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- bu batt ery-opera te d equ ip ment to co nserve battery power; many transist orized portable radios use class B output stages for this reason. Class C operation is confined almost exclusively to rf power amplifiers whic h can be nonlinear; this means, in practice, all rf powe r stages in a CW or FM transmitter, as well as in an AM transmitt er which uses high-level ~o dulati on (if mo d ulat ion is applied before the final output stage, all the followi ng stages must be linear, req uiring A, AB, or B operation). Class C is hardly ever use d in SSB transmitters, since th e no nlinearity wo uld destro y th e modulatio n of the signal. How and Why Do Ampli fier s Misbeha ve? Amplifiers, like all ot her physical objec ts, faithfully follow Murphy's Fir st Law that says, " If any thing can possibly go wro ng, it will." In consequence, they co n tinu ally misbehave. Fortunately, an amp lifier is a relatively simple device, so th at on ly a few t hings in them can possibly go wrong, and the result is that mos t amplifier misbehavior can be traced to a fairly small num ber of poss ible causes. However, any specific case of "wronggoing" may be due to not just one but several of the possible causes all acting togeth er, which sometimes makes it difficult to find and fix the trouble. One of the first thi ngs whic h can go wro ng is for the specific amplifier to be badly design ed; in th is case the designer has failed to take into acc ou nt all the tricky littl e in teractions which can affect his circuit's operatio n , an d you as an operator are just about hopelessly lost. The cure is to forget it, and start over wit h a properly designed amplifier (if you're the designer as well, then it's back to the drawing bo ard with our sy mpathy). Since this kind of problem, though it's the most common in th e case of new or one -of-a-kin d circuits, is not with in the range of the F CC exam, we 'll ignore it t he rest of the way in and assume that th e misbehaving amplifier was properly designed and built and did , in fact, work perfectly at some t ime before the troubles began. Anot her fre quent wrong-goer is th e user of the equipment ; in this case no thing at all is wro ng with the ampli fier itself, but it's being used unde r con dit ions which never were inten ded by the designer. This is th e mos t freq uen t cause of amplifier misbehavior - and we 'll get back to it in much more detail a little lat er. The fin al frequent cause of trouble is failure of one or more co mponents in the circuit. A blown tube or transist or is relatively easy to find and fix v t a bypass capacitor which has changed in value (b ecause of age or overheating) just enough so that it no longer does its job properly may provide ma rc t han its share of hair-pulling before it's detected. From the standpoin t of the operator, the on ly one of these three probable causes of problems which he can do anything about is the second - misuse of the equipmen t. The cure for the firs t is to redesig n the circ uit , and for the third is to locate and re pair the defect ive par t . While bo th of these are legitimat e activities for a rad io ama teur , neit her of them is invo lved direc tly with the operating of the equipment. Because of this, and also because misuse accounts for most of the amplifier pro b lems an d bad signals in exis tence at any one instant, we' ll concentrate on just th e one problem. Before we can talk about "misuse" of an amplifier, we must know what the proper use for t hat ampl ifie r is. Fo r instance, the purpose of a linear powe r amp lifier used to bring a single-sideband signal from .lOW up to SOOW is defined, and "proper use" of thi s particular rig wo uld be that use whic h acc omplishes the power am plifica tion without distortio n and unwanted spurious output signals. In general, the amp lifier's name (such as "linear," "power," etc .) to gether with the designer's specifications for inp ut an d outpu t levels will tell you all yo u need to know about th e circuit's pur pose, and proper use amoun ts to "that use which accomplishes th e purp ose of the circuit." Misuse, t hen , can be defined as any use which defeats th e circuit's int ended pur pose. In most cases, this boils down to one or more of th ree major mistakes : 72 73 MAGAZ INE
1. T he amp lifier is adjusted to operating conditions such as bias or voltage levels which are outside the range anticipa te d by th e desig ner. 2. In put signals are app lied at levels either lower or higher tha n th e range intende d by th e designer. 3. More output is at tempted than the designer intended the circ uit to produce. Let's see ho w these three mistakes affect several t ypical amplifier cir cuits. For starters, let 's see what hap pens when a class A amplifier inte nded for low-distortion amp lific ation of an audio signal is misused. Such an amp lifier might be in the speechamplifier chain c f any phone rig, or in a receiver. If the amp lifier 's bias can be adjuste d, and is set to a value higher t han the designe r intended , the sta ge will draw less current than it was designed for, and so cannot develop it s rated po wer out pu t (o r voltage output, if it's a voltage am plifier). Simil arly, if th e bias is set too low, a "normal" input signal may overcome the bias and change the operati on co ndt ion fro m class A to class AB. The effect of bia s adjust ment, th en, is to reduce ' the allowable input and out pu t signals - ,and if the value is very far from the in tended level, excessive distortion may be introduced in to all signals (Fig. 7). . /\ Fig. 7. Shown here are the effects of overbias and underbias on a class A amplifier, with a triangular input signal of maximum rate d level which could be reproduced linearly were th e bias proper as indicated by the "X" in the center of each amplifier transfer line . Over bias causes the negative-going peaks of the signal to " bottom " against the cutoff point, which is actually class AB operation, while underbias causes amplifer to saturate and flat-top the signal. SLOW SCAN TV KIT 1 P.C. Boards onl y (2 req .lea. KIT 2 P.C. Boar ds {Both] & a ll parts (u nwired ) KIT 3 P.C. Boar ds Wired & tested Power Supply Kit - No chassis Complete Monitor - I n handsome cab inet Now for the f rst ti me an 5.5. monitor designed for your own custom installation. Available in 3 economy kits. All kits tube type with in stru ctions. CRTs. Power Suoolies . and cabinets not included. E.K.Y. VI DEO VISION CO. Box 15, Stock ho lm NJ 07460 $9 .95 $90.00 $125.00 $54.95 $298.00 DUAL GATE MOSFET PRE-AMPS t o 175 M Hz. $21.95 ppd. to 300 M Hz. $25.95 ppd. • A vailable from 5 MH z. to 3 00 MHz. Bandwidth is approximate ly 3% of f requ e n c y. • Voltage ga in 30 t o 40 DB depending o n f requen c y. • Two Dual G a te MOSFET amplif ier stages with each having a tuned in p ut and tuned output. Each Daul Gate MOSFET is actually an integrated cascade ctrcutr thus g iving you 2 cascade ci rc u its equivalent to 4 t r iOd es. • Exceptionally lo w noise (2.5 DB at 175 M Hz .) . gr eatly redu Ced crOss modulation and 10 times t h e dy nam ic range (s igna l hand lin g capability) of the best b i·polar transistors. Also supe ri or to preamps u sing junction FETs and S ingl e Gate MOSFETs. • Pane l mounted m a n u al gai n c o nt ro l. • Type BNC ,nput and output recep t acl e s for minimu m loss at U H F . S t andard I m p ed an ce is 5 0-75 o hms. • Carefully tuned at o u r lab orat o ry wi th sw eep generator and oscilloscope for the best bandpass characteristlc. • F lJll w ave UHF d iodes protect inplJttrans'stor. • Operates On 6 to 16 vo lts DC, 5 to 1 5 Ma . New Y ork City an d S tat e residents add local sal es ta x . VANGUARD LABS Dept. H 196·23 Jamaica Ave., Hollis, NY 11423 FEB R UARY 19 71 73
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CONTENTS where. Then carne the day
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BOOKS I ADVAN CED CLASS STUDY G U I
Page 12 and 13:
Irving M. Gottlieb W6HDM M~W 931 Ol
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situ ated on a PC board (Fig. 4) di
Page 16 and 17:
See your DEALER for DRAKE products
Page 18 and 19:
Practical circuit applications usin
Page 20 and 21:
INPUT )1 c I SO MHz 1N4381 IOO~ 450
Page 22 and 23:
The Mos t Powerful Signals Under th
Page 24 and 25: CA 3028 J.lL 923 '" 4.7 K ., 680 4
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Page 28 and 29: Theodore Cohen W4UMF 8603 Conover P
Page 30 and 31: ~ ~ Installation of the switch and
Page 32 and 33: A ~Sy CQ, CQ, CQ, t his is K3FEN, R
Page 34 and 35: Messy and m ickey mouse, bu t wh en
Page 36 and 37: The tuning indicat or shown in Fig.
Page 38 and 39: Being qui te activ e on two me ter
Page 40 and 41: "'12V "' 12-24V .60 10K lOOK .I IN3
Page 42 and 43: ON.. EFFICIENCY ORGANIZATION and MA
Page 44 and 45: Bill Hoisin gton K 1eLL Far Over F
Page 46: installation of on e or more low-no
Page 49 and 50: STEP UP TOTELREX Professionally Eng
Page 51 and 52: " 4 ~ O Y " 4 ~ OY ( I U 6 2 11 0'
Page 53 and 54: etter than the "Hot-Water 100 " ...
Page 55 and 56: y Ken W. Ses$ion~ K6MVB DU LD A 8 .
Page 57 and 58: Motorola HEP Semiconductor Devices
Page 59 and 60: see the A1ann for the best in USED
Page 61: esults. The transmitter adj ustme n
Page 65 and 66: What Is An Amplifier? Several acres
Page 67 and 68: When it comes to grea t used mo bil
Page 69 and 70: of am plifier o pera ting con ditio
Page 71 and 72: New advanced 4-band, Communication
Page 73: LI NEAR NON-LINEAR Radio Amateurs R
Page 77 and 78: many do not) you can either speak a
Page 79 and 80: SOLID STATE ENCODERS for quick and
Page 81 and 82: WHEN IT COMESTO ANTENNA SYSTEMS...I
Page 83 and 84: Richard Tashner WB2TCC 163-34 2 1 R
Page 85 and 86: A. E. McGee, Jr. K5LLI 281 5 Materh
Page 87 and 88: John G. Harder W5QKO/AI4QKO 6200 A
Page 89 and 90: The LOUJ-OhTn JJleter ALL RESISTORS
Page 91 and 92: Photos I and 2 show the meter built
Page 93 and 94: powers of ove r 100W across the ban
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Page 97 and 98: 4.000 BIT MEMORY D $8.50 from IBM C
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