NATIONAL 7.1. UIIIIIl It111t. Radio-Trician (Trace Mark Registered U. B. Patent Office.) LESSON TEXT No. 11. (2nd Edition) THE VACUUM TUBE

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1 NATIONAL RADIO llnstitute 7.1 UIIIIIl It111t _,`mmuuuuunuuuum 1 nnu uuunuuunu.ugr = Radi-Trician (Trace Mark Registered U. B. Patent Office.) LESSON TEXT N. 11 (2nd Editin) THE VACUUM TUBE AS AN AMPLIFIER ';: R m p111p1p tp nnippp t111fÌ1.r (Öriginatrs f Radi Hme Study Curses Established Washingtn, D. C.

2 "Yu are just as big as the things yu d, just as small as the things yu leave undne. The size f yur life is the scale f yur thinking." -Wdrw Wilsn. Clearness. SOME GOOD STUDY HABITS A Persnal Message frm J. E. Smith The habit f insisting n clearness f thught is es- sential t study. Mental vagueness is mental weakness. Never leave a lessn with a vague idea f it. A gd way t vercme mental cludiness is t establish the habit f writing ut the vague pints. If yu can express yur thughts in writing, it will help yu clear them up. Cpyright 1929, 1930 by NATIONAL RADIO INSTITUTE Washingtn, D. C. W125C22530 Printed in U. S. A.

3 Radi-Trician's (Trade Mark Registered U. S. Patent Office.) Cmplete Curse in Practical Radi NATIONAL RADIO INSTITUTE WASHINGTON, D. C. THE VACUUM TUBE AS AN AMPLIFIER Thus far in the study f "Practical Radi", ur study f elementary principles has built a fundatin preparing us t take up ne f the mst imprtant subjects cnnected with Radi-the actin f a three element vacuum tube as an amplifier. In the previus lessn, the actin f the vacuum tube as a detectr was studied. In this lessn, the actin f the three element vacuum tube as an amplifier will be discussed. Withut this adaptin, the usefulness f the vacuum tube in radi wuld be cnfined t detectin r rectificatin. Befre prceeding with a detailed study f the actin f a vacuum tube as an amplifier, let us briefly review sme facts. T understand the actin f a vacuum tube, the fllwing facts shuld be brne in mind. A current f electricity is simply a flw f electrns, the electrns flwing in ne directin, which makes a current. Electrns are small charges f negative electricity. All material cntains electrns. There are tw kinds f electricity-psitive and negative. Like electricity repels and unlike attracts. The fllwing additinal facts must nw be grasped befre the actin f the vacuum tube can be thrughly understd. It has been discvered that metals, if heated, will thrw ff int space sme f the electrns which the metals cntain. Furthermre it has been discvered that the htter the metal, up t a certain degree f heat, the mre electrns it discharges. These electrns travel at a high rate f speed. If air r any ther gas be present in the space arund the metal, the electrns strike the minute particles f the air r gas and are sn stpped. At this pint, it wuld be well t bring ut a pint in regard t the directin f flw f current and the directin f flw f electrns. The directin in which electric current is said t flw is a matter f arbitrary decisin. The electric current is said t flw frm psitive t negative in the exterir part f a battery circuit. When the early scientists decided which 1

4 way the electric current flwed, they did nt have knwledge f the existence f electrns s they called ne terminal f a battery psitive and the ther negative, and since the psitive value indicates a strnger frce than a negative value, the current was said t flw frm psitive t negative. Since that day, the electrn thery has been definitely established. It has been definitely established that the electrns flw frm the negative terminal f a battery t the psitive terminal in the exterir part f the battery circuit. The student shuld firmly bear this in mind and nt becme cnfused as t the directin f flw f electrns because this is definitely settled. T prevent any seeming cntradictin we shall adhere t 0 e e/ e p l ó/ e / \ ($01/ 0 00/ r 0\ O p O Fig. 1-The Electrn Discharge frm the Ordinary Filament. The arrws n this much -simplified drawing shw the directin taken by the electrns that are emitted frm the heated tungsten filament, A. the ppular way f expressing the directin f flw f current-that is, frm the psitive t the negative in the exterir part f a battery circuit. THE FILAMENT There are at present three general types f materials used in the cnstructin f the filament: tungsten wire filament, the xide cated tungsten filament and the thrium cated tungsten filament. Frmerly, all vacuum tubes used the tungsten type f filament. It was later discvered that by cating the tungsten filament with the xide f certain metals that the number f electrns given ff by the filament fr a certain amunt f cur - 2

5 rent passing thrugh the filament wuld be materially increased. Fr a given number f electrns thrwn ff, less current wuld be required t heat the filament naturally, the filament culd be perated at a much lwer temperature, with a cnsiderable increase in the life f the filament, befre burning ut. The latest advance in filament cnstructin cnsists in cating the tungsten filament with thrium. This gives a further increase f electrn emissin, and in rder t have a certain amunt f electrn emissin, a further saving in the amunt f current required t heat the filament is btained O 0 ` n O ` O O0O O n O O O C 0 _ e Q Fig. 2-The Electrn Discharge frm the Thriated Filament. Nte the greatly increased emissin f electrns frm the tungsten filament, A, which has been cated with a minute layer f thrium, B. This is illustrated in Figures 1 and 2. In Figure 1, it will be nticed that the number f electrns, represented by the small circle, is nt nearly as great as the electrns represented in Figure 2. ACTION OF THE VACUUM TUBE A vacuum tube cnsists f a cntainer, usually glass, frm which the air has been pumped. In this glass tube r bulb is munted the filament, the grid, and the plate. In sme A. C. tubes the usual filament is replaced by an indirectly heated cathde cnsisting f a metal xide cated cylinder; this thrws ff the electrns as it is heated by an internal filament. By applying sme facts previusly learned, we can nw take up in detail and understand the actin f the vacuum tube. Since the air is pumped frm the glass tube r bulb (hence 3

6 the name vacuum), the passage f the electrns thrwn ff frm the filament will nt be stpped. Since there is nly a very small amunt f air r gas within the glass tube, electrns thrwn ff by the filament will be accelerated, because there are n small air particles which impede the electrns in becming detached frm the filament. In Fig. 3, the filament is heated s that it becmes red r white ht. This is dne by the electric current furnished by the "A" battery r by a step -clwn transfrmer with AC tubes. Suppse that the filament is ht and the grid and plate are nt cnnected t utside circuits. The electrns are thrwn ff frm the filament and strike bth the grid and the plate. These acquire a negative charge, as they have acquired electrns, which are negative charges f electricity. The space inside F the tube has als a negative charge as the space is filled with negative electrns. Like charges repel each ther and hence the negative charge n the plate, the negative charge n the grid, and the negatively charged space inside the tube are all repelling the electrns which the a ht filament is trying t thrw ff. As =11111 each electrn is thrwn ff f the filament, it adds its charge, either t the Fig. 3 plate, grid, r space. The strnger charge causes a strnger repulsin f the escaping electrn. In a very shrt while, the repulsin is strng enugh t prevent the escape f any mre electrns frm the filament. Figure 4 shws an "A" battery, used t heat the filament and a "B" battery, with its psitive terminal cnnected t the plate f the tube, and its negative terminal cnnected t the filament circuit. (The use f the grid will be shwn later). By cnnecting the battery as shwn in the figure, tw things have been dne. First, a psitive ptential has been placed n the plate; secnd, a metallic circuit cntaining a battery has been made utside f the tube frm the plate t the filament. This leaves nly the space between the filament and the plate inside the tube t cmplete the circuit. The "A" battery is used simply t heat the filament. The heated filament thrws ff electrns. The plate is psitive and attracts the electrns which are negative. The 4

7 electrns travel thrugh the space (n air r gas particles being present t hinder them as it is a vacuum) frm the filament t the plate. There is then a flw f electrns frm the filament t the plate and a flw f electrns is an electric current. Thus, the cmbinatin f the heated filament, the vacuum, and the psitively charged plate, has caused a current t flw; that is, in effect, it has cmpleted the circuit which cntains the "B" battery, plate and filament. The actin f the "B" battery is cmparable t a pump. When it frms part f a circuit, it pumps electrns ut f its negative terminal and int its psitive terminal. In the circuit arrangement shwn in Fig. 4, the "B" battery pumps the Fig. 4-Diagram f the Insulated Grid in the Vacuum Tube. electrns cming frm the filament, t the plate t the + terminal f the "B" battery and ut f the - terminal f the "B" battery t the filament. The filament again thrws them ff and they g t the plate, being attracted by it as it is psitive and the electrns flw arund the circuit. This flw f electrns cnstituting a current f electricity can be measured by the ammeter, A, placed as shwn. This ammeter culd als be placed between the plate and the psitive terminal f the "B" battery. Cnsider the effect f changing the number f cells in the "B" battery. Changing the number f cells in the battery wuld change the psitive ptential n the plate. If the psitive ptential n the plate became greater, it wuld have a greater attractin fr the flying electrns in the tube, and hence in a given 5

8 time, mre electrns wuld arrive at the plate and be pumped arund the circuit by the "B" battery. An increased flw f electrns means an increased flw f current. In the same way, a decreased ptential n the plate wuld cause a smaller current t flw. This change in current with a change in plate ptential des nt fllw Ohm's Law; that is, dubling the plate ptential des nt duble the current as it des in a whlly metallic circuit. If there is nt an increase in filament current and temperature, nly a certain amunt f electrns can be thrwn ff by the filament, and hence a pint will be reached when the plate ptential is raised, where n mre electrns can be attracted frm the filament t the plate. Since the electric current is said t flw in the ppsite directin frm the electrns, this electric current is then said t flw frm the psitive terminal f the "B" battery, thrugh the plate, and then by means f the electrn current, thrugh the filament, and back t the negative terminal f the "B" battery. The electrns pass frm the filament t the plate and grid. Neither the plate nr grid can emit electrns as they are nt heated. This means that the electrns can pass nly ne way thrugh the tube; an electric current can pass nly ne way thrugh the tube. This is exactly what the crystal detectr des. A vacuum tube with nly a filament and plate (grid cnnected t the plate r nt built int the tube) may be used as a detectr in place f the crystal. Such a tube may als be used as a rectifier f alternating currents, because it allws current t pass nly in ne directin. Nw we will explain hw the grid greatly imprves the actin f the tube, befre taking up the actin f the vacuum tube as an amplifier. ACTION OF THE GRID As has been explained, the plate current may be cntrlled by variatin f the filament current and temperature and als by variatin f the plate ptential. It was discvered by DeFrest that putting a third element in the tube gives a mre sensitive methd f cntrl. This third element is the grid. It must be remembered that the grid is a lattice -like cnstructin which surrunds the filament and is, therefre, between the filament and the plate. The illustratins s far d nt shw any cnnectin t the grid. The electrns, which are emitted by the filament, pass thrugh and arund this grid and are nt impeded in 6

9 psitive their passage t the plate. Nw, suppse we shuld cnnect a small battery (shwn as "C" in Fig. 5) acrss the filament and grid with the negative terminal cnnected t the grid and the. terminal cnnected t the filament. This wuld make the grid negative with respect t the filament, r in ther wrds, a negative charge will be placed n the grid. Let us study the effect f this charge n the grid in the diagram. The electrns trying t leave the filament represented by the arrws are negative. The grid is charged negatively by the "C" battery. Remembering the fact that "like charges repel and unlike charges attract," we readily see that the electrns are repelled and frced back t the filament; a small number, r -c-bar 2 V 6 V. 20- /OO V. fl"bat I--- B"BAT. Fig. 5-This Diagram Illustrates the Actin f the Negative Charge n the Grid. nne, ever get acrss t the plate. Hence, in this cnnectin, the tube lets little r n current acrss frm the plate t the 2 - filament. What wuld happen if we suddenly were t reverse the terminals f the "C" battery which is charging the grid? Let us investigate this in Fig. 6. In this case, the grid wuld have a psitive charge and the negative electrns wuld be strngly attracted acrss frm the filament t the grid. When they get this far n their jurney, they begin t feel the greater attractin f the higher psitive vltage charge n the plate and they pass thrugh the spaces in the grid in a flying effrt t get t the plate, which receives them "with pen arms", s t speak. 7

10 The attractin f the psitive charge n the grid draws many times mre electrns frm the filament than wuld rdinarily leave it, and thus the density f the stream is increased many times. The current flwing acrss frm the plate t the filament, f curse, is a direct current, and is knwn as the plate current f the tube. T sum up the actin f the tube in a few wrds, we might say that the plate current f the vacuum tube can be cntrlled by the vltages applied t the grid. In Figures 5 and 6, the "C" battery allws a ptential t be placed n the grid. This can be made strnger r weaker by changing the ptential f the battery. It can be made psitive r negative by reversing the cnnectin f the battery. Making the psitive ptential f the grid higher causes it t attract the electrns with mre frce, and causes a greater current -C-317 T Fig. 6-The Actin -BAT. B -BAT. f the Psitive Charge n the Grid. t flw in the tube. By making the negative ptential f the grid higher, the electrns are impeded in their prgress, resulting in a decreased amunt reaching the plate and a crrespnding decrease in the plate current. Since the grid itself has a very small surface and des nt catch many f the flying electrns, mst f the electrns g past the grid and reach the plate when there is a psitive ptential n the grid. If the negative ptential is made large enugh, its repulsin f the electrns will entirely stp their flw and hence stp the passage f any plate current. Because f the nearness f the grid t the filament, a slight change in the ptential f the grid 8

11 Il I 1 I.11 makes a large change in the plate current. The effect f changing the grid ptential is, therefre, much greater than btained by changing the plate ptential. Thus, it is pssible t have a small amunt f energy acting n the grid cntrl a greater amunt f energy in the plate circuit. In Figure 7, we have an arrangement whereby it is pssible t quickly vary the amunt f vltage and the plarity f the vltage that is applied t the grid. When the mvable arm f the ptentimeter, P, is at C, there will be a negative ptential applied t the grid, equal t the ptential difference between the pints A and C. When the arm is at B, a psitive ptential will be applied t the grid. Since the grid -return, filament end f the grid cnnectin r circuit, cnnects t the filament, at pint A, which crrespnds t the middle cell f the "C" battery, the grid vltmeter will nt indicate any ptential difference between the grid and filament when the arm f the Battery= B A 400 hm Ptentimetei filament 6.V ) vltmeter Plate MA. O A Battery B Battery Plate 17//meter Fig. 7-Circuit Diagram f Testing Set fr Obtaining Tube Characteristics. ptentimeter is half -way between C and B. The ppsite vltages between AB and AC just balance each ther. As the ptentimeter arm is mved alng the resistance, the vltage applied t the grid. which is usually called bias vltage, causes a change in the plate current as registered by the plate milliammeter. By changing the applied grid r bias vltage, the plate current is changed. Suppse that n current is flwing in the plate circuit when the ptentimeter arm is at C in Fig. 7 and when the arm is at B, a flw f 6 milliamperes. At intermediate pints between C and B, the grid vltmeter and plate milliammeter reading culd be recrded fr future reference. The readings culd be recrded smething like that shwn in Table N. I. 9

12 In the left-hand clumn, the grid vltages are shwn and n the same line in the right hand clumn, the crrespnding plate current is shwn. Different tubes f the same type, r different types f tubes will give different readings. Later n in this lessn, we are ging t make use f this table and the figures therein in rder t learn mre abut the actin f the vacuum tube. S far in ur study f the actin f the vacuum tube, we have fund that with a negative ptential applied t the grid, the electrns passing between the filament and plate are retarded, resulting in a decrease f plate current. Furthermre, with a psitive ptential applied, we fund that this psitive ptential assisted the plate in attracting the negative electrns, resulting in an increase f plate current. Hwever, we verlked an imprtant fact. In Figure 6 when the psitive terminal f the "C" battery is cnnected t the grid with a psitive charge applied t the grid, it can be nted that a few TABLE N. I Grid Vlts Plate Current Milliamperes Grid Vlts Plate Current Milliamperes f the electrns will be stpped by the grid, and since the "C" battery psitive is cnnected t the grid, these electrns will pass frm the grid t the psitive terminal "C" battery, and thence t the filament, cmpleting a circuit. This results in a very few f the electrns which shuld g t the plate being utilized in the grid circuit. If there were nly a certain number f electrns being given ff by the filament and sme were utilized in the grid circuit, when the grid becmes psitive, then, this wuld result in less electrns reaching the plate and a very slight reductin in the plate current. This, we want t vercme because when the grid is psitive with respect t the filament, the plate current will be increased in the same 10

13 amunt that the plate current is decreased when a negative ptential having the same value as the psitive ptential is applied t the grid. Befre prceeding further, it will be well t clear up anther statement fr the student. When the grid is spken f as being psitive, r negative, with respect t the filament, the negative end f the filament is referred t as this is the end which the electrns enter in their passage thrugh the filament frm the "A" battery. Remember this, as it is a very simple but imprtant fact. We have nt yet settled ur prblem f preventing a current frm flwing in the grid circuit when the grid is psitive with respect t the filament. Hw can we arrange things when the grid becmes psitive with respect t the filament, a grid current will nt flw? Simple enugh, but read these next few sentences carefully. We fund that when the grid was slightly negative that the grid repelled sme f the electrns and prevented them frm reaching the plate, resulting in a very slight decrease in plate current. The mre the grid became negative, the mre the plate current was decreased until finally the plate current culd be cut ff entirely. Hwever, it is nt desirable t d this. Suppse that the grid is permanently maintained at sme negative value s that any changes in the ptential applied t the grid never allw the grid t becme psitive with respect t the filament. T make this pint clearer, let us say that we apply a negative ptential f tw vlts t the grid. If the negative grid ptential is increased ne vlt mre r t three vlts, we wuld have less plate current flwing than when tw vlts negative were applied. Als, if the negative grid ptential is decreased frm tw vlts t ne vlt negative, there wuld be mre plate current flwing than when tw vlts negative were applied t the grid. Nw refer t Fig. 8. Here we have an alternating current generatr, G, cnnected t the grid and filament f a vacuum tube. A cell, C, which delivers, let us say, tw vlts, is inserted in series in the grid cnnecting wire. Ntice that the negative terminal f the cell, C, is cnnected t the grid f the tube. As in the case given in the preceding paragraph, let us say the generatr delivers ne vlt f alternating current. When the vltage f the generatr is trying t frce current frm A in the directin f C and t the grid, it is ppsed by the vltage f the cell, C, which is trying t frce current 11

14 in the ppsite directin. When the vltage f the generatr is trying t frce current frm B in the directin f the filament, it is assisted by the cell, C. As the cell, C, delivers mre vltage than the generatr, then the grid is always negative with respect t the filament. The vltage f the generatr merely adds t and subtracts frm the effective vltage f the cell which is applied t the grid. Here, then, is the slutin f ur prblem. By always keeping a negative ptential applied t the grid, the grid never becmes psitive in respect t the filament, whenever changes in ptential ccur. Changes in the grid ptential will cause crrespnding changes in the plate current. In Figure 8, the alternating current generatr causes the vltage applied t the grid t vary between minus ne and minus three vlts. Since the nrmal negative vltage f the Fig. 8 cell, C, is decreased and increased by the actin f the generatr vltage, the vltage that is actually applied t the grid is said t vary r swing in a negative and psitive directin with respect t its nrmal vltage. This being the case, we cme t an imprtant fact. When a three element vacuum tube is used as an amplifier, the grid must always be maintained at such a negative ptential that any variance f this ptential will never allw the grid t becme psitive with respect t the filament. This fr the purpse f preventing grid current frm flwing. Later we shall learn mre abut hw this is dne in a receiving set. THE CHARACTERISTIC CURVE Here we have a drawing f a curve n a piece f crsssectin paper frm which we are ging t make use f the 12

15 I i i I i I reading f the grid vltage and crrespnding plate current readings which were shwn in Table N. I. The base line CB crrespnds t the vltage that is applied t the grid by mving the ptentimeter arm alng CB in Fig. 7. We fund that as the arm was mved tward B, the plate current increased, s we let the line CD f Fig. 9 represent this increase. Frm Table N. I, we fund that with the arm at C r the maximum negative vltage, we did nt have any plate current flwing. Nw in rder t make these values serve anther purpse, we will plt them n the sheet f crss-sectin paper Ki 5 i- T I-5.6' I - I r -X C _-6 54_3-2_I Grid Vltage Fig. 9 Lcate -12 n the base line CB and g up n this line until the hrizntal line ppsite fr.1 n CD is intersected. This is the first hrizntal line abve the base line. Stp here and make a dt as shwn in Fig. 9. The hrizntal lines, represent values f plate current and the vertical lines grid vltages. Nw lcate the remaining pints as btained frm the values given in Table. N. I. When all the pints have been 13

16 3 lcated, draw a line cnnecting all the pints and the curve XY is the result. This is called a characteristic curve f the plate current and grid vltage. It is btained as just explained. First, it is necessary t have a circuit arranged such as in Fig. 7; then, make nte f the values such as in Table N. I, and finally, plt the curve as shwn in Fig. 9. The imprtant pint abut this curve is that it is nt a straight line. The lwer prtin f the curve t the left f and abve and crrespnding t -7.5 grid vlts is mre hrizntal and nt as steep as abve and crrespnding t zer grid vlts. Als, again, abve and crrespnding t +10 grid vlts, it is mre nearly hrizntal than between -7.5 and +7.5 grid vlts. In ur study f detectin, in the previus lessn, we fund that the tube perates n ne f the bends r "knees" f the curve, such as at K r Kl, when using the "C" battery methd f detectin. Shuld we desire t perate the tube as an amplifier, it is necessary t perate it n the straight prtin smewhere between these pints. Why and hw d we d this? First, we learned earlier in this lessn that when a vacuum tube was wrked as an amplifier, we wanted changes in the grid ptential t cause crrespnding changes in the plate current. If the grid vltage is increased and decreased in a like amunt, we want the plate current t increase and decrease in the same manner. Next, we learned that it was necessary t keep the grid at a permanent negative value in rder t prevent a grid current frm flwing which wuld interfere with the plate current increasing in the same prprtin that it was decreased fr varius changes f the grid vltage. By referring t Fig. 9, we see that between K and Kl, the line is almst straight. At the pint n curve abve and irer crrespnding t -3 grid vlts, the tube culd he satisfactrily perated as an amplifier. If the grid bias negative vltage is decreased t --1 vlt, the plate current wuld be increased frm 2 milliamperes t 2.65 milliamperes. If the negative grid vltage is increased t -5 vlts, the plate current wuld be decreased frm 2 t 1.35 milliamperes. We then have the cn- 3 ditin under which an amplifier must perate; the grid is maintained at such a negative vltage that any changes which might ccur in this ptential will never allw it t becme psitive 14. r-- r

17 TUBE CX-1z UX-12 CX-299 LUC -199 CX-22 UX-Iz CX-322 UX-2z2 TABLE N. 2 AVERAGE CHARACTERISTICS OF RECEIVING TUBES +F 'A E ÿtl.ä. ô w F CX-3A UX-2 A 5 «^ ú º 7 P. w U. 1.1 z5 3'3 DETECTOR V q ' r. â U.. á, ô i ill 7:3'-(5 C AMPLIFIER h+ C U..,7_,--- 2a Ó Lie, ii p N;- t Ñ ñ,w Ñ i a+ á3 á0.'o,,.. Ñ t 45 V I t 45V Screen -Grid Vltage -E CX-301A UX-21 A CX-34 UX-z4 5 O 25 CX-326 UX-zz C-327 L'X C-324 UX-22 CX II2A UX-112 A CX-371A UX-171 A CX X H * * Screen -Grid Vltage --75,.., a Iß. t D. C. A. C I CX LX CX-350 LUC -25 7' _ g í5 *When used in resistance -cupled amplifier with.25 meghm plate resistr.

18 with respect t the filament. By applying this negative ptential and perating the tube upn the straight prtin f its characteristic curve, the changes in grid vltage prduce crrespnding changes in the plate current. It will be well t nte here that it is nt necessary t have a circuit such as shwn in Fig. 7 and plt a curve such as shwn in Fig. 9 befre we can use a particular tube as an amplifier in a receiving set. The manufacturer des all this fr us in the labratry and tells us just what grid vltage t apply fr a certain plate vltage. This infrmatin is usually given in the sheet f directins which accmpanies the tube when it is bught, r else given in a tube chart. Such a tube chart is shwn in Table N. 2. Take fr instance the UX-201-A tube. Under the clumn "Amplifier Plate Vltage," we find the figures 90; under the clumn fr "Amplifier Grid Bias Vltage," the figures 4.5. When a plate vltage f 90 vlts is applied t the tube, a negative vltage f 4.5 vlts shuld be applied t the grid, and fr 135 vlts n the plate. 9 vlts negative shuld be applied t the grid. By applying the prper negative grid vltage t the tube, when a certain plate vltage is applied, the tube is caused t wrk upn the straight part f its characteristic curve where prper amplificatin takes place. ACTUAL WORKING CONDITIONS Nw let us refer t Fig. 11. Here we have the circuit diagram f part f a receiving set. The appraching signal wa're which is sent ut by the transmitting set strikes the antenna, induces a vltage in it, and causes a current t pass thrugh the cil L t the grund. The current in passing thrugh the cil L sets up a magnetic field which changes accrding t the changing signal vltage in the antenna. This changing field induces a vltage in the cil L2. The variable cndenser C cnnected acrss the cil L2 tunes r causes the circuit L2 C t be respnsive t the same frequency as the signal vltage induced in the antenna. There is then a crrespnding alternating vltage in the circuit L2 C and since ne terminal f the cil L2 and the cndenser C cnnects t the grid, and the ther terminal f the cil L2 and cndenser C cnnects t the negative terminal f the "C" battery and then t the filament, this alternating vltage causes changes in the applied ptential between the grid and filament. Any change in the grid ptential will cause changes in the plate current, 16

19 therefre, the amplifier tube V.T. merely repeats the changes f the grid ptential in the plate circuit. As the "C" battery always maintains the grid at a negative ptential, as previusly explained in regard t Fig. 8, the plate current rises and falls in prprtin t the changes in the grid vltage. There are tw imprtant things t remember here. First, the antenna signal current, which flws thrugh L merely induces a vltage in the secndary circuit L2 C and this in turn is impressed n the grid, resulting in a change in plate current. The antenna current des nt flw thrugh the secndary circuit and then thrugh the tube and plate circuit. In flwing thrugh the primary cil L, a vltage is induced in L2 C circuit which affects the grid ptential resulting in a change in plate current. Each part f the circuit "repeats" the changing currents and vltages. The secnd pint is that the vacuum tube amplifier V.1'. is a vltage actuated device. By this, we mean that what we desire is t have a change in the ptential that is applied t the grid. The "C" battery maintains the grid at a certain negative ptential when there is n signal vltage n the antenna. When a wave appraches, the net result is that the effective vltage f this "C" battery is changed, resulting in a changed grid ptential and a crrespnding change in plate current. Thus, it can be seen that the change in plate current depends upn a change in the vltage applied t the grid. It is nt always necessary t use a "C" battery in rder t maintain the grid at a slight negative ptential r bias. This negative bias can be btained in anther way, illustrated in Fig. 12. Ntice that the "C" battery is left ut, and the grid - return cnnects t the negative terminal f the "A" battery. 17

20 Previusly, it was stated that when the grid was spken f as being negative r psitive in respect t the filament that the negative end f the filament was referred t. Since this is true and the grid -return in Fig. 12 cnnects directly t the negative terminal f the "A" battery, then the grid is slightly negative with respect t the negative end f the filament. This is true due t the vltage drp in the filament rhestat. The main pint t remember abut the amunt f negative "C" battery r bias vltage is that it must always be greater than the signal r alternating vltage that is applied between the grid and filament in rder t prevent a grid current frm flwing. FORM OF PLATE. CURRENT Earlier in this lessn, we learned that the plate was maintained at a psitive ptential by being cnnected t the psitive terminal f the plate r "B" battery in rder t attract the electrns thrwn ff by the filament. The electrns flw in ne directin nly, frm the filament t the plate. Therefre, any change in the grid ptential merely causes a rising and falling f the plate current. The plate current never reverses its directin f flw. Sme writers refer t the plate current as "alternating". This is misleading t the student, as the wrd alternating indicates a reversal in directin f flw. A better way is t say "the alternating cmpnent f the plate current". This varying r alternating cmpnent f the plate current cmes in due t the fact that when a vacuum tube is prperly used as an amplifier, the frm f the plate current resembles the frm f the alternating vltage applied t the grid. By referring t Fig. 13, this can be mre easily understd. In this figure, we have illustrated a characteristic curve f a tube. The grid is maintained at a negative value f 3 vlts. This is shwn as the pint B n the curve. This pint als represents the steady plate current f 2 milliamperes flwing. Suppse that while the grid is maintained at this ptential, we impress n the grid an alternating vltage causing its ptential t vary between minus 1 and minus 5 vlts. In rder t illustrate the changing values in grid vltage and crrespnding changes in plate current, the "wavy" curves Eg and Ip have been used. Since the letter E usually represents vlts, then t indicate grid vltage, the letter g is added. The letter I is 18

21 a ild used t represent current, hence Ip represents plate current. As the grid negative vltage is decreased frm -3 t -1 vlts caused by the applied alternating vltage, then the part f the wave r curve frm B t D represents this change. The effect f decreasing the negative grid vltage results in an increase f plate current as shwn by the part f the plate current curve frm B t E. As the effective grid vltage begins t increase, due t the applied alternating grid vltage, it reaches its maxi- mum negative value at A, s that the part f the curve r wave frm D t F represents this change. Due t this increase in negative grid ptential, the plate current decreases frm E t G. By carrying this explanatin further, the balance f the grid vltage and plate current curves are cmpleted. The main pint t nte abut these curves is that the plate current curve Ip is larger than the grid vltage curve Eg, and Fig. 12 that it als is the same general shape. Since this varying plate current has the same shape as the curve Eg representing the alternating grid vltage, it is cmmn t speak f the varying plate current as "alternating", r better yet, "the alternating cmpnent f the plate current". AMPLIFICATION FACTOR -f- The amplificatin factr f a vacuum tube is its theretical 5 - vltage amplifying pwer. The symbl p used in representing this amplificatin cnstant is knwn as "mu" which resembles ur small letter "p" as used in the alphabet. The amplificatin factr f a tube depends, fr example, n the mesh f the grid, diameter f the grid wire, and resistance between the grid and the plate. In practice, it is generally fund that the amplificatin cnstant des nt vary. Fr extreme limits f grid vltage and plate vltage, this cnstant des change 19 A e

22 I 1 I slightly. The vltage amplificatin factr r amplificatin factr, as it is usually called, is generally given in the directin sheets which accmpany a tube and in tube charts, such -- as in charts shwn in Table N. 2. As a practical example, suppse the amplificatin factr f a certain tube is given as 8 as determined by examining Table N. 2. This simply means that whenever a grid vltage is applied t the grid -filament 6 L L 4 3 G E B i /P A I -i D t^7 i-- F I i i I I i -/5 -/0 fy i / a + 5 Grid Vltage Fig /.5 circuit, it is increased eight times in the plate circuit. The amplifying pwer f the tube is eight times. This amplificatin factr is regarded as the cnstant because it cannt be changed by altering any f the circuit arrangements r parts within the circuit. It depends upn the gemetrical arrangement f the elements within the tube. SPACE CHARGE In Fig. 14 is shwn in elementary fashin the distributin f electrns between the plate and the filament; we will 20

23 cnsider the electric frces acting n tw f the electrns A and B. The electrn A is urged t the plate by tw frces, the attractin frm the plate and the repulsin frm all f the electrns between it and the filament; it will undubtedly g t the plate. Electrn B, althugh attracted by the plate, is repelled by all the electrns between the plate and itself; whether it will mve twards the plate r re-enter the filament depends upn the relatin between these tw frces. It is evident that clse t the surface f the filament, the effect f all the electrns in the space between the filament and the plate (cnstituting the space charge) will practically neutralize any effect n the plate, unless the plate vltage is high enugh t give a frce + f attractin greater than the repulsin B A frce exerted by the space charge. Thus, it can be seen that it is necessary t use a Q high vltage applied t the plate f the tube e in rder t vercme this space charge ef- 0 fect which causes the electrns near the fila- ment t be repelled t a certain extent. As the electrns prgress in their travel frm the filament t the plate, they are naturally accelerated due t the attractin f the plate and due t the fact that the electrns which Fig. 14 have vercme the space charge assist them. PLATE RESISTANCE AND IMPEDANCE The plate resistance f a tube is due t the wrk which the electrns emitted frm the cathde (filament) must perfrm in mving frm the cathde t the ande (plate). Let us cnsider the case f a single electrn emitted frm the cathde. In mving thrugh the cathde surface, it has t d an amunt f wrk equivalent t the electrn affinity, and in mving frm the cathde t the ande, it has t d wrk in vercming the space charge effect and difference in ptential between the cathde and ande. This may smetimes assist the electrns in mving frm cathde t ande. If these were the nly frces exerted n a large number f electrns escaping frm the cathde, the applicatin f a small vltage between cathde and ande wuld almst immediately give rise t the saturatin current (ttal amunt f plate current btainable) and the resistance f the tube wuld be very lw fr all values f cur - 21

24 rent less than the saturatin current. This is nt the case, since the electrns in the space exert a mutual repelling frce n ne anther. This is the space charge effect previusly explained and causes by far the greatest expenditure f energy n the part f the electrns in mving t the ande. This expenditure f energy causes a slight heating f the ande. In small receiving tubes, this heating f the ande never becmes enugh t be visible unless excessive vltages are applied t the ande. The true direct current resistance f the tube is, f curse, given simply by the rati f the ttal amunt f wrk dne t the square f the current, that is, by the rati f the plate vltage t the plate current. The alternating current resistance, n the ther hand, is given by the slpe f the plate current characteristic curve, and since the characteristic curve is nnlinear, the alternating current and direct current resistances are nt the same. The impedance (alternating current resistance) f the tube is given by the rati f the alternating vltage acting between the filament and the plate, t the alternating current cmpnent in the plate circuit. CASCADE AMPLIFICATION Frm ur discussin s far, we have learned that the vacuum tube amplifier merely increases and repeats changes in vltage applied t the grid in the plate circuit. The real advantage f the vacuum tube as an amplifier is due t the fact that several amplifier circuits r stages can be prperly cnnected, ne fllwing the ther, and the riginal signal vltage repeated and amplified many times. Where several stages f amplificatin are t be used, it is cmmnly referred t as cascade amplificatin. If scillatins (alternating current f high -frequency) are applied t the grid f ne tube, we have learned that this creates variatins in the plate current. If the plate current f this tube passes thrugh the primary cil f a suitable transfrmer, then variatins f current thrugh it create crrespnding changes f ptential at the terminals f the secndary circuit f the transfrmer (see Fig. 15). We can apply this amplified ptential t affect the grid f a secnd tube, and again cuple the transfrmer f the secnd tube t the grid f a third tube and s n. The great imprtance f this cascade arrangement is that the resultant ampli - 22

25 Iß ficatin f the stages is the prduct, and nt merely the sum, f their separate amplificatin. Nw, suppse each tube and its transfrmer amplifies the ptential variatins ten times, then tw in series r cascade, amplify 1U0 times, and three, 1,000 times and s n. Hence, by the use f several stages f amplificatin, we can btain enrmus amplificatin f the riginal ptential acting n the antenna. By this means, then, it is pssible t have vltages acting n the detectr which therwise might be s weak as t be ineffective in perating the detectr. It might appear t the student that with such enrmus values f amplificatin that the latter tube wuld be verladed. This wuld be the case if it were nt fr the fact that the vltage induced in the antenna by the radiated wave is s very small. In fact, this signal vltage acting n the antenna is Fig. 15-Three Stages f Radi -Frequency Amplificatin and Detectr Circuits. represented in micrvlts (millinths f a vlt). Ordinarily, it takes abut 5 micrvlts ( ) t actuate the grid f a vacuum tube s as t give an appreciable change in the plate current. Frm this, then, it can be seen that with 5 micrvlts acting n the grid f the first tube in a cascade amplifier, the amplificatin culd be increased by several stages by 1,000,000 befre 5 vlts wuld be acting n the grid f the last tube. In cases where cnsiderable vltage is acting n the grid, it is necessary t use larger tubes which require higher grid bias in rder t prevent verlading. Remember, always, that it is necessary t have a tube which will require a negative bias great enugh in rder that the applied signal vltage is never greater than the bias vltage. If the signal vltage shuld be greater than the bias vltage, then the grid wuld becme 23

26 psitive with respect t the filament, resulting in a slight grid current, which wuld result in a slight decrease f the plate current instead f a prprtinal increase when the grid is psitive. It is highly desirable that the plate current shuld never be decreased by a grid current, and fr this reasn, tubes f varying characteristics are manufactured s as t prvide tubes which will allw a greater amunt f grid signal vltage t be applied withut drawing a grid current, resulting in a frm f distrtin. CLASSIFICATION OF AMPLIFIERS An amplifier generally cnsists f tw r mre vacuum tubes s arranged that the varying signal vltage is impressed upn the grid f the first tube, thus prducing a variatin f the plate current in this tube; this varying plate current is made t prduce a varying vltage between the grid and filament f the secnd tube, and, similarly, the varying vltage is relayed frm the secnd tube t the detectr tube wherein detectin takes place. Since the functin f the detectr changes the frm f plate current, making the audi -frequency part f the signal vltage available, it can be fllwed by several stages f audi amplificatin. The stages which precede the detectr are usually referred t as radi -frequency amplifying stages, because the frequency f the signal vltage is varying at a radi -frequency 'r abve the range f audibility, which is in the neighbrhd f 10,000 cycles. The stages fllwing the detectr are usually referred t as audi -amplifying-frequency stages that amplify the signal vltage which is varying at an audi -frequency, r belw smething like 10,000 cycles. Frm this brief descriptin, it is plain that the signals must be "repeated" frm ne tube int the next. Amplifiers, either fr radi -frequency r fr audi -frequency, are divided int the fllwing classes, accrding t the arrangement used fr "repeating": (1) Transfrmer -repeating amplifiers. (2) Resistance -repeating amplifiers. (3) Inductance -repeating amplifiers. A tube, tgether with all c -acting apparatus used fr amplifying purpses, is knwn as "a stage f amplificatin"; an amplifier cnsisting f a certain number f stages f amplificatin. The tw terminals f the amplifier upn which the incming signal vltages are impressed are knwn as the "input" 24 fi 7

27 terminals, while the tw terminals acrss which exists the amplified signal vltages are knwn as the "utput" terminals. TRANSFORMER -REPEATING AMPLIFIERS Transfrmer -repeating amplifiers are used fr amplifying bth radi -frequency and audi -frequency signals, and we will discuss their principle f peratin by referring t Fig. 16 which is intended t represent an audi -frequency transfrmer repeating, tw stage amplifier. The radi -frequency transfrmer practically always uses a tuning cndenser acrss the secndary cil, and thus this cil has quite an appreciable current; whereas the secndary f the audi -frequency transfrmer has very little current. The audi -frequency varying ptential is cnnected at Ph and stepped up by means f the transfrmer Tl, after which it Fig. 16-Circuits f Tw Stage Audi -Frequency Amplifier. is applied between the grid and filament f the first tube; this prduces a crrespnding variatin f the plate current f tube 1. The varying current flwing thrugh the primary Pl f the transfrmer T2 induces an alternating vltage in the secndary Sl. This vltage is applied t the grid and filament f the secnd tube, and thus the varying signal vltage is repeated frm the first int the secnd tube, and finally frm the secnd tube, varying plate current is caused t affect the ludspeaker (L.S.) RESISTANCE -REPEATING AMPLIFIERS The circuit diagram f Fig. 17 shws a three stage resistance -repeating r resistance -cupled amplifier. The incming 25

28 signal vltage is applied t the pint Pb and is caused t affect the grid f Tube 1 thrugh the means f the high resistance R. The grid and filament f Tube 1 are cnnected acrss the resistance R, thrugh the cmparatively large cndenser Cl; a grid leak resistance rl is cnnected frm the grid t the filament. In an amplifier f this type instead f using transfrmers t pass the energy frm ne tube t the next, the vltage drp acrss a resistance in the plate circuit (R, R1 and R2) supplies the change in ptential t the grid f the next tube. In rder t insulate the grids f the amplifying tubes frm the "B" battery, blcking cndensers Cl, C2 and C3 are inserted between the plate and grid. Grid leaks rl, r2 and r3 are als necessary t prvide a leakage path fr the charges which wuld therwise accumulate n the grids f the tubes. Tube / ñ Gz Tube 2 C Tiibe.3! CS Fig. 17-Circuit Diagram f Three Stage Resistance Cupled Audi - Frequency Amplifier. The variatin f the grid ptential f Tube 1 will cause a crrespnding variatin in the plate current in this tube, and hence a varying difference f ptential will exist acrss the high resistance Rl. Since the pint 0 is at cnstant ptential, it is plain that the ptential difference between the pints K and O will be varied and, as the battery resistance is cmparatively lw, the variatin f this ptential difference must necessarily be very nearly the same as that acrss Rl. The grid and filament f tube 2 are cnnected acrss K and O thrugh the cmparatively large cndenser C2, and any variatin in the ptential difference acrss K and O will be impressed n the grid f tube 2; in ther wrds, the signal 26

29 will be repeated int the secnd tube by means f the repeating resistance Rl. In a similar manner, the signal will be repeated frm tubes 2 t 3 where it will be picked up n the speaker. The resistances R, R1 and R2 are generally in the rder f 100,000 hms. The capacity f the cndensers Cl, C2 and C3 generally lies between.01 and.1 micrfarad. The larger the capacity f these cndensers, the better the lw -frequency currents will be passed. The leak resistances rl, r2 and r3 which make the tubes stable depends upn the type f tubes used, they may be anything between 250,000 and 1,000,000 hms fr tubes nw generally used as amplifiers. Fig. 18-Circuits f an Inductance Cupled Audi -Frequency Amplifier, Smetimes Called Impedance Cupling. INDUCTANCE -REPEATING AMPLIFIERS This type is similar t the resistance -repeating amplifier, except that instead f a resistance in the plate circuit f each amplifying tube, an inductance r impedance cil is used whse reactance, at the frequency fr which the amplifier is designed, is high. The thery upn which the repeating actin frm tube t tube is based exactly the same as fr the resistance -repeating amplifier. This methd f repeating has an advantage ver resistance -repeating in that the repeating inductance ffers but little ppsitin t the flw f direct current thrugh the plate circuit, and hence the "B" battery may be f lwer vltage than if resistance -repeating were used. The inductance type f repeating amplifier circuit is shwn in Figure

30 PUSH-PULL AMPLIFIER When a large amunt f vlume is required frm a radi receiver r lud -speaker, the set must amplify the signal sufficiently t prduce the required signal energy t be delivered t the speaker. The pwer is the last stage f an amplifier and may be s large that the vacuum tube in that stage f amplificatin is verladed, resulting in distrtin. One way t avid distrtin f the signal frequency is t use a pwer tube. The mst ppular methd, hwever, is t use the pushpull amplifier, as shwn in Fig. 19, which generally replaces the secnd stage f amplificatin. The input transfrmer T1 uses a split secndary winding having a tap used as a cmmn filament terminal at the electrical center. This center -tap is cnnected t a "C" battery t the filaments f the tubes in the usual way. The tw grid A Fig. 19-Circuits f Push -Pull Audi -Frequency Amplifier. terminals are cnnected t the grids f the tw tubes. The utput transfrmer T2 has a split primary winding with plate terminals at its tw ends and a "B" battery terminal brught ut frm the electrical center f the primary winding. This utput transfrmer has a single secndary winding which is cnnected t the lud -speaker. Since the signal ptentials n the grids f the tw tubes B and D are 180 degrees ut f phase (ne is always + r - with respect t the ther), the fundamental plate currents in the tw plate circuits differ by 180 degrees. It will be evident frm the freging that the current directins at any in - 28

31 stani in the primary f T2 add tgether since an increasing current thrugh the upper half f the winding prduces the same effect as the decreasing current in the lwer half, and vice versa. The current that flws t the lud -speaker frm the secndary f T2 is the result f the vltages f the tw tubes acting tgether. With tw pwer tubes cnnected as shwn, the distrtin is smthed ut, as this arrangement prduces a current curve almst perfectly symmetrical. Hwever, true push-pull actin can nly be btained when the tw tubes have the saine characteristics (matched tubes), and the input and utput transfrmers have accurately lcated center taps. TEST QUESTIONS Number Yur Answers and Add Yur Student Number Never hld up ne set f lessn answers until yu have_ anther set ready t send in. Send each lessn in by itself befre yu start n the next lessn. In that way, we will be able t wrk tgether much mre clsely, yu'll get mre ut f yur curse, and better lessn service. 1. Name the three general types f materials used in the cnstructin f the filament f vacuum tubes. 2. What effect des placing a negative charge n the grid have n the plate current? 3. Upn what prtin f the characteristic curve shuld the tube act when used as an amplifier? 4. In Fig. 13, what is the plate current when the grid vltage is 3 vlts negative? 5. What is the meaning f the amplificatin factr f a vacuum tube? 6. What is cascade amplificatin? 7. Name three classes f amplifiers. 8. What are the input and utput terminals f an amplifier? 9. What is the purpse f the blcking cndenser in a resistance -cupled amplifier? 10. Draw a circuit diagram f an inductance -cupled audi frequency amplifier. 29

32