t voltage risesabr&ptly.When the inp&t signal falls back belo# its drop/o&t level, P1 s#itches 4off4 and P>s#itches 4on4. he o&tp&t voltage of the Schmitt trigger drops to +ero almost instantly.his cycle of events #ill then be repeated in definitely, as long as the inp&t signal isapplied.

8ontin&e #ith ransistor &torial Part 3

8opyright I>::- / ony van 2oon, =


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Part 3:

"Learn about common-collector bipolar junction (*T

transistor amplifiers and apply this knowled#e to thecircuits that you desi#n."

2e#ritten by ony van 2oon (=


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'igh inp&t impedance $o# o&tp&t impedance =oltage gain appro5imately e9&al to &nity 8&rrent gain appro5imately e9&al to hfe

By contrast, notice that #hile the common/emitter andcommon/base amplifiers provide high voltage gain, they offeronly lo#/to medi&m inp&t impedance. he applications for

these circ&its are governed by these characteristics.

6i"ital A*%lifiers:

ig&re >is the schematic for a simple * common/collectorLemitter/follo#er digitalamplifier. he inp&t signal for this circ&it is a p&lse that s#ings bet#een +ero volts andthe positive s&pply voltage. When the inp&t of this circ&it is at +ero volts and thetransistor is f&lly c&t off, and the amplifier4s o&tp&t is also +ero volts//indication +erovoltage phase shift.

When an inp&t voltage e5ceeding Q-:: millivolts (the minim&m for#ard bias for t&rn/on)appears across the inp&t terminals, the transistor t&rns on and c&rrent $flo#s in loadresistor 2$, generating an o&tp&t voltage across 2$. nherent negative feedback ca&ses theo&tp&t voltage to ass&me a val&e thatfollowsthe inp&t voltage. he inp&t voltage is e9&alto the inp&t voltage min&s the voltage drop across the base/emitter 0&nction (-::millivolt).

n ig. > schematic, the inp&t (base) c&rrent is calc&lated as7

Beca&se the circ&it can have a ma5im&m voltage gain of one, it presents an inp&t

impedance calc&lated as7

nserting the val&es sho#n in ig. > yields7he circ&it has an o&tp&t impedance that appro5imately e9&als the val&e of the inp&tsignal so&rce impedance (2S) Beca&se the circ&it sho#n in ig. ? e5hibits all of thecommon/collector amplifier characteristics previo&sly disc&ssed, it behaves like a &nity/gain buffercirc&it. f high/fre9&ency p&lses are introd&ced at its inp&t, the trailing edge ofthe o&tp&t p&lse #ill sho# the time constant decay c&rve sho#n in ig. ?. his responseis ca&sed by stray capacitance 8S(representing #ith the circ&it4s load resistance.When the leading edge of the inp&t p&lse s#itches high, P1 s#itches on and rapidlysourcesor feeds a charge c&rrent to stray capacitance 8S, th&s prod&cing and o&tp&t p&lse#ith a sharp leading edge. 'o#ever, #hen the trailing edge of the inp&t goes lo#, P1s#itches off and effective capacitor 8Sis &nable to discharge orsinkthro&gh thetransistor.'o#ever, 8Scan discharge thro&gh lead resistor 2$. hat discharge #ill follo# ane5ponential decay c&rve #ith the time to discharge to the ?"N level e9&al to the prod&ctof 8$and 2$.Relay 6rivers:

he base digital or s#itching circ&it of ig. > can be p&t to #ork driving a #ide variety ofresistive loads s&ch as incandescent filament lamps, $3D4s, or resistors. f the circ&it is todrive an ind&ctive load s&ch as a coil, transformer, motor, or speaker, a diode m&ch be


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incl&ded to limit an inp&t/voltage s&rge that co&ld destroy thetransistor #hen the s#itch is closed.he schematic in ig. !is a modification of ig. ? #ith theaddition of diode D1 across the load, in this case a relay coil,and s#itch S1 in the collector/base circ&it. t can act in either

the latchin#or non-latchin#modes. he relay to be act&atedeither by the inp&t p&lse or s#itch S1.2elay 2E14s contacts close and are available for s#itching

either #hen a p&lse #ith an amplit&de e9&al to the s&pply voltage is introd&ced or S1 isclosed. he relay contacts open #hen the inp&t p&lse falls to +ero or S1 is opened.*rotective diode D1 damps relay 2E14s s#itch/off voltage s&rge from s#inging belo# the+ero volt s&pply level. ;ptional diode D> can also be incl&ded to prevent this voltagefrom rising abo&t the positive po#er s&pply val&e. he addition of normally open relay >(2E>) makes the circ&it self/latching.ig&re sho#s a same relay driver circ&it organi+ed for an ** transistor, a >?:-BJ. ::('feval&e of P1). 8onsider a relay re9&ires an activating c&rrent of 1:: m< and has acoil resistance of 1>: ohms. he effective inp&t impedance of the circ&it (R in) #ill be7

1>: 5 >:: >!,::: ohms. ;nly an inp&t operating c&rrent of 1L>::of1:: milliamperes or :. milliamperes is re9&ired.8irc&it sensitivity can be f&rther increased by replacing transistor P1 #ith the Darlingtonpair of P1 and P>, as sho#n in ig. -. his circ&it represents an inp&t impedance ofabo&t 1 megohm and re9&ires an inp&t operating c&rrent of abo&t 1> microamperes (&


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n the ig. "circ&it, consider that 81 is f&lly discharged so that the 21/81 0&nction is at+ero volts and relay 2E1 is off (contacts open) #hen the po#er s&pply is connected.8apacitor 81 then charges e5ponentially thro&gh 21, and the increasing voltage is fed to

the relay circ&it thro&gh Darlington pair P1 and P>. hat ca&ses relay 2E14s contacts toclose after a time delay determined by the prod&ct of 21 and 81.8onsider that capacitor 81 in the ig. Ccirc&it is also f&lly discharged #hen the po#ers&pply is connected. he 0&nction of 21 and 81 is initially at the s&pply voltage, and therelay contact close at that moment. 8apacitor 81 then charges e5ponentially thro&gh 21,and the decaying voltage at the 21/81 0&nction appears across the coil of relay 2E1. hecontacts of 2E1 open after the delay determined by 21 and 81 times o&t.

onstant2urrent !enerators:

< BJ can serve as a constant/c&rrent generator if it is connected in the common/collector topology and the po#er s&pply and collector terminals f&nction as a constant/c&rrent path, as sho#n in ig. . he 1:::/ohm resistor 2> is the emitter load. he seriescombination of resistor 21 and +ener diode D1 applies a fi5ed .-/volt reference to thebase of P1.


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he is a -::/millivolt (:.-=) base/to/emitter drop across P1, so volts is developedacross emitter resistor 2>. .8onse9&ently, the val&e of the c&rrent can be changed by varying either of theseparameters.

he ig. 1:circ&it takes this concept a step f&rther. t can be seen, for e5ample, that thecirc&it of ig. #as invertedto give a gro&nd/referenced, constant/c&rrent o&tp&t.


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he most important feat&re of the constant/c&rrent circ&it is its highdynamic o&tp&t impedance//typically h&ndreds of kilo/ohms. heprecise magnit&de of constant c&rrent is &s&ally &nimportant inpractical circ&its. he circ&its sho#n in ig. 1: and 11#ill #orksatisfactorily in many practical applications.

f more precise c&rrent generation is re9&ired, the characteristics ofthe reference voltage of these circ&its can be improved to eliminatethe effects of po#er so&rce variations and temperat&re changes.< simple #ay to improve the circ&its in igs. and 1: is sho#n in

ig. 11. 2esistor 21 in both circ&its can be replaced #ith a /milliampere constantc&rrent generator. (he symbol for a constant/c&rrent generator is a pair of overlappingcircles.) With a constant/c&rrent generator installed, the c&rrent thro&gh +ener diode D1and the voltage across it is independent of variations in the s&pply voltage.r&e high precision can be obtained if the ind&stry standard reference +ener diode D1 isreplaced #ith one having a temperat&re coefficient of > millivoltsLM8 to match th base/to/emitter temperat&re coefficient of transistor P1. 'o#ever, if a +ener diode #ith those

characteristics cannot be located, satisfactory res&lts can be obtained by s&bstit&ting afor#ard/biased light/emitting diode, as sho#n in ig. 1>.he voltage drop across $3D1 is abo&t > volts, so only abo&t 1.! volts appears acrossemitter resistor 21. f the val&e of 21 is red&ced from 1::: to >": ohms, the constant/c&rrent o&tp&t level can be maintained at milliamperes.

Analo" A*%lifiers:

he common/collectorLemitter/follo#er amplifier can amplify


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ig. 1!has an inp&t impedance of abo&t : kilohm.Both the ig. 1? and 1! circ&its offer a voltage gain that is slightly less than &nityA thetr&e gain is given by7Where Rb >L3ohms and 3is the emitter c&rrent in milliamperes. With an operatingc&rrent of 1 milliampere, these circ&its provide voltage gains of :. #hen the R load !."

kilohm, or :." #hen the load is 1.: kilohm. he significance of these gain fig&res #illbe disc&ssed shortly.


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'ootstra%%in":

he relatively lo# inp&t impedance of the circ&it in ig. 1!circ&itcan be increased significantly by bootstrappin#as ill&strated in ig.1. he !"/kilohm resistor 2? is located bet#een the 21/2>0&nction and the base of transistor P1, and the inp&t signal is fed to

P14s base thro&gh capacitor 81.otice, ho#ever, that P14s o&tp&t signal is fed back to the 21/2>0&nction thro&gh 8>, so that almost identical signal voltages appearat both ends of 2?. 8onse9&ently, very little signal c&rrent flo#s in2?. he inp&t signal sees far greater impedance that the tr&e

resistance val&e.o make this point clearer, consider that the emitter/follo#er circ&it in ig. 1has aprecise voltage gain of &nity. n this condition, identical signal voltages #o&ld appear atthe t#o ends of 2?, so no signal c&rrent #o&ld flo# in this resistor, making it appeare9&al to 2in, or 1 megohm.*ractical emitter/follo#er circ&its provide a voltage gain that is slightly less than &nity.

he precise gain that determines the resistor amplification factor, or


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he circ&it in ig. 1Ccirc&it has a d&al or split po#er s&pply, and its o&tp&t is direct/

co&pled to a gro&nded load. he series connected * and ** transistors are biased ata 9&iescent +ero volts val&e thro&gh the voltage divider formed #ith resistors 21 and2> and diodes D1 and D>. 3ach transistor is for#ard biased slightly #ith silicon diodesD1 and D>. hose diodes have characteristics that are similar to those of the transistorbase/emitter 0&nctions.

8apacitor 8> ass&res that identical inp&t signals are applied to each transistor base, andemitter resistors 2? and 2! protect the transistor against e5cessive o&tp&t c&rrents.


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ransistor P1 in ig. 1Cso&rces c&rrent into the load #hen the inp&t goes positive, andtransistor P> sinks load c&rrent #hen the inp&t goes negative. otice that inp&t capacitor81 is non/polari+ed.ig&re 1sho#s an alternative to the circ&it of ig. 1C designed for operation from a

single/ended po#er s&pply and an in igs. 1C and 1 are slightly for#ard biased bysilicon diodes D1 and D> to eliminate crossover distortion problems. ;ne diode isprovided for each transistor.

f these circ&its are modified by s&bstit&ting Darlington pairs, fo&r biasing diodes #ill bere9&ired. n those variations, a single transistor amplifier diode stage replaces the fo&rdiodes, as sho#n in ig. >:.he collector/to/emitter voltage of P in ig. >: e9&als the base/to/emitter voltage drop

across P (abo&t -:: millivolts, more or less) m&ltiplied by (2? Q 2!)L2!. h&s, iftrimmer potentiometer 2? is set to +ero ohms, abo&t -:: millivolts are developed acrossP, #hich then behaves as a silicon diode. 'o#ever, if 2? is set to its ma5im&m val&e of!" kilohm, abo&t ?.- volts is developed across P, #hich then behaves like si5 seriesconnected silicon diodes. rimmer 2? can set the voltage drop across P precisely as#ell as ad0&st the 9&iescent c&rrent val&es of the P>/P? stage.

8ontin&e #ith ransistor &torial Part 1: ;Po,er A*%lifiers;



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Transistors TutorialPart 4:

by ony van 2oon (=


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he 8lass < circ&it amplifies a&dio o&tp&t #ith minim&mdistortion, b&t transistor P1 cons&mes c&rrent contin&o&sly//even in the 9&iescent state//giving it lo# efficiency./b.

lass A' Fun&a*entals:


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c&rrent flo#s in the speaker &nder 9&iescent conditions.

evertheless, a slight for#ard bias can be applied #ithtrimmer potentiometer 2? so that P1 and P> pass modest9&iescent c&rrents to prevent crossover distortion. dentical

inp&t signals are applied thro&gh 81 and 8> to the base of theemitter follo#ers, #hich avoid a split/phase drive.When an inp&t signal is applied to the ig. ! circ&it, thepositive s#ing drives ** P> off #hile driving * P1 on.ransistor P1 acts as c&rrent so&rce #ith a very lo# o&tp&t(emitter) impedance if feeds a faithf&l &nity/gain copy of theinp&t voltage signal to the speaker. he transistorcharacteristics have little or no effect on this response.Similarly, negative s#ings of the inp&t signal drive P1 off andP> on. Beca&se P> is a ** BJ, it becomes a c&rrent sink#ith minimal inp&t (emitter) impedance. t also prod&ces a

faithf&l &nity/gain copy of the voltage signal to the speaker,again #ith P>4s characteristics having little or no effect on thecirc&it4s response., and neither inp&t nor o&tp&t transformers arere9&ired. %odification of this circ&it, as sho#n in igs. /aand b, #ork from single ended po#er s&pplies. n ig. /a, oneside of the speaker is connected to the amplifier thro&gh high/val&e blocking capacitor 8? and, and the other end isconnected to gro&ndA in ig. /b, one side is connected to 8?and the other side is connected to the positive s&pply.


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*ractical amplifiers incl&de a pre/set trimmer potentiometer in series#ith D1 and D>. his component makes it possible to ad0&st biasedvoltage over a limited range. $o#/val&e resistors 2! and 2 in series#ith the emitters of P> and P? provide some negative D8 feedback.he impedance of the ig. ! circ&it e9&als the prod&ct of the speaker

load impedance and the c&rrent gain of either P1 or P>. he circ&itcan be improved by replacing transistors P1 and P> #ith Darlingronpairs #hich #ill significantly increase the circ&it4s inp&t impedanceand increase the amplifier4s collector load capacity.ig&res " to sho# three different #ays of modifying the ig. -circ&it by replacing individ&al transistors #ith Darlington pairs. ore5ample, in ig. ", transistors P> and P? form a Darlingron *pair, and P! and P form a darlington ** pair. here are fo&r base/emitter 0&nctions bet#een the bases of P> and P!, and the o&tp&tcirc&it is biased #ith a string of fo&r silicon diodes, D1 and D!, in

series to compensate for the Darlingron pairs.

ig&re C, P> and P? are a Darlington * pair, b&t P! and P are a complementary pairof common/emitter amplifiers. hey operate #ith 1::N negative feedback, and provide&nity/voltage gain and very high inp&t impedance. his0uasi-complementaryo&tp&t stageis probably the most pop&lar 8lass , and D?.inally, in ig&re , both pairs P> and P? and P! and P are complementary pair of&nity/gain, common/emitter amplifiers #ith 1::N negative feedback. Beca&se the pairsprod&ce o&tp&ts that are mirror images of each other, the circ&it has a complementaryo&tp&t stage. otice that this circ&it has only t#o silicon biasing diodes, D1 and D>.

A*%lifie& 6io&es:

he circ&its in igs. - to incl&de strings of t#o to fo&r silicon biasing diodes. 3ach ofthose strings can be replaced by single transistor and t#o resistors config&red as anamplified diode, as sho#n in igs. 1:.he o&tp&t voltage of the circ&it, =o&tcan be calc&lated from the form&la7 =o&t =B35 21Q 2>L2>f resistor 21 is replaced by a short circ&it, the circ&it4s o&tp&t #ill be e9&al to the base/emitter 0&nction diode voltage of P1 (=B3). he circ&it #ill then have the thermalcharacteristics of a discrete diode.


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f resistor 21 e9&als 2>, the circ&it #ill act like t#o series/connected diodes, and if 21e9&als three times 2>, the circ&it #ill act like fo&r series/connected diodes, and so on.herefore, the circ&it in igs. 1: can be made to sim&late any desired #hole or fractionaln&mber of series/connected diodes, depending on ho# the 21L2> ratios are ad0&sted.ig&re 11sho#s ho# the circ&it in ig. 1: can be modified to act as a f&lly ad0&stable


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amplifier diode, #ith an o&tp&t variable from 1 to ." timesthe base/emitter 0&nction voltage (=B3)

'ootstra%%in":

he main p&rpose of the P1 driver stage in ig. -, the base complementary amplifier, isto give the amplifier significant voltage gain.


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a res&lt, an &ndefined signal voltage appears at the lo# end ofresistor 2>, and :. times that &ndefined voltage appears at thetop of 2>. n other #ords, only one/tenth of the &nkno#n signalvoltage is developed across 2>. herefore, it passes one/tenthof the signal c&rrent that #o&ld be e5pected from a 1/kilohm

resistor.his means that the is tentimes greater (1:/kilohms) than its D8 val&e, and the signalvoltage gain is increased correspondingly. n practical circ&its,bootstrapping permits the effective voltage gain and collectorload impedance of P1 to be increased by the factor of abo&tt#enty.ig. 1?is the schematic for an alternative version of ig. 1>#itho&t one resistor and one capacitor. n this circ&it. S*F21 ispart of P14s collector load, and it is bootstrapped thro&ghcapacitor 8>.

allo#s the circ&it to #ork over a #ide s&pplyvoltage range. he feedback resistors can be toincrease the gain and inp&t impedance, b&t at the e5pense of increased signal distortion.ransistor P1 can be replaced #ith a Darlington pair if very high inp&t impedance isdesired.


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po#er amplifiers.An %o,er a*%lifier:

mprovements in the po#er/handling capabilities of monolithicintegrated circ&its have permitted po#er amplifier to beintegrated on a single silicon s&bstrate or chip. he techni9&es

for designing integrated circ&it po#er amplifiers are similar tothose for discrete device circ&its. t t&rns o&t that thesimilarities bet#een discrete and 8 po#er amplifier designsare closer than for most other linear circ&its.ig&re 1"is a simplified circ&it diagram for the $%?C:, an 8po#er amplifier, dra#n in the man&fact&rer4s data book style.

he $%?C: #as developed by ational Semicond&ctor 8orporation for cons&merapplications. t feat&res an internally fi5ed gain of : (?! dB) and an o&tp&t thata&tomatically centers itself at one/half of the s&pply voltage. (resistor 21 is

formed by t#o >/kilohm resistors and 2> has a val&e of >/kilohms). egative D8feedback, thro&gh resistor 2>, balances the differential stage #ith the o&tp&t at halfs&pply, beca&se 21 2>.he o&tp&t of the differential amplifier stage is direct co&pled into the base of P1>, #hichis a common/emitter, voltage/gain amplifier #ith a constant c&rrent/so&rce load provideby P11. nternal compensation is provided by the pole/splitting capacitor 84. *ole/splitting compensation permits #ide po#er band#idth (1:: F'+ at > #atts, C ohms).he collector signal of P1> is fed to o&tp&t pin C of the 8 thro&gh the combination of


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emitter/co&pled P" and the 9&asi/complementary pair emitterfollo#ers PC and P. he short/circ&it c&rrent is typical 1.?amperes.

8ontin&e #ith ransistor &torial Part ::- / ony van 2oon, =


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"Learn about audio power amplifiers and apply this

knowled#e to your circuits desi#ns and e/periments."


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!N at one/tenth of its ma5im&m o&tp&t po#er level.< typical 8lass B amplifier is sho#n in ig. >/a. t has a pairof BJs, P1 and P>, operating 1C:M o&t/of/phase driving acommon o&tp&t load, in this e5ample another speaker. n thistopology, the BJs operated as common/emitter amplifiers

drive the speaker thro&gh p&sh/p&ll transformer >. < phase/splitting transformer 1, provides the inp&t drives for P1 andP> 1C:M o&t/of/phase.he o&tstanding characteristic of any 8lass B amplifier isthat both transistors are biased off &nder 9&iescent conditionsbeca&se they are operated #itho&t base bias.


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P1 and P> pass modest 9&iescent c&rrents to preventcrossover distortion. dentical inp&t signals are appliedthro&gh 81 and 8> to the base of the emitter follo#ers, #hichavoid a split/phase drive.When an inp&t signal is applied to the ig. ! circ&it, the

positive s#ing drives ** P> off #hile driving * P1 on.ransistor P1 acts as c&rrent so&rce #ith a very lo# o&tp&t(emitter) impedance if feeds a faithf&l &nity/gain copy of theinp&t voltage signal to the speaker. he transistorcharacteristics have little or no effect on this response.Similarly, negative s#ings of the inp&t signal drive P1 off andP> on. Beca&se P> is a ** BJ, it becomes a c&rrent sink#ith minimal inp&t (emitter) impedance. t also prod&ces afaithf&l &nity/gain copy of the voltage signal to the speaker,again #ith P>4s characteristics having little or no effect on thecirc&it4s response.

, and neither inp&t nor o&tp&t transformers arere9&ired. %odification of this circ&it, as sho#n in igs. /aand b, #ork from single ended po#er s&pplies. n ig. /a, oneside of the speaker is connected to the amplifier thro&gh high/val&e blocking capacitor 8? and, and the other end isconnected to gro&ndA in ig. /b, one side is connected to 8?and the other side is connected to the positive s&pply.


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he impedance of the ig. ! circ&it e9&als the prod&ct of the speakerload impedance and the c&rrent gain of either P1 or P>. he circ&itcan be improved by replacing transistors P1 and P> #ith Darlingronpairs #hich #ill significantly increase the circ&it4s inp&t impedanceand increase the amplifier4s collector load capacity.

ig&res " to sho# three different #ays of modifying the ig. -circ&it by replacing individ&al transistors #ith Darlington pairs. ore5ample, in ig. ", transistors P> and P? form a Darlingron *pair, and P! and P form a darlington ** pair. here are fo&r base/emitter 0&nctions bet#een the bases of P> and P!, and the o&tp&tcirc&it is biased #ith a string of fo&r silicon diodes, D1 and D!, inseries to compensate for the Darlingron pairs.ig&re C, P> and P? are a Darlington * pair, b&t P! and P are acomplementary pair of common/emitter amplifiers. hey operate#ith 1::N negative feedback, and provide &nity/voltage gain and

very high inp&t impedance. his0uasi-complementaryo&tp&t stage is probably the most

pop&lar 8lass , and D?.inally, in ig&re , both pairs P> and P? and P! and P are complementary pair of&nity/gain, common/emitter amplifiers #ith 1::N negative feedback. Beca&se the pairsprod&ce o&tp&ts that are mirror images of each other, the circ&it has a complementaryo&tp&t stage. otice that this circ&it has only t#o silicon biasing diodes, D1 and D>.

A*%lifie& 6io&es:



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f resistor 21 e9&als 2>, the circ&it #ill act like t#o series/connected diodes, and if 21e9&als three times 2>, the circ&it #ill act like fo&r series/connected diodes, and so on.herefore, the circ&it in igs. 1: can be made to sim&late any desired #hole or fractionaln&mber of series/connected diodes, depending on ho# the 21L2> ratios are ad0&sted.ig&re 11sho#s ho# the circ&it in ig. 1: can be modified to act as a f&lly ad0&stableamplifier diode, #ith an o&tp&t variable from 1 to ." times the base/emitter 0&nctionvoltage (=B3)


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'ootstra%%in":



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signal c&rrent that #o&ld be e5pected from a 1/kilohm resistor.his means that the is tentimes greater (1:/kilohms) than its D8 val&e, and the signalvoltage gain is increased correspondingly. n practical circ&its,bootstrapping permits the effective voltage gain and collector

load impedance of P1 to be increased by the factor of abo&tt#enty.ig. 1?is the schematic for an alternative version of ig. 1>#itho&t one resistor and one capacitor. n this circ&it. S*F21is part of P14s collector load, and it is bootstrapped thro&ghcapacitor 8>. allo#s the circ&it to #ork over a #ide s&pplyvoltage range. he feedback resistors can be toincrease the gain and inp&t impedance, b&t at the e5pense of increased signal distortion.

ransistor P1 can be replaced #ith aDarlington pair if very high inp&timpedance is desired.


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techni9&es for designing integrated circ&it po#er amplifiers are similar to those fordiscrete device circ&its. t t&rns o&t that the similarities bet#een discrete and 8 po#eramplifier designs are closer than for most other linear circ&its.ig&re 1"is a simplified circ&it diagram for the $%?C:, an 8 po#er amplifier, dra#n inthe man&fact&rer4s data book style. he $%?C: #as developed by ational

Semicond&ctor 8orporation for cons&mer applications. t feat&res an internally fi5ed gainof : (?! dB) and an o&tp&t that a&tomatically centers itself at one/half of the s&pplyvoltage. (resistor 21 isformed by t#o >/kilohm resistors and 2> has a val&e of >/kilohms). egative D8feedback, thro&gh resistor 2>, balances the differential stage #ith the o&tp&t at halfs&pply, beca&se 21 2>.he o&tp&t of the differential amplifier stage is direct co&pled into the base of P1>, #hichis a common/emitter, voltage/gain amplifier #ith a constant c&rrent/so&rce load provideby P11. nternal compensation is provided by the pole/splitting capacitor 84. *ole/

splitting compensation permits #ide po#er band#idth (1:: F'+ at > #atts, C ohms).he collector signal of P1> is fed to o&tp&t pin C of the 8 thro&gh the combination ofemitter/co&pled P" and the 9&asi/complementary pair emitter follo#ers PC and P. heshort/circ&it c&rrent is typical 1.? amperes.

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by ony van 2oon (=


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distortion, b&t transistor P1 cons&mes c&rrent contin&o&sly//even in the 9&iescent state//giving it lo# efficiency./b.

lass A' Fun&a*entals:


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evertheless, a slight for#ard bias can be applied #ithtrimmer potentiometer 2? so that P1 and P> pass modest9&iescent c&rrents to prevent crossover distortion. denticalinp&t signals are applied thro&gh 81 and 8> to the base of theemitter follo#ers, #hich avoid a split/phase drive.

When an inp&t signal is applied to the ig. ! circ&it, thepositive s#ing drives ** P> off #hile driving * P1 on.ransistor P1 acts as c&rrent so&rce #ith a very lo# o&tp&t(emitter) impedance if feeds a faithf&l &nity/gain copy of theinp&t voltage signal to the speaker. he transistorcharacteristics have little or no effect on this response.Similarly, negative s#ings of the inp&t signal drive P1 off andP> on. Beca&se P> is a ** BJ, it becomes a c&rrent sink#ith minimal inp&t (emitter) impedance. t also prod&ces afaithf&l &nity/gain copy of the voltage signal to the speaker,again #ith P>4s characteristics having little or no effect on the

circ&it4s response., and neither inp&t nor o&tp&t transformers arere9&ired. %odification of this circ&it, as sho#n in igs. /aand b, #ork from single ended po#er s&pplies. n ig. /a, oneside of the speaker is connected to the amplifier thro&gh high/val&e blocking capacitor 8? and, and the other end isconnected to gro&ndA in ig. /b, one side is connected to 8?and the other side is connected to the positive s&pply.


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val&e resistors 2! and 2 in series #ith the emitters of P> and P?provide some negative D8 feedback.he impedance of the ig. ! circ&it e9&als the prod&ct of the speakerload impedance and the c&rrent gain of either P1 or P>. he circ&itcan be improved by replacing transistors P1 and P> #ith Darlingron

pairs #hich #ill significantly increase the circ&it4s inp&t impedanceand increase the amplifier4s collector load capacity.ig&res " to sho# three different #ays of modifying the ig. -circ&it by replacing individ&al transistors #ith Darlington pairs. ore5ample, in ig. ", transistors P> and P? form a Darlingron *pair, and P! and P form a darlington ** pair. here are fo&r base/emitter 0&nctions bet#een the bases of P> and P!, and the o&tp&tcirc&it is biased #ith a string of fo&r silicon diodes, D1 and D!, inseries to compensate for the Darlingron pairs.ig&re C, P> and P? are a Darlington * pair, b&t P! and P are a

complementary pair of common/emitter amplifiers. hey operate #ith 1::N negative

feedback, and provide &nity/voltage gain and very high inp&t impedance. his0uasi-complementaryo&tp&t stage is probably the most pop&lar 8lass , and D?.inally, in ig&re , both pairs P> and P? and P! and P are complementary pair of&nity/gain, common/emitter amplifiers #ith 1::N negative feedback. Beca&se the pairsprod&ce o&tp&ts that are mirror images of each other, the circ&it has a complementaryo&tp&t stage. otice that this circ&it has only t#o silicon biasing diodes, D1 and D>.

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f resistor 21 e9&als 2>, the circ&it #ill act like t#o series/connected diodes, and if 21e9&als three times 2>, the circ&it #ill act like fo&r series/connected diodes, and so on.herefore, the circ&it in igs. 1: can be made to sim&late any desired #hole or fractionaln&mber of series/connected diodes, depending on ho# the 21L2> ratios are ad0&sted.ig&re 11sho#s ho# the circ&it in ig. 1: can be modified to act as a f&lly ad0&stableamplifier diode, #ith an o&tp&t variable from 1 to ." times the base/emitter 0&nctionvoltage (=B3)


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'ootstra%%in":



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signal c&rrent that #o&ld be e5pected from a 1/kilohm resistor.his means that the is tentimes greater (1:/kilohms) than its D8 val&e, and the signalvoltage gain is increased correspondingly. n practical circ&its,bootstrapping permits the effective voltage gain and collector

load impedance of P1 to be increased by the factor of abo&tt#enty.ig. 1?is the schematic for an alternative version of ig. 1>#itho&t one resistor and one capacitor. n this circ&it. S*F21is part of P14s collector load, and it is bootstrapped thro&ghcapacitor 8>. allo#s the circ&it to #ork over a #ide s&pplyvoltage range. he feedback resistors can be toincrease the gain and inp&t impedance, b&t at the e5pense of increased signal distortion.

ransistor P1 can be replaced #ith aDarlington pair if very high inp&timpedance is desired.


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techni9&es for designing integrated circ&it po#er amplifiers are similar to those fordiscrete device circ&its. t t&rns o&t that the similarities bet#een discrete and 8 po#eramplifier designs are closer than for most other linear circ&its.ig&re 1"is a simplified circ&it diagram for the $%?C:, an 8 po#er amplifier, dra#n inthe man&fact&rer4s data book style. he $%?C: #as developed by ational

Semicond&ctor 8orporation for cons&mer applications. t feat&res an internally fi5ed gainof : (?! dB) and an o&tp&t that a&tomatically centers itself at one/half of the s&pplyvoltage. (resistor 21 isformed by t#o >/kilohm resistors and 2> has a val&e of >/kilohms). egative D8feedback, thro&gh resistor 2>, balances the differential stage #ith the o&tp&t at halfs&pply, beca&se 21 2>.he o&tp&t of the differential amplifier stage is direct co&pled into the base of P1>, #hichis a common/emitter, voltage/gain amplifier #ith a constant c&rrent/so&rce load provideby P11. nternal compensation is provided by the pole/splitting capacitor 84. *ole/

splitting compensation permits #ide po#er band#idth (1:: F'+ at > #atts, C ohms).he collector signal of P1> is fed to o&tp&t pin C of the 8 thro&gh the combination ofemitter/co&pled P" and the 9&asi/complementary pair emitter follo#ers PC and P. heshort/circ&it c&rrent is typical 1.? amperes.

8ontin&e #ith ransistor &torial Part


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Part 5:

by Tony van Roon (VA3AVR)

"Learn about the audio amplifiers in stereos! tuners! tape1cassette! and

players! and apply your knowled#e to e/periments or desi#ns."


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ransistors are the key components in many differentkinds of a&dio preamplifiers, amplifiers, and tone/

control circ&its. 2ecent articles in this series have disc&ssed theoperation principles and applications for discrete bipolar 0&nction

transistors (BJ). 3arlier articles have covered s&ch s&b0ects aslo#/po#er amplifier circ&its, m&ltivibrators, and oscillators.

Au&io A*%lifier 'asics:

< modern stereo amplifier system has t#o closely matched high/fidelity a&dio amplifierchannels. ypically each of those channels offers s#itch/selectable inp&ts for s&ch signalso&rces as a t&ner, tape/player, 8D/player, =, %S, etc. 3ach also provides a singleo&tp&t signal to a high/po#er lo&dspeaker. o analy+e one of those systems, it is &sef&l todivide the system into three f&nctional circ&it blocks, as sho#n in ig. 1.

he first of these blocks is the selectorLpreamplifier. t permits thesystem listener to select the desired inp&t signal so&rce, and it

a&tomatically applies an appropriate amplification level andfre9&ency correction to the signal to condition it for the secondcirc&it block, toneLvol&me control.

he toneLvol&me/control block permits the listener to ad0&st thefre9&ency characteristics and the amplit&de of the a&dible o&tp&t tos&it his individ&al taste. his block might also contain additionalfilter circ&its incl&ding one specifically designed to screen o&tscratch and r&mble.

he last section of the amplifier system is the po#er amplifier. tmight be able to prod&ce po#er levels from a fe# h&ndred

milli#atts to h&ndreds of #atts.


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or pie+oelectric ceramicLcrystal.%agnetic transd&cers typically offerlo# o&tp&t impedance and a lo# signalsensitivity of abo&t > millivolts. heiro&tp&ts m&st be fed to a high/

impedance preamplifier stage #ithnear/&nity voltage gain.

%ost microphones have a near flatfre9&ency response, so they can be matched to simple, flat/response preamplifier stages.ig&re >sho#s a &nity/gain preamplifier circ&it that #ill #ork #ith most high/impedanceceramic or crystal microphones. t is an emitter/follo#er (common/collector) amplifier#ith an inp&t net#ork bootstrapped by 8> and 2?. t has a typical inp&t impedance ofabo&t > megohms. he combination of 8 and 2 deco&ples the amplifier from the D8po#er s&pply.ig&res ? and ! sho# alternative preamplifier circ&its that #ill match magnetic

microphones. he single/stage circ&it of ig. ?gives !-dB (5>::) of voltage gain, and#ill #ork #ith most magnetic microphones. he t#o/stage circ&it of ig. !, ho#ever,gives "-dB of voltage gain, and it is intended for preamplification of the o&tp&t of very/lo#/sensitivity magnetic microphones.

RAA Prea*%lifier ircuits:

he replay of a constant/amplit&de >:'+ to >:F'+ variable/fre9&ency signal that hasbeen recorded on a phonograph disc #ith conventional stereo recording e9&ipment #illgenerate the nonlinear fre9&ency response c&rve sho#n in ig. . 'ere, the dotted linesho#s the ideali+ed shape of this c&rve, and the solid line sho#s an act&al shape.35amination of the ideali+ed (dotted) version of the c&rve in ig. #ill sho# that theresponse is flat bet#een :: and >1>: '+. 'o#ever, it rises at a rate of -dBLoctave (>:dBLdecade above >1>: '+), and falls at a -dBLoctave rate bet#een :: '+ and : '+.he response then flattens at fre9&encies belo# :'+.

here are good//b&t diffic&lt to e5plain//reasons #hy the precise ig. recording c&rvesare &sed. 'o#ever, all yo& really need to kno# is that they make it possible to prod&cedisc recordings #ith e5cellent signal/to/noise ratios and #ide dynamic ranges. he c&rves#ere applied d&ring record pressing.he important point to be made here is that #hen a disc is replayed, the o&tp&t of thepick&p device m&st be passed to the po#er amplifier thro&gh a preamplifier #hosefre9&ency e9&ali+ation c&rve is the mirror image (e5act inverse) of the one &sed to makethe original recording.


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from the record to conform tothe 2: k'+response.

ig&re "is the schematic for an amplifier #ith those net#orks that #ill #ork #ith anymagnetic phono cartridge. t gives a 1/volt o&tp&t from a -/millivolt inp&t at 1F'+, andprovides e9&ali+ation that is #ithin 1 dB of the 2, #ith 8> and 2, and

8? and 2- forming the feedback resistor capacitor e9&ali+ation net#ork. he o&tp&t ofthe emitter/follo#er b&ffer stage, transistor P?, can be controlled by vol&me controlpotentiometer 21:.he 9&ality of reprod&ction of ceramic or crystal phono cartridges is generally lo#er thanthat of magnetic cartridges, b&t they prod&ce far higher amplit&de o&tp&t signals.8eramic and crystal phone cartridges #ill #ork #ith simple e9&ali+ation preamplifiers//one reason #hy those cartridges #ere installed in so many lo#/cost record players.

ig&re C and sho# alternative phone cartridge preamplifierLe9&ali+ation circ&its thatcan f&nction #ith #ither ceramic or crystal phono cartridges. Both circ&its are designedaro&nd transistori+ed emitter/follo#er o&tp&t stages P1 and P>. he o&tp&t of the circ&it

in ig. Ccan be controlled by vol&me control potentiometer 2!, and that of ig. iscontrolled by 2.

he preamplifierLe9&ali+er in ig. C #ill #ork #ith any phone cartridge #hosecapacitance is bet#een 1::: and 1:,:::p. #o/stage e9&ali+ation is provided by theresistance/capacitance net#ork made &p of 81, 8>, 2>, and 2?.*reamplificationLe9&ali+ation for this circ&it is typically #ithin 1.- dB of the 2


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he alternativepreamplifierLe9&ali+er sho#n inig. #ill #ork only #ithphono cartridges #hosecapacitance val&e are bet#een

::: and 1:,:::p beca&se thiscapacitance is part of thecirc&it4s fre9&ency responsenet#ork. he other part of thenet#ork is formed by 81 and2?.


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t is f&lly D8/isolated from the o&tp&t of the preamplifier by capacitor 81, and from theinp&t of the tone/control circ&it by 8>.


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Passive Tone ontrol:

< tone/control net#ork permits the listener to change th systemamplifier4s fre9&ency response to s&it his o#n mood or taste. 'ecan, for e5ample, boost or red&ce the lo#/fre9&ency (treble)sections of a m&sical selection to emphasi+e the so&nds of

specific sections of the orchestra.one/control net#orks typically consists of simple resistive/capacitive filters thro&gh #hich the signals are passed. Beca&sethese net#orks are passive, they ca&se some signal atten&ation.one control net#orks can, if desired, be #ired into the the

feedback loops of simple transistor amplifiers to give the system an overall signal gain.hose are kno#n as activetone control circ&its.ig. 1>/asho#s a typical passive basstone/control net#ork, and ig. 1>/bthro&gh ig.1>/dsho# the e9&ivalent of this circ&it #hen control potentiometer 2? is set to itsma5im&m boost, ma5im&m cut, andflatpositions, respectively. 8apacitors 81 and 8> areeffectively open circ&ited #hen the fre9&ency is at its lo#est bass val&e. t can be seen

from ig. 1>/b that the boostcirc&it is e9&ivalent to a voltage divider formed by dividing1: kilohms by 1:1 kilohms. his arrangement res&lts in a lo# resistive val&e of abo&t1:: ohms that only slightly atten&ates bass signals.he ig. 1>/c cutcirc&it, by contrast, has a voltage divider e9&al to 1:: kilohms dividedby a 1 kilohm #hich gives a signal atten&ation of abo&t !: dB. inally, in ig. 1>/d #henpotentiometer 2? is set to theflatposition, it #ill have : kilohms of resistance above the#iper and 1: kilohms belo# it.his circ&it resistance val&e is e9&al to 1:: kilohms divided by 11 kilohms.

t gives a signal atten&ation of abo&t >: dB at all fre9&encies. : dB or cutrelative to theflatsignals.

ig. 1?sho#s a typical passive trebletone/control net#ork together #ith its e9&ivalentcirc&its &nder ma5im&m boost, ma5im&m cut, andflatoperating conditions. his circ&italso provides abo&t >: dB of signal atten&ation #hen potentiometer 2? is in the flatposition, and it gives ma5im&m treble boostor cutval&es of abo&t >: dB relative to itsflatperformance.

inally, ig. 1!sho#s ho# the ig. 1>/a and 1?/a schematics can be combined to make acomplete bass and treble tone/control net#ork he 1:/kilohm resistor 2 has been addedto minimi+e &n#anted interaction bet#een the t#o connected circ&it sections. he inp&tto this net#ork can be taken from the circ&it4s vol&me control, and its o&tp&t can be fed tothe inp&t of the po#er amplifier.

Active Tone ontrols:

< tone/control net#ork can be incl&ded in the feedback path of a transistor amplifier sothat the system #ill have an overall signal gain (rather than atten&ation) #hen its controlsare in theflatposition. hese net#orks can be simplifier versions of the basic circ&itsho#n in ig. 1!. ig. 1is the schematic for an active tone/control circ&it.


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< comparison of igs. 1 and 1> #ill reveal that the bass control section of ig. 1 is asimplified version of ig. 1>/a. t can be seen that the t#o capacitors 81 and 8> of ig.1>/a have been replaced by the single :.:?& capacitor 8> of ig. 1. Similarly, thetrebleversion of ig. 1?/a, #ith resistors 21 and 2> eliminated. 2esistors 2? and 2!balance the performance of the t#o section of the ig. 1 control circ&it.

An Au&io +i>er:

< m&ltichannel a&dio mi5er is an attractive modification that can be added to thevol&meLtone/control section of an a&dio amplifier. his mi5er permits several differenta&dio signals to be mi5ed together to form a single composite o&tp&t signal. hismodification #ill be of val&e if, for e5ample, yo& #ant to hear the front/door b&++er orthe so&nds of a baby crying in a child4s room #hile yo& listening to m&sic.

ig&re 1-is the schematic for a three/channel a&dio mi5er that #ill provide an overallgain of one bet#een the o&tp&t and each inp&t channel. 3ach inp&t channel incl&des asingle :.1 & capacitor and a 1::/kilohms resistor, to provide an o&tp&t impedance of 1::kilohms. he n&mber of inp&t channels to this a&dio mi5er can be increased by addingmore capacitors and resistors #ith the same val&es as 81 and 21.

he mi5er sho&ld be located bet#een the o&tp&t of the tone/control circ&itry and theinp&t to the po#er amplifier. ;ne inp&t sho&ld be taken from the o&tp&t of the tone/control circ&it, and the other inp&ts sho&ld either be gro&nded or taken from the desiredso&rce.


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8ontin&e #ith ransistor &torial Part 9


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