Transistors - Circuits and Servicing

14 Pages·0.938 MB·English

by  B. R. A. Bettridge, A. M. Brit. I. R. E.

Transistors
trade informhtion
CIRCUITS AND SERVICING
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Transistors
Circuits and Servicing
INTRODUCTION
THE transistor has now passed from the this way, most transistor circuitry is not
11 stage of being a scientific curiosity to difficult to follow.
being a component found in an increasing From the circuit designer's point of view
number of pieces of apparatus likely to it might be preferable to start from scratch
come into the hands of service engineers. and think purely in terms of transistors and
If one can judge from the trend of events their particular properties, but the service
in the U.S.A., where published figures show engineer will understand transistor circuits
a greater sale of transistors in the first six more readily by regarding them as develop-
months of 1956 than in the whole of the ments of the valve circuits with which he
two previous years, this process is likely to is already familiar.
continue at an accelerated pace in this
country too. It is not too soon, therefore, Power Supplies
for service engineers to make themselves The provision of H.T. and bias supplies
familiar with transistor circuitry in prepara-
to the electrodes is examined, with particu-
tion for the jobs they will have to tackle,
lar reference to the elaborations necessary
and that is the purpose of this booklet.
to stabilize bias against temperature varia-
The physics and fundamental operation of tions-an important point that is not met
transistors have already been well covered
with in valve circuitry.
in various publications, but this kind of
Audio frequency amplifying and output
information, although of great interest, is
by no means essential, or even helpful,
to a service engineer confronted with the Six transistors from the Brimar range, with an
task of dealing with a piece of equipment ordinary ruler below them for comparison.
employing transistors. The emphasis is
therefore placed on the behaviour of these
devices in circuits and on the detailed
examination and explanation of typical cir-
cuits. Point -contact transistors will be re-
garded as being unlikely to be met with
in any domestic equipment.
The booklet begins with general informa-
tion about the devices themselves, the terms
and symbols used in connection with them,
and their electrical characteristics. Where
it is helpful, their behaviour is compared
and contrasted with that of thermionic
valves and it will be found that, treated in
1
stages are dealt with individually and illus-
trated by reference to a typical gramophone
amplifier circuit. This information will, of
course, apply equally to the A.F. end-,,pf
radio receivers.
The higher frequency end of radio sets
is then described, starting with I.F. ampli-
fiers, continuing with detectors, and leading
on to methods of achieving automatic gain
control. Finally, frequency changing stages
of various kinds are also discussed. I
Since "hybrid" sets using both valves and
transistors are likely to be fairly common
for some time to come owing to frequency
limitations of existing transistors, it is THE NATURE OF THE TRANSISTOR
necessary also to describe transistor D.C.
Shown above are G.E.C. germanium junction
convertors, which enable H.T. for the ther-
mionic valve stages to be obtained from the transistors, and junction and point contact
diodes.
low -voltage supply used for the valve fila-
ments and transistor H.T.
ALTHOUGH point -contact transistors are ture of the device. The piece of germanium,
unlikely to have any practical applica- which is called the base, is represented by
coWveirthed, tthhee boFcoairukcllute iTtt irsa ccaisonpmgecptletethdo broyu gghivly- pPinoyweteerrN mtreawenmdsiaisartkoteret. twtiroiandnesliyins tu odsroe dmw dehesirtciihvce eissq funroiopmwm tbehneetg p, iotnhinneit n j-ugcn otcnott iabocent tawhrheei crhehop rprieazssosenentstaeld lf iobnryew ,ta haredn ds lctohuperir netwngt oli nwbeehsa.i rsTsk heaarnst
ing information about signal tracing, fault transistor, and consequently a brief descrip- arrow -head and is called the emitter, whilst
finding, precautions to be taken against tion of its nature and the manner in which the other, which passes reverse current,
damage when making measurements and it functions will be found useful as a lead-in bears no arrow -head and is called the col-
simple methods of testing transistors. to a treatment of the junction transistor. lector. These terms, incidentally, have a sig-
By putting the facts clearly and simply, nificance relating to the operation of the
this little booklet will enable any service Physical and Electrical Properties device, as will be seen later.
engineer at present competent to deal with The point -contact transistor may be re- The junction transistor is similar to the
valve equipment to feel equally at home garded as an elaboration of the germanium point -contact type in that it, too, consists
with transistorized gear. It is quite possible crystal diode which consists of a piece of of two rectifiers so devised that forward
that some service engineers previously tak- germanium with a pointed, springy wire (a current in one modifies the reverse resis-
ing an interest in this subject have been "cat's whisker") in contact with its surface. tance of the other. So that, although the
dismayed and discouraged by the almost This combination acts as a rectifier, its elec- construction is quite different, the same
exclusively theoretical approach of much trical resistance being low in one direction symbol is used for it, and the same terms
of the existing literature. Prospective readers and high in the other. are employed to describe its electrodes.
may rest assured that the approach in this With the whisker made positive, current
booklet will be made against a practical A single ex- flows easily from it to the germanium, but JunctionMtoraren sCisotomrsplehxa Tvyinpegs more than
bpaecrskognraolulyn db uthirlto ruagdhioouts, etthse a anudt haomr phlaifviienrgs aEmdipslwean ofMaztdhae whihgehn s oit tihs amt oadnley n ae gmaitnivuete t hceu rrreesnits tpaanscsee sis. these three electrodes have been devised,
using circuits of the types to be described range of tran- If a second whisker is placed on the surface but they will certainly not be widely used
anTd haenraely isse do.ne point about transistor cir- scilsutdoers, Rw.hFic. ha nind- ooff tahne ginecrmh aonfi uthme w fiirthsitn t oa ffeowrm th ao usseacnodnthds iann de qwuiilpl mtheenret fdourrei nngo tt hbee ncoexnts iydeearre do rh tewreo.
cwuoitrst hw hwichhil eis toof msuecnht iiomnp iotr itnan acdev tahnatc eit oisf A.F. types. rine cotnifey irnegct icfoienrt accatn, iatffiesc tf othuen cdh tahraatc tceurirsrteicnst Earxec slutidlli nmga tnhye svea mrieotriee sc oomf jpulnecxt itoynp etrsa tnhseirse-
the context of future articles to drive it of the other. tors in existence, and readers who have seen
hcteuorrmrmeens.t o-Tofhp aiesr astiegisdnatdlh eavvtoiclett,haegaen tardat n tthshiisent okcrionngistroinal Binecllouwdi nisg a sdeiloedcetiso,n otrfa Mnsuisltloarrsd, semanid -coan pduhcottoor s-, cuTrrheen tn aintu oren eo fr etchtei feieffrec(tt hiast thiast wfoirtwh athrde rrmaetafeey rg efrenoecwle tn ht,o as tus turhfcaehc s etuy bbpajeersrci tae hrs, a adsl ilafoflyure,s aegddry,o ewbtcen.-,,
evlaelvcetr,o wdiel,l oasn liyn mthisel ecaads.e of the thermionic transistor, compared with ordinary matches. wwhhiisckhe rt hpeo soittihveer) rwedhuisckees rt hwe ohuigldh rneosrismtaanlclye comFoer tduinffaitceulylt, tot hfoellocwom. plication is more
schuoFrrurelondmt g t eotth tiehneto cs ttoahnrett, rhoaltb heietlr eeocffto rcroeod,nes,it dhineesritneregaad td hoeerf eicnxu hrorinbeenit t cw iisrh ceaun si tme pmaadareya t nbee egc iamrtciavudeiet. Tatonh duin,s ,fi nlcuu serunreictne-t aifnapc pftaaurcreten, wtd tehhsiaccnrh ir paetfaifole.n cTsth oedfs eem tvaeaitlrhsi oooudfss b toeefrh mamvsa iaonruuer-,
the voltage. It will be necessary to come able circumstances, amplification can** be rather than the fundamental mode of opera-
back to this point on numerous occasions, achieved. This two whisker device consti- tion. Similarly, the effect of using another
and the sooner it is accepted and its impli- tutes the point -contact transistor. substance such as silicon in place of ger-
cations realized, the sooner will transistor The symbol used for the point -contact manium is mainly to raise the permissible
circuits seem as clear and logical as valve transistor is given in Fig. 1(a) and it will operating temperature, leaving its general
circuits. be seen to follow naturally from the struc- behaviour unaltered.
2 3
This state of affairs has resulted in little components, but those in which the wires
So that, from the point of view of Contacts are then made to each region, standardization of shape, size or connection, are brought out in line can alternatively be
examining their operation in circuits, there the n -type material becoming the base, and manufacturers' literature should always fitted into holders.
is no need to deal with many types whilst one of the p -type zones becomes the be consulted regarding the last point.
When dealing with electrical behaviour,
stoe piallruastetrlayt;e a tshienmgl ea ltly,p aincadl fcoars eth wisi lpl usurpffoiscee eqmueisttteior na nodf twheh iocthh ezro tnhee icsolulescetdo rf.or Tthhee Valve Analogy there are sufficient points of similarity
ab ep c-hno -spe ng.ermanium alloy type transistor will esceemstst.iltetdeI rna asf napcdat rwttr aohnfi sctihhset fo omrsr amntuahyfea bcectuo urllsienecgdto cproronis-- tohueTt chfooelrrle es cpisteo scro icamol nei dnceeocnmttiiomfinco ianst ipuorsnau caetliiltcyhe es iirnn b gtyhle aadt btetorxeuttewre wmemehaenkel enytr athrneasclnipossmfitusoptlro,as rra sians naoddrn et sth hbieserianmnigsdi oupnsaeiracdtni vcatuolaoll gpvaireeelrsys-
The Junction Transistor nected so that the electrode coded as col- colour code or by different spacing. The form functions already familiar in valve
taebrBliese tftiooc rshe a odvfe eja ualin nlcigttt iwloeni tb htar ctahknegs reiosletuocnrtdrsi ckitanlo icswhalderedascgir-e- lbaemuctpt ow%r ifftiuhcan ttchioteino cnsousnf afveser sen mutinoidtnteearrl tlahyne msde vacdiocene d dvieteivorisncase,. smaller transistors are essentially wire -in praTchtiece.electrodes of a transistor are
analogous to those of a thermionic triode,
othfatt hite iirs nbeacseicssasrtyru tcotu kren.ow Twoh uatn dise mrsteaanndt The n -p -n Transistor Fig. 2.-Basic transistor circuit configurations with the emitter corresponding to the
bmyatenr i-atyl,p eanda tnhde caps -ety opfe gesremmai n-ciuonmd uwctiolrl allBoyyingstarnt i-ntygpewiimthp pu r-ittyypetomeaittehreiarl saidned, ccoiomrmciuttpietadsr.edo( faw)i stChimo cmiolanmrvoeknnit nieodmns,aitlt etwhriethrtmraiboninsaiissct ovdrae ltavanieldss cltehacetth ovora dtloev ,te ht,he t eha nbeo atdsreea. ntosiF stihtgoe. r 2g m rsihdao ywa nbsd,e h tuohswee ,dc lo iikln-e
sabebursTtvo ehtlh ueteot e m ai pldeluumtrsaitiltrxy agt tuieesr r tevmh ieora tfmun aaiaul tlmtmye iran .i unntoen a-pcroosnptdaoutrectitooornf, taaArvsam annil se-airpbsi tlc-eonarn st y imapnnead.y JtbhaTeips hamenayecd soaeeur nwerta rhndyio,icoth c abosruemett smknosuoosnwmilnyneg ctvcraoaatlmnhvsoemid;setoo -nr(fc oc)lalaotncwhdooem drceomm(oovnmra ccolvoonelml;emc(ot(oobrr)n gactrnrooaoumndnsdmeie)sodtonrv b gaaralnvisdede). tf""hicgrrorsemote um adnnoidfdnef edmr eeonmestm tic tiuittretscereru""d,i, t i sncho wonsowfhimigncueh tari amtthtieeo(sa ns)sig.incsaa Ttlllh heiedse
of other elements confers a limited conduc- them have found their way over here and The H.T. positive line is earthy. fed in between base and emitter and taken
tance, and the material then becomes a it is advisable to know something about out between collector and emitter, so that
semi -conductor. Conduction can take place them. the emitter is common to both input and
in two different ways according to the Actually, all that need be said is that output circuits. This corresponds to the
nature of the added impurity. they behave in the same way as p -n -p most usual arrangement in valve practice,
If arsenic or antimony is added, electrons transistors, but the polarities of H.T. and where the cathode is earthy, the grid the
are made available to carry current, and bias supplies have to be different. All cir- input electrode and the anode the output
the material is described as n. -type, the cuit illustrations based on p -n -p types will electrode.
"n" signifying the negative polarity of the therefore be equally applicable to n -p -n
current carriers. The addition of indium or types provided that the polarity signs are
gallium leads to the production of current reversed. The symbol for the n -p -n type is Other Modes of Use
carriers whose polarity is positive, and the the- same as that for the p -n -p type, except The second at (b) is the "common base"
material is known as p -type. that the arrow on the emitter points away circuit, where the input is between emitter
The carriers are in fact positive "holes", from the base, as shown in Fig. and base and the output between collector
but their precise nature need not be dis- By varying the dimensions of th1e( bb)a.sic and base. This corresponds to the grounded
cussed here since, for our purposes, we may sandwich, the ratings of transistors may be grid valve circuit, more correctly called
quite conveniently treat them as if they modified, a scaling down giving improved common grid, which is used mainly in
were electrons having a positive instead of a frequency performance, and an increase in U.H.F. pre -amplifiers. Finally at (c) there
negative charge. size resulting in higher power -handling is the "common collector" use, where the
A p -n -p transistor consists of a sandwich capacity. base and collector constitute the input
of n -type material between two, regions of terminals and the output is taken between
p -type material. This is made in the case High Degree of Purity emitter and collector. This is analogous to
of the germanium alloy type by taking a the valve cathode follower circuit. The
The chemical purity of the materials used
thin wafer of n -type germanium and alloy- for the transistor assembly has to be con- collector (or anode) is, of course, earthy.
ing indium into it from either side, so that It will be noticed that in (a) and (c) the
trolled to fantastically close limits, and it is
two zones of p -type material are formed, therefore not surprising that it should be transistor symbol is turned on its side,
separated by a narrow zone of the unaltered and, this is standard practice in such cases.
extremely sensitive to contamination. One
n -type material.
of the major problems in transistor manu-
facture is the provision of suitable encap-
Fig 1.-The standard symbol for a transistor sulation (casing) to give effective hermetic
in a circuit diagram: (a) the p -n -p (and point - sealing without sacrificing the essential small
contact) transistor; (b) the n -p -n transistor. size and robustness of- the device, and at
The arrow on the emitter points in the direction present individual manufacturers favour
of conventional current flow, not electron flow. different solutions.
There is as yet no standard technique for
envelopes as there is in the case of ther-
mionic valves, for instance, some makers
using glass, and others metal in conjunction
with glass, ceramic or plastic. With the
higher -power types there is the further
problem of obtaining satisfactory heat con-
duction from the system to the outside.
4 5
a rectifier with forward voltage applied so sunffice to say that the increase in collector
that its resistance is very low and, in the current is due to collection by the nega-
absence of any limiting resistance, current tively polarized collector of most of the
high enough to destroy both transistor and positive holes injected into the base region
meter flows in the circuit. by the passage of emitter current. If it
could collect all of them the current gain
Current -limiting Resistance alpha would be unity.
For the next measurements, the range
2 of the collector meter must be increased How it Amplifies
to 0-5 milliamps (0-5mA) with the collector It is not obvious at first sight how a
voltage Ve still remaining at -6V. The device, which has already been described
emitter circuit is first completed by plug- as current operated, should be able to
ging into the + 1.5V tap. The presence amplify if the current change in the output
COMMON (OR EARTHED) BASE OPERATIONS of the 1,50012 resistor is necessary since circuit is slightly less than that causing
the emitter -base junction is also a rectifier it in the input circuit. The matter becomes
and + 1.5V applied directly between clear when it is appreciated that amplifi-
emitter and base would cause excessive cation cannot properly be defined purely
current to pass. In this part of the circuit in terms either of current gain or voltage
a reversal of polarity would do no harm, gain except when measured between
IN explaining the operation of the the -1.5V tap a current of about 4pA correct polarity with no limiting resistance identical impedances. .
thermionic valve the common cathode flows which increases to a minor degree being the dangerous condition. The fact that this is often ignored in
).rrangement is always taken, and the less only, perhaps to 511A, when the voltage is The current in the emitter circuit Ie will valve amplifiers should not be allowed to
frequently used configurations follow easily increased to - 6V. Under these conditions not differ measurably from lmA, which is obscure the basic fact that amplification
from this. In the case of the transistor, the collector -base junction is behaving as the value to be expected from 1.5V across is a raising of power level and should
the common emitter mode, though normally a rectifier connected in its reverse direction 1,5001, showing that the emitter -base
used, is not the best starting point and the so that the current consists of the inevit- contribution to the total resistance is
most straightforward appreciation of the able leakage current of such a device. negligible. In fact, for this type of tran-
relationship of currents in the various elec- This collector current, in the absence of sistor it is in the neighbourhood of 2551.
trodes arises from first looking at the any emitter current, is called Ieo, and With this L of lmA it is found that the
common base circuit. although so small it ii- of considerable collector current Ic is also ImA, or very
importance in circuits to be described later. close indeed to it. A very accurate
Earthed Base Operation Its value is highly dependent on the tem- measurement would show it to be 985µA,
A consideration of the currents flowing perature of the transistor, and it approxi- consisting of the initial 5µA, due to Ico
mately doubles itself for every 10 deg C plus a further 980µA brought about by
ushndoewrn vinar iFoiugs. c3o wndiiltli oilnlsustirnatet htehe smeto-uspt rise, so that a value of 5ILA measured at the passage of emitter current. Leaving
important facts about the common base a normal room temperature of 20 deg C the emitter circuit unchanged, and changing
operation of transistors. The circuit, which would rise to about 40/1A at 50 deg C. the collector voltage back to 1.5V, would
It should be remembered that such a rise cause negligible change in collector current,
iioasnfs e taara y gstiieynlnypge e mro aGual-dtpE emuT urap3ptot ecbsreoys np afn no-enyr c h-rtpeiema dtdrs aevenrli sfaii,n sctteoosrurne isstsauitsbectldhse idfnrio stmseimpapatneioyrant uiinrne ct hrmeeai sdgeehvt ibicnee bittrshoeeulfg,h squt urariobteuo nuadtp iabnrygt smish eovnwetr iysnh ghoi wgthh at.ht athRt eae fitimynpepidec damla envtcahelou doesf i ostfh i imns eecaxisrccueurseis-t Fteshiasiergtot hr3m.e.d-eT abTshauhesreee 1cm,- i5cer1hcncatu fri2ato cdfrteeesrstiicshsrtteoiicb rcs elo idommf i imants poth n-tenh b e-a tpeesmxe,tti rtfatoenorr-r
mcuertreernst tot othe teamppitetedr anbda tctoerlileesctor.supplying (ambient) temperature. of A1 m cehgaonhgme. in the tap to the emitter current to a safe value.
It will be noted that the supply to the Dangerous Voltages battery to + 3V causes 2mA to flow in therefore be measured in terms of power
collector is of negative polarity, as opposed No damage can result to the transistor the emitter circuit, and the collector gain. Approached from this direction,
tcoorrtehsepopnodsiintigv eelescutrpopdley, trheaqt uisi rtehde ' bayn otdhee, umnedaesru rtheme ceonnt doift iIoenos, bjuust ti td iess ccroinbveedn fioenr tt haet cnuearrrelyn t2 minAcr.easEesa cbhy fau rftuhrethreri n9c8re0apseA toof tchoemremiosn n boa dseif ftricaunlstiystoirn a smepeilnifgie hs osiwnc teh ea
of a thermionic valve. This means, of /this stage to mention two departures from 1 mA in the emitter circuit brought about current change in its low impedance emitter
course, that one can no longer take for them which can cause trouble. The first by moving the tap another 1.5V in the represents a much lower power level than
granted that the H.T. line in a radio set is the application of a voltage of the correct positive direction results in an increase of a similar change in its high impedance
iHs.Tp.o psiotilvaer.ity Iwf itlhl edespeetndis ontr atnhsei sttyopriez eodf, pTohliasr citayu bseust ba esyhoanrpd rthisee mina kleeor,s 'anrdat itnhge. ncoeallrelyctorl mcirAcuit.(actually 0.98mA) in the coTlleoc toobr.tain gain in a practical transistor
ttryapnessi satnodr inne ugsaet,i vbee iwngit hp othsiet imveo wrei tcho mn -mpo -nn cinocnrseeaqsuee innt hleeaot ,p wrohdiucche idn leiatsds ttuor na fcuaruthseesr Current Gain atom bpel imfiaetrc,h iendp tuot tahned, iomuptepduatn cciersc uoift se mhiatvteer
pmc e-nru t-saptin a tclywapseaesy.s sd abTmeha iksgeep qtotu eibnsot itmohn itnradon fssiisnptoocrlea arnitidyn m"dreuvoinrce e- ahwiesaatyd w"e scittorhonydseittdiilol bnyhs itaghrheee rsh Teetoa uoti,pn gasnodef ttfhheacett ccuhrTarehnnegt era iitnnio tt hhoeef thceeom lclieottcretrroe rsi sptootnhade ircnehgfoa crneug r0er.e9 on8ft, aicsinrcdiul lciutosltlrdeaicattegodrra ,im na nFdiog tf.h 4e, a wwahytiw ctohh - isssthaogiews ds oIt.hnFee.
mreveaesrsuarli.ng instrument may result from ohifg thh ev oelxtcaegses itvheer ceu irsr eimntm. edWiaitthe sbureffaikcideonwtlyn aitndis a cltahloleudg hth teh icsu irsr ean lti,t tglea ilnes os ft hthaen turnaint-y apmoipnlitf itehra.t aItt isnhteorumlde dbiea tme efrnetqiouneendc iaets tthhies
The first measurements are taken with of the junction without the intervention sistor and is designated by the Greek letter impedance figures already mentioned no
the emitter circuit left open and with the of thermal run -away. alpha (s). We are more concerned here longer hold good, and the emitter im-
with the use that can be made of this pedance may have risen to about 200C1,
meter in the collector circuit set to a 0-50 The second danger is the application of
microampere (0-5011A) range. When the a voltage of incorrect polarity. In this behaviour than the reason for it, so it will while that of the collector may have fallen
collector circuit is closed by connecting to case the collector -base junction behaves as .7
6
rswtpaqoohut siiaaecosrsh eo ilfsii s rtaaotp lotteprht araeonsrxoseiffmf3oo0ar3r,mt0e0e0, l00egy10ri 20 1v.f2eo:nr1dT .mibvhyeiad ttaceaphdkpi ibnrnoyggprp2iaut0ht0ree-, ncuaseinpirtgoouclndiuag teiaiitb o dcalnjeunu ridsrstc e insunnoirgtmr ecctihnhulaetat r fnoe ltgmofofew i wtbiitnnyehg r ecn bnoei ilgaanlse av;ct aitithvnoleve rfe acb echuimtaar,s rsite t hstineteosrt hibniiaggTsh haeitrs m i fnheiigtageunrhrsmeerse tidwhniapeat utheta tifvhmreee pqmeudieenannpntcuicoiteen sew.tdri atahns s rfoeocdrcumucreerrd-, iermsem neniatet ntcweseri s ttishhsa e nro ycop o ewclnolhe r-ecrcnetiosr ccrp huoiseint cedokdipin,en gwng c h-tocihlliielrse c eccumtiiorticert utdcei.urt C, r crbaueerrne--t
that further bias can have no _effect. which incidentally no longer gives correct cause short-circuiting either of the bias
matching from the previous stage, cannot resistors RI or R2 results in excessive bias,
Ti is an I.F. tSrtaangse fGoraminer with a step- apIptl iiesd c bleeatwr fereonm ethmisi tthteart aa nsidg nbaals sei mopf lay demrivietters aon md guacihn issi ganccaolrdcinugrrleyn dtiminintoishethde. wdeitshtroyc otlhleec ttroarnsicsutorrres.nts high enough to
down turns ratio of 12:1 whose primary transistor would be rectified, and only the Variation of emitter bias thus constitutes
winding L 1 is tuned by Cl and fed from positive half -cycles would affect the col- a method of controlling gain, and this is Operating Frequency
the F.C. stage. Its secondary winding L2 lector current. To allow both halves of the the basic factor in most A.G.C. systems. In choosing a replacement transistor for
which is closely coupled to LI is untuned signal to be passed on it is necessary to These will not be considered in detail until
an I.F. amplifier the most important charac-
aI.nFd. afemepdlsi fiynitnog t htrea enmsiistttoerr. oTfh TeR fu1n, cthtieo fni rosft pwahsisceh a c satena bdey abdiadse cdu trore onrt idnitmo itnhies hemedi tbteyr lcaotveerr,e da.fter certain other points have been teristic to take into consideration is the
tpheids atrnacnes foofr mtheer Fis. Cto. mouattcphu tt htoe htihgeh liomw- tshigen palo.sitive and negative half -cycles of the Making DD.C.C. .m Meaesausureremmeenntsts in the circuit fburnyedq fue5re. n wOcyhw icciuhnt -giot ftfoi fs it gchuoernev auersntuifiaeiclnliyat l tsoyc mmonbedoailtsiizuoenrdes
impedance of the emitter of TR1 and in In the practical circuit in Fig. 4 bias in Fig. 4 is straightforward, collector vol- this parameter it represents a frequency
the process a current step-up of twelve times current is provided for the emitter of TR1 tages being read directly between collector considerably higher than that at which use-
takes place between the I.F. signal in LI from a positive tap on the battery via R1 and base leads, and collector currents by ful gain can still be obtained. In fact a
and that induced in L2. and L2, a low impedance, return path to breaking into the circuit at points x and y. transistor should be chosen with a f7 figure
There is, of course, no true amplification earth for I.F. being provided by the capaci- If there is no current despite the presence not less than two or three times the re-
hsaemree b eexctaeunste a ws ew hea hvaev leo sgta iinn evdo litna gceu rtore tnhte. teomr iCtt2er. Tciorc auviotsi,d T cRou2p ilsi nsgu pbpeltiwede ethnr tohueg thw .oa oitf ivso nltoatg ree,c eeiitvhienrg tehme ittrtaenr sbisiatos.r is faulty or quOirende ofipnearla tpionign ft rsehqouuelndc yb.e noticed regard-
However, this increased signal current in the separate resistor R2 decoupled by C4. The By breaking the circuit at the junction ing the circuit in Fig. 3; both le and Ic
emitter circuit of TR1 causes a signal at normal bias for such a circuit lies between of L2 and R1 it is possible first to make flow in the base lead of the transistor, but
aXlm 0os.9t8)th teo fslaomwe inc uthrree nctollleevcetlor( aacntdua iltlys 0sh.5o wannd i s1 .a0bmoAut, 0a-n7dm iAn .t hAe hpiagrhtiecru vlaarl ucea sies cceorntanienctitnhga at bmiailsli acmurmreentetr ibsetawveaeinla Rbl1e ,a bnyd icnu rorepnptosiiste dthierectsimonalsl, sdoi ftfhearetn tchee nbeett wbeaesne
external circuit consisting of L3, the pri- merely wasteful of current, but a lower one earth, and then to check whether the tran- them. This small order of base current,
mary winding of T2. We therefore have a reduces gain due to a property of the tran- sistor will pass it, by inserting the meter which is associated with emitter and collec-
current gain of twelve between the tuned sistor not yet described, but which is briefly between R1 and L2. If the transistor will tor currents perhaps 50 times as large as
circuits L1, Cl and L3, C3, and if we take as follows. not pass any emitter bias it means that its itself, should be noted for future reference.
their impedances as being equal this repre-
sents a power gain, too, of twelve.
A similar step-up of current results be- Emitter -base Resistance
tween L3 and L4 followed by transfer in We have seen that the emitter -base path
the transistor of this current to the higher has the low resistance value of 2512 when
impedance of L5. The outpu% transformer lmA is flowing and this low value is main-
T3 is of suitable ratio to ni etch into the tained for higher values of Fat lower
detector circuit. values of le, however, the emiaer i esistance
Returning for a moment to the circuit increases appreciably so that at 025mA it
in Fig. 3, and reversing the ,olarity of the is 10012 and at 0.025mA it is 1,00012. A
battery feeding the emitter, we should find corresponding rise takes place in the already
Fig. 4.-A practical circuit diagram of a two - tage transistorized I.F. amplifier using earthed -
based operation with p -n -p transistors, which is not a common method. It serves to illustrate
the author's explanation of how stage gain is achieved, and how bias can be applied.
a
HT -12V
from F.C.
to detector
stage
HT+.
GB-
GBI-1-5V
8
A further alternative, and one that may Bearing in mind what has already been
possibly supersede all the others is hie, the said about the effect of impedance levels
3 "e" denoting common emitter. If this when determining amplification, it is
convention is adopted, lin will be used for apparent that amplification in a common
common base current gain in place of the emitter transistor when properly matched
is due both to the current gain acb between
present symbol a.
base and collector and to the impedance
COMMON (OR EARTHED) EMITTER OPERATION transformation betvieen input and output.
Valve Analogy
If no other consideration than optimum
It is interesting to compare the common gain is taken into account the input trans-
emitter transistor amplifier in Fig. 5 with a former T1 must have a ratio matching the
corresponding one using a valve. In the input to 1,000 ohms, so that if the previous
valve case, current in the anode circuit is stage is a common emitter transistor with
FROM a practical point of view, common emitter. In this circuit a definite current Ib controlled by a voltage applied to the grid an output impedance of 50,000 ohms
emitter operation of transistors is most mar be fed to the base via a high resistance and a voltage bias is provided so that
conveniently illustrated by examination of taken to H.T. negative. positive and negative half -cycles of the input the correct ratio would be 50.000 .1
their behaviour in such a circuit as that In such a circuit as Fig. 5 the condition signal may be dealt with. In the absence of or 7:1 approximately. 1.000
shown in Fig. 5. However, a more com- shown in the table where Ib=zero could not bias the anode current is high, and when a A lower ratio than this is frequently used
plete understanding results if, in the first exist unless R1 were made infinite (open negative bias is applied it reduces it. in the interests of low distortion, 4-5:1
place, an approach is made from what has circuit), and the condition Iv= -1-5/AA could In the case of the transistor, current in being typical. This deliberate mis-match
already been learned about common base never occur in any case. the collector circuit is controlled by current lowers gaia somewhat by feeding the tran-
operation where currents were considered in supplied to the base, and a current bias is sistor from too high an impedance, but at
two separate branches, each supplied from Common Emitter Operation provided to achieve correct amplifying con- the same time it tends to mask variation in
its own battery. ditions. In the absence of bias current the input impedance and so gives better
Although the currents are now fed into collector current is very low, and bias is linearity.
Current Relationships tbheet weleeecntr tohdeems dmifufesrte snttillyl ,f othlleo rwel tahtieo nsasmhipe supplied to increase it. There is little point These variations are of the same nature
in comparing polarity of suppliss since in as those mentioned in the common base
In this case current in the emitter, was fundamental rules, and the figures in the
the transistor case they depend on whether amplifier, in that there is a great rise in the
dtslreheeecrnetite teoer scrmm arcielionltsneetiedsdsrti as aIbtnlcemacodte ot p,oes wltfru ysahso i acllaesnontldn y cas utbtmharynreo ett uhnv lentea ailtvn ukeo eatqlh gtuoaeegaf c clet ou htolore-f- tca-a4hnb9odl5se /Ie Ascnt A imtlol ru hegsositplv detea c gktaio veboe adulsy.pe. AvcIaunflr,du rt eiehfnse Rt r oeol fffi os 5- ra50edµ,0j ARu as,lnt eIidesd apc uapArr ri-essnon m-tnp c, i oogirfnh atdtrn uob nele o -aepfl xl t-orpnaw entyacspntiseectd eois rfi susr, os maemsda .o dtphepisof oscerod tm htoe- viecsaua fslrluyroee wtn ooit nf ae gcic.nhopineuAvot emvi moiynip d ipesaod nuractsnaeucb aeolle flw yethh qieimusni pp elmofofrweetacn cntt ,uti,srs r ainennnocdett
emitter current multiplied by a factor taos vlaarryg elb m, cuosrrte tsapkoen dpilnagc ev airnia Itieo.ns 49' times tghreea vt odletgargeee ocfo snitmroilla roitfy v baeltvweese,n thtrearnes iissto ar epveremni swsihbelere i th iisg dhieffricuclut rtroe nmtakoep vereartyio snmailsl
slightly less than unity called alpha. Current This large ratio between current changes
and valve circuits. transformers capable of passing the current
iaIncn ,dt hb eueq tb uiata swle ta olse tnahdoe t w dcioaffnse snriedonetecreded ba isen t bwdeeeitenangil .Isem aanldl itno c oblele cetoqru aandl btoas ae- cbanu itn i ftacits beo fs hgorewant ionu Bdtpeeutfato iirlm ei tpc ieosd nnasenicdceeesrssi,an rwgyh ttihoc ehm ceidnrictfiufoeinrt coinofp nFusiitdg ae.n r5d- without saturation.
This can now be investigated by tabulat- 1 -a ably from those given for common base Matching the Load
ing a set of values for these various currents importance in the common emitter circuit operation. Precise values depend on a num-
in a typical transistor which, to give easily and is usually given a symbol in its own ber of factors which will not be gone into The ratio of the output transformer T2
handled figures, will be taken as having right. Unfortunately, there is as yet no here, but it may be taken that for a small depends on the nature of the load.
Iecx0p=re--s5sµeAd iann md aic=r o0 .-9a8m.ps,A alnl dc uthrer epnotssi taivree satratnicdlaersd opfr alictteicrae tiunr eth ries freers ptoec itt aansd a v',ar/i3o uosr agnecnee riasl apbuorpuot s1e, 0tr0a0n soishtmors t,hew ihnpilustt itmhapte odf- Oanbovtihoeurslys,imiiflarthiosnestagthee risemfaeerdkisn gaboinvteo
or negative sign indicates whether they are mob so that the reader must be prepared apply, but if it is operating as an output
the output approximates to 50,000 ohms.
flowing into or out of the transistor. for the present to recognize any of these stage feeding headphones or a loud -speaker
algWebhreanic e sxupmre sosfe lde ,i nIc t hanisd c Iobn mveunstti oanlw, athyes symbols as signifying the same thing. 4 Fig. 5.-Simptlhee ctoramnsmistoonr emAi.tFt.er mamodpleif ioefr tghaein r abtuiot fmoru mst abxei mchuomse pno nwoetr .for optimum
be zero, since the total inward current Transistor Characteris Table coiprceuraittiouns.i nPgositive base bias is applied via Rl. The method of calculation in the case of
must always equal the total outward current. a transistor class "A" output stage is simple
This table shows that each change in base and needs no reference to characteristic
current of 5µA corresponds to a change in 1e -le x .98 'CO Total I, Ib curves and drawing of load lines. Suppose
collector current of 245/IA, that is to say, the transistor has a maximum collector
49 times as large. This leads to the idea [phone] +0 dissipation of 60 mW and that it is to be
that by using the base as control electrode, [phone] run at this figure. If the H.T. supply is
agdaivna nbteatgwee meni gbhats eb ea ntadk ceonl loefc ttohris c cirucrurietns.t ++17,50000 --793850 --55 --978450 --1105 c6oVll,e cthtoer bciuarsr emntu osft 1b0em aAdj,u asntde dth teo cgoirvreec at
A further point to be noted is that at load will then be 6 X 1.000 ohms that is 600
16=250,aA the base current is zero so that a=cornmon base current gain factor (alpha). to
under these conditions the current may be = emitter current. ohms, and the output power will be very
regarded as flowing into the emitter and out Ib=collector curent. close indeed to 30 mW which represents an
of the collector from the two batteries in Ibo=collector leakage current. efficiency of 50 per cent and is the maxi-
series. This leads naturally to the kind of Ib=base current. mum thepretically obtained in a class A
circuit shown in Fig. 5, where there is only All currents quoted in pA. Ambient temperature stage.
one battery connected between collector and assumed to be 20-25 deg C.
11
10
Incidentally, the value of the bias resistor this effect and achieving bias stability, the
to give 10mA collector current would be one shown in Fig. 6 is the most usual
30,000 ohms approximately because the base for transformer -coupled circuits. In this a
current has to be about 1/50 of the collector voltage divider RI; R2 is connected across
current, which is 0.2mA, and this flows the H.T. supply, and R3 is inserted in the
when 6V is applied to 30,000 ohms. emitter lead. It is by-passed by C2 to
The same output would be obtained with prevent A.F. degeneration.
a 12V H.T. supply by biasing the base A detailed analysis of the working of this 4
through a 120,000 ohm resistor to give a arrangement is not possible here and it will
collector current of 5mA, making the out - suffice to say that a high value of R3 com-
put load 12 X51.000 ohms, i.e. 2,400 ohms. tbhien egdre watiethst ad elogwre eto otaf ls vtaabluileiz oafti RonI,, bRu2t gsiinvcees
R3 reduces the effective collector voltage, LOW FREQUENCY TRANSISTOR AMPLIFIER CIRCUITS
Effect of Temperature and oftl, R2 increase the H.T. current drain,
some compromise is necessary in practice,
The biasing arrangement shown in Fig. 5
and the values shown in the figure are fairly
has a serious limitation in that it is only
satisfactory in equipment operating in a typIinc atlh.is circuit the base is maintained by
very small range of temperatures, and a
mnOefoe drIeecd oc. owmTitphhle itc etarmoteupdbe rlceai triucsur deit,u ewis t hnoie ctahhre lh yian sac lbrweeaaesynei tedhaiterit ovhno lsli tntahege, eraenddii ssviinadc epero u1tne-1ndt.e5iraV lo pdneiefrfgaetaritneignv ceceo onto-f IthNre ttieho eenl eloacfst rttho cedh ecasup lrteredern at t rodeelaatat siioilmendsph leiifpxisae dmin wi nthaaye- onefT gpah t-einv fe -i pHrs .ttTr sa. tnlaisngiese t ioiss rt syb poiaipcsaeelrd.a tiinn gth fero mma an n6eVr
m10e dnetigorneeesd C a.s doubling for every rise of ltceraasunss steihs iattonsr e0m-w2iVitltl ebre tatawlksoeee tnos ubbfaefsi ecini ea tnnhtde encmeuirigrttehenbrt,o tuhtreo- coofm lomookni negm iattt etrr acnonsinsetcotrio onp.eraInti othni si,n t wthoe adbelsyc irnibdeedp einnd Cenhta opft etre m3 pseor aatsu rtoe cbhea rnegaesso. nA-
In the common emitter arrangement of hood of 1 - 1-5V negative to earth line by currents only were considered, one flowing voltage divider, R2, R3 holds the base a
tthFbohheifieeag l2 sd.tt 0 rr5c aa au dnnttrhes sraigieissbsrnt etocotoe ruuar.sr n triC deslS n mwaatunl Aoptadeupr r kgiosbti mssynte hc ge,et on hf alwtoelster oa tcpehr tkxrteoeoia rnmimv ncgiptup seperlinorerotea,iin nnottu httno raoiaefstfl viopcRnoofir 3fltctlse euuentceettrt i rnobaoerdlyfn , c itttnuhh drgeertu e hdtienounr tscotsor.pel otaaerwscemerdedoupr sc vestirho nRaglett3au.gr beAeea smrdneiry is-ot ettiope n rw -caercimlerlaiao tnbtssedeser bcfcthioherestIlatwmnlne eg accbenteuneoasri r lrbniy eaangsnsn ietttndhh gwa eeen pemcsdrrue aeeirctc mrtoseteihninrctod,tta e w,algr nt,acn htdiih nre tcec oru mh oaiccatttbsahin.oueg irsbte ebse ei tlsitwanw .rc egetoeheennner- lacacRinuu tv4tdrrlorr+eeel Rntnltaetht5 g sek( seiaen ceto httpifrutasa saan n nlle slmo1ymisi. os5taioetsVbrt tleo otnruhw metle eg 0u wasa-sdati3ttim.mhvoeetAau T tkvt)oh e aa tielopsua p psevrrutr aehafolcfc udiilcraieuionbe cosenleest,f
15µA base current as shown in the table. venient to continue treating the operation effect on gain.
If the temperature rises to 40 degrees C, Ico in these terms which, amongst other advan-
rises from 5µA to 20µA, making the total Servicing Danger tages, make for easy comparison with Analogy With the Valve
base current 30/tA and causing the collector When making measurements in this type of corresponding valve' circuits. Only when
current to become nearly 1.75mA. circuit it is most important that R3 should examining D.C. bias conditions is it neces- Resistance in the emitter lead produces
theT hheig ihnecrr ecaoslel ewcitlolr n cout rsrteonpt hmeerea,n bs ehciaguhseer nleeavde rt ob ev esrhyo rht-ecaivrcyu eitmedit, tseirn acne dth cios lwleocutoldr seamryittetro andgi vceolleccotnosri dceurrarteionnts, atond hseepraer natoe nreesgiasttaivnec efe iend t h-bea ccakthoidne ltehaed soaf ma vea wlvea,y a nads
dthises itpraatniosins,t owrh sictihl lr amisoerse t hseo ttehmapt ear aftuurrteh oefr caulsrore inntasd avnids aablmleo tsot scheortrati-nc idrcaumiat gReI., siInt cies athpep reemciiatbteler cerurrorre nist ianst roofd uthcee ds abmy ere mgaargdninig- tboy r ae sctoaprea cfuitlol rA o.Ff .l ogwai nim itp meduasnt tb.e. shIunn ttheids
rttirthisasienesl sfiein.sf t Ofoceurfc rttr hcweeani lnstl e obvce"eccrruaourlmns m. eae wctIhunaom ysdou"sm l aaoetnfi vdcceoo d mnaednbsidtatir totiohnnyegs tcdahcureroroprset srnn aRetn,a3 sri.ilnsy t oothtrh ewe wro hwuolodler td hosef nathbeop uaHsts .T12.s mvufofAlict,ai egtnoet ttuuondI dmnee tearhn sti tshtih eoce noc cn oaotngelldxaetiic,tn iti ootthrn iescs u oiprmfer eruphnsoate.pr tsma aneptt p frwaocipttr hitah itanet ptbthoaae crgtkii clvwiunehle aaiarcr hsictm ayhsaaesl,a l tn1 hdd0ee0 gd rouraehaiselmi neosgf ffie snc tlehte egoftfa utiiimnnvsppehur ufoten vieitmneddg--
aF isgt.a 6b.i-liCzeodm bmiaosn seumppitltye rc Airc.Fu.i ta.mpBlaifsieer buiasisn igs isbniepiTitanihatglielo yrdner oiiisspn pneeltdihgt itlaslicger ilodbaslatetsne rtoghweerc i rnateogsse itt sohbto eearc l talir nuathssnteees aiHsddt. ioTos-rf. Apemo.Ftei.tntetiara mlifp slmidfiifaeflrelsr aennacdne rda rrbeaeldytwi oeexecnereecdebsia v2see0rs0matVnhde. piCmeldW,pa ebnhducaeetnn. ic nteh tsehoeu srcicgaens iaetl oisfc foae mdh ietgosh t fhirmeo mpbea dsaea l novciwae
derived from R1, R2 and emitter bias from R3.
tchaep tarcaintosirs tiosr d.amIfa, gheodw beyve trh, eth eex dtreac ovuopltlaingge Practical A.F. Amplifiers snoeucrecses,a sryu cahlsoa sto ain ccrluydstea lR1p.ickT-uhpe, upitpeirs
across R3 and forms a short-circuit across With these reminders, we can proceed to limit to the gain which can be achieved
the resistor, immediate destruction of the an examination of the circuit in Fig. 7 by this stage in the absence of help from
transistor results from the application of the which, with slight variations, can be used a matching transformer is set by the current
full H.T. voltage across the emitter -base to illustrate most of the features likely to gain factor of the transistor itself. This
junction in the direction of good (forward) be encountered in the products of indi- is analogous to the R.C.-coupled valve case,
conduction. vidual manufacturers in their gramophone where the voltage magnification factor
it
input output This examination of the basic points of amplifiers and the A.F. portions of their sets the upper limit.
common emitter operatitm will enable the portable radios. The coupling arrangement is similar to
analysis of practical radio and amplifier The circuit shows an amplifier in which that used in the valve circuit, but in arriving
circuits in succeeding articles to be easily the first stage is R.C.-coupled to a second at appropriate values for the components
understood. The common collector stage driving a class "B" output stage via different considerations have to be borne
arrangement, which is less frequently used, a phase -splitting transformer. Negative in mind. In the valve case the requirement
will be dealt with as a particular case when feed -back is taken from the speech coil to is to develop maximum voltage across the
it arises in a practical circuit. the base of the driver transistor. The use grid circuit of the following stage, whilst
12 13
in the transistor case it is to drive maximum proportion of the signal current goes suitable because the average current in a The output transformer is chosen so as
current into the base circuit of the following straight to earth instead of flowing in the transistor under class "B" conditions varies to present a load to each collector which
base circuit of TR2, but there is no dis- with the amplitude of the signal, and the will cause neither the collector dissipation
staWge.ith a valve, this leads to a load im- turbance to D.C. conditions as there would capacitors needed to provide a low im- nor the maximum peak current of the
pedance that is high in comparison with be if the slider were connected to the base. pedance path for the signal would charge transistors to be exceeded. In the circuit
the internal impedance of the valve, and Bias for the driver stage follows con- up and cause "blocking". A low resistance of Fig. 7 the collector load impedance is
this load is provided mainly by the anode ventional lines, the base being held at bias network without capacitors is therefore arranged to be 66 ohms for each transistor,
feed resistance shunted to as small a degree slightly more than -1.5V above earth line essential, even though this has an adverse so that the peak current with a 6V supply is
as possible by the succeeding grid leak. by R7 and VR1, and with 750 ohms total effect on temperature stabilizing. sx 1,000---=90mA (approximately). This cor-
Matching the Load esmtabititleisresresaitstanacpeproxthime atterlaynsis2tmorA. curTrehnist heIlnd taht e1 d4i0amgrVa mne ogfa tFivige. t7o tehaer tbha lsiense abrye re6s6ponds to a power output of 50)2<6
value is adequate to drive the output stage, the drop across RIO, and emitter resistances 270mW for the pair.
With the transistor, a low load impedance taking into account the current step-up of of only 3.3 ohms are used. It is possible
relative to its internal impedance is required, the Transformer T1. The whole of the to reduce bias variations with temperature
and this is provided mainly by the input emitter resistance is shunted by C4 to by replacing RIO by a combination of a High Efficiency Output Stage
impedance of the following stage, shunted prevent negative feed -back at this point. resistor and a thermistor having a negative
to as small a degree as possible by the temperature coefficient. For this comparatively large output,
resistance feeding the collector and by the owing to the high efficiency of the class
The Output Stage
bias network of TR2. "B" system, the max dissipation in each
Thus R6 needs to be high in comparison The output stage consists of two tran- A Compromise transistor will be about 60mW, which is
wwiitthh tthhee ionuppuut ti mimppeeddaannccee o of fT TRR21, .but Tnhoet swisotourlsd ibne c lcaosms "pBle" tpeulysh c-uptull.off,Idebaullty i ntheya disTtoo rctiloenar i tupi sa nuys ureaml taoi nsienlge c"ct rao srsa t-ioov feor"r nporotgrabmeymoned mathteeririalrattihneg .meaOnn v oarlduienaroyf
value of the coupling capacitor in the valve practical case this is not possible because the driver transformer Ti lower than that output power, and consequently the dissi-
cimircpueidtaniscee avt athluea ltoewde ostn r ethqeu ibreads ifsre tqhuaetnciyts idt iwstoorutilodn i.ntrTohdiusc tey pinet oolfe draisbtloer t"icorno isss c-aouvseerd" winhstiachn cwe,o iunl dt hgei vcei rpceurifte scht omwantc thhien gr.atioF oofr pnoartimoanl, owpielrla btieo ng rtheeartely i sr ead guecneedro, usso mthaartg iinn
ssuhcocueelddi nbge gmridu cleha klo. wIenr t hthe atrna nthsiastt oorf c tahsee bwyh itchhe hlaarsg ael rveaadriya tbieoenn i mn einnptiuotn eimd pase dtaaknicneg Tim1peisda3n c:1e, owf thhiec hd trriavnesrf torramnssistthoer toou otpnuet- of Tsahfies tmy.argin is increased still more when
tswcbmhtoaieioTagtn hvsiehtm . ercetsohpo lueten o ndVodwauuRinastticpur1odeua ns itlos stic f hma foCedopn 3oreen apdTmertactReuhntdsi2 clto e.i tbnn ooee ofa ,l Afvtoa hnwotseh ii tdepcnh orcveaemrlocetpelaesudsarlmiiiridnnneegedggr pcomtbhlupuaieTertlc rlr hettiearahitnasement os nspcv,ei oewsaocrntnireotv dhsmret sshoni oewtnatraieeoto ,elr wonssskwtao a atlie ynhntrdh e obcia nfpiltra ga arasfsonovsgvro cec "iisuisAdrmriciro"onuea.nfinglt lt o isetfiims gobinfsiita anttelsoosar,t tmosvwhuauhairftrkiipfdcaiiucnth itsig oeqinsnu satitatrlpryapein nahrdsobiisgxott(hiouh-m} rt eisX ma5i t4r,ep- 0)loeay0wdr o0e1afn ,no0 chi0idnetm0sr pi tvoosuae.h tmncmtausrTse.afkshlri ousvmtmsaai lnntuchoeeear,, tdnbbhoeeae,Frgs meraee paeaoetldplsfay -TtCr beqaRm autauco2tpsk te we t irdfasahr. ktowiucemrohser s dpkthilisanelscog ise,pwp aaveesteiir aoic tt nh Rhu racs1auno1taii .ltnl hlgtyoes w Ttahhri4liee5sl
Fig. 7.-Several features that have been described will be recognized in this diagram, which shows Fig. 8.-Three kinds of output circuit that can be used to couple the output from a pair of
the circuit arrangement of a transistorized A.F. amplifier. transistors in push-pull to the speech coil of a speaker.
14 15
reduces distortion and at the same time In Fig. 8(c) series connection of the tran-
lowers the output impedance of the sistors across the H.T. supply and a centre
amplifier. The capacitor C7 introduces a tap on the latter enables push-pull operation
measure of phase correction and prevents to be employed without either output trans-
positive feed -back with possible oscillation former or tapped speech coil. In addition
at higher frequencies. Since with a class there is no difficulty in taking negative 5
"B" output stage, the current from the feed -back from the speech coil to earlier
battery varies with the signal, good de - stages, using the same method as is shown
coupling is essential, and this is provided in Fig. 7. Bias is supplied by the voltage
by the large capacitor C5 across the battery divider RI, R2, R3, R4, so that under
and the decoupling combination R8, C6, no -signal conditions each transistor draws A COMPLETE TRANSISTOR RECEIVER
which feeds all supplies to the first stage a small current.
and the bias current to the second stage. Proxided that the transistors are matched,
the junction point of the emitter of TR3
and the collector of TR4 will be at the
Output Circuits
same potential as the centre tap of the
Several methods are possible for coupling battery, and no current will flow in the pRACTICALLY all the essential features of this voltage drop the effective H.T. volt-
to the speaker, and output arrangements speech coil. On applying a signal from of transistor radio circuitry may be age on TR4 is 20V, so that the load for
tarraen ssfhoorwmner i nis Foimg.itte8.d anAdt t(hae) ttwheo ohualtvpeust tdhreiv derni vaeltrernstaategley, tTo Rpa3s sa na dh iTgRhe4r cwuirlrle bnte, eoxnpel saiinngelde bryec tehiev ethr oarnodu ga hc oenxsaimdeinraattiioonn ooff maximum output is 20 x2.15,000 = 8,000 ohms,
of a centre -tapped speech coil constitute which will in each case pass through the possible variants. The set selected for this derived as explained previously on page 11
the collector loads. The design of such a speech coil. purpose is the American "Regency" receiver in Chapter 3, when discussing class "A"
loudspeaker does not present great difficulty Although, as the speech coil is in its which was the first commercially available stages. This high value of load enables high
because the impedance required is low. emitter lead, at first sight it may appear transistorized pocket radio in the world, and gain to be realized in this stage compared
A disadvantage of this arrangement is the that TR3 is operated with common col- since a few of these sets have found their with the class "B" stages described in the
difficulty of applying negative feed -back, but lector, it will be realized that this is actually way into this country, this circuit informa- last chapter, but the output power is
it simplifies the circuit and saves a major common emitter operation, because the tion may be of direct use to service small, in the neighbourhood of 25mW. A.F.
component. The capacitor C8 is necessary input signal is applied between base and engineers who may be asked to deal with feed -back through the resistance of the H.T.
to limit the rise in high note response which emitter and the output is taken between faults that develop in them. battery is minimized by shunting it with
occurs when a loudspeaker is fed from a collector and emitter. The strangeness of C16.
high impedance source. their appearance in the diagram is due to A Commercial Receiver The secondary of the I.F. transformer T3
In Fig. 8(b) centre -tapped speech coil the battery position on the collector side The circuit is shown in 9 and it feeds the signal to the crystal diode detector
is again employed, but in this case the of the load. employs four transistors. The Ffiigrs.t is a self - D1, whose load consists of the volume
clteroaadnnsns.eisctTtoihrosins a ,c rwier ciutuhsiet tdhh aeisn l iocnaohdmer iemnn ott hnfee ce doe ml-lbeiacttcteokrr, 2o5pA0ermmatWep l oiafnrie es rinmso wwila ibrth epi nrfiagnr cm hipailgdehes,e tbro u pttho sowinseecr ei lttlhhuaesny- oafonscudirltlhatt hiinisr gda cfoarmereqmueoIn.ncFy .- e-cmhamaitntpgelerif rci,elrassths e"a An'sde"c ootunhted- cdpoeovnneterlnootlp ieVsd R sah3cu.ronstesT dV htRoe 3 Ae, aw.rFthh.i lvemi taho Ced uI1l.a3Ft..i ocnoTmhi-es
which gives it an advantage over the trated above a separate analysis would put stage. I'ktection and A.G.C. are pro- parallel path for some of the rectified cur-
arrangement in Fig. 8(a), but the stage gain cover no new points and will not therefore vided by a crystal diode. The transistors are rent through R11 will be considered later
is smaller and the feed -back does nothing be pursued here. The earlier stages of of n -p -n type so that the H.T. line, which when examining the A.G.C. arrangements,
to reduce distortion in the driver stage radio sets do, however, raise a number of is supplied from a 22.5V hearing aid battery, but in the meantime it should be noted that
which has to provide higher power than new points, and these will be examined in is positive. The use of a circuit with n -p -n a slight positive bias is provided from the
in the common emitter case. the next chapter. transistors for our illustration will serve to H.T. positive line via R11 and R4 which
remind readers of the existence of the alter- improves the sensitivity of the diode to weak
native type, and it underlines the fact that, signals.
apart from polarities, circuits are identical
Low Intermediate Frequency
in the two cases.
The A.F. signal is fed via C14 to the base TR3 functions as a common emitter
of the output transistor TR4, and the output amplifier at an I.F. of 262Kc/s, the input
signal is taken from the collector through and output transformers having tuned
the matching transformer T4 to the loud primary windings and closely coupled
speaker. Biasing arrangements are conven- untuned secondaries with a step-down turns
tional, the voltage divider R12, R13 supply- ratio. H.T. to the collector is obtained via
ing the base, and R14 producing the neces- RIO, and C11 provides decoupling to the
sary D.C. feed -back to give stability against emitter.
temperature changes. R14 is shunted by As already mentioned, base bias to this
C15 to prevent reduction of gain due to stage is derived from the emitter resistance
A.F. feed -back. of TR4 instead of from the more usual
The collector current for this stage is voltage divider. This places the base i-.bout
about 2.5mA, so that the emitter is 2.5V positive with respect to earth line, so
about 2.5V positive to earth line, and it will that to establish its emitter at the correct
be noticed that this voltage is used to feed potential, TR3 passes a little less than 1 InA
the base of a previous stage, with conse- through its 2-7kn emitter resistance R8. The
quent saving of two resistors. As a result earthy end of the secondary of T2 is
16 17
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