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TransistorndFETCurve racerDesign of an adapter that permits you todisplay the characteristic curves of anytransistor or FETon conventional scope.

BY DANIELMETZGERAsst.Prof. or Electronics echnologyMonroe ounty ommunity ollege, ichigan

Curve racer adaptor being used to >produce amily of curves on scope.

immediate display of the characteristic curves of any tran-sistor or FET on a conventional oscilloscope. The followingare some of the uses to which the curve tracer can be put:O Determine the a.c. and d.c. beta, collector-to-emittersatrrration voltage, collector broakdown voltage (up to 35volts), and dynamic collector output resistance of a transis-tor.O Observe the effects of temperature on transistor chaacteristics, either by applying heat externally or overbiaFig. 1. Schematic of tracer. 01, 02, and Q3 comprise staircase genorator, while04 and 05 form Darl ington emitter-fo l lowr provid ing current to driv tho baseof th tosttransistor. Collector sw6p voltagecomes from D7-D1O ull-wave bridge.

R l -3 3 0 o h m,% W re s .R2-500,000 oh mpo t("Stabi l i ty" ontro l)R3 .R21-1 0,000ohm,% W res.R4-1 00 ohm r immer "1-VStep Ad j . " )R 5 - 3 3 o h m , % W r e s .R6-1 0,000ohm rimmer "No.of Steps")R7,Rl 8-1 000 ohm,% W res.R8-1 800 ohm.% W res.R9-1 000 ohm rimmer("Step-Position" ontrol)Rl 0-47 ohm,% W res.R1 -470 ohm,1 W res.R1 -2700 oh m 1/ zW res.Rl 3- 1 00 ohm,10 W res.R1 . Rl 6. Rl 7, R24-1 00,000ohm,% W res.Rl 5-25.000 ohm.% W res.(selected, ee ext)R1 -2000 ohm,1/z res. selected)R20-5000 ohm,% W res. selected)R22-2O,OOO hm,% W res. selected)R23-50,000 ohm,% W res. selected)R25-200.000 ohm.% W res. selected)R26-500,000 ohm,% W res. selected)R2 7 -5 0 0 o h m.2 W p o t("Ect-.*.)" control)C1 0.05 pF ,20 0 V ceramic aPacitorC2-0.2 pF ,20 0 V MylarcapacitorC3 .C4-300 ;rF,25V elec. apaci torC5-500 yF,25V elec. aPacitorS1-2-deck, 4-pole, -pos. elector sw.(3 poles,4pos. sed)Centra lab A-1 13 or equiv.S2-S.p. 3-pos. electorsw .(Centra lab 461 or equiv.)S3-2 deck,11-pos. elector w. (Centra-lab PA-1 05 or equiv.)S4, S5-S.p .s . t . oggle w.52

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01 O3-2N1 67 1 unijunct ionransistor02-2N3638 "p-n-p" i l iconransistor04, 05-2N3569 "n-p-n" i l icon ransistor

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N IOTHING provides more information about a transistor| \ tna" it s family of collector characteristic curves. Typi-cal characteristic curves are often given in manufacturers'data sheets, but these are of limited value because transis-tor characteristics may vary by as much as a factor cf threeor four either way from.unit to unit. Temperature varia-tions also cause considerable change in transistor character-istics, especially with germanium types.The adapter to be described will allow you to obtain an

sl-FuNcrloN swlrcH s5-r5=o oR srEP| - N P N 3 - N - C h . R92-PtrP 4-P-Ch. STEP POSITIONftlTl-Transformer: 117 V pr i . ; hree' l 2- Vsec.at 0. 1 A each StancorP-8351D1-D1 -Si l icon diode, ' l00.p. i .v . l N628or equiv.)

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transistor and allowing it to generate its own heat.Observe th e base-emitter input characterist ics of aObserve the E os I curves of diodes, zener regulators,photocells.Selectively match transistors on input or output char-Spot transistors with low breakdown or high saturationwell as those with low beta or high leakage; thetests are usually not available on meter-type tran-

Circuit DescriPt ione basic function of the adapter is to supply a continu-sweeping voltage of the proper polarity to the collec-of the transistor under test, while a staircase waveformd through a resistor to the base to produce the "steps"current. A different line in the family of curves isat each s tep of the s ta i rcase. For s impl ic i t y thevoltage is an unfiltered, full-wave rectified 60-Hzobtained from 71 (Fig. 1). Fig. 3 shows the rela-of the collector sweeping voltage to the base driv-voltage. Although they are shown in synchronism, thereparticular need to keep them in step, since the locus ofcreated by the two voltages will be the same for

repetition rate. Bl , Q2, and B3the staircase generator, while B4 and B5 form ainput impedance Darlington emitter-follower tothe necessary current to drive the base of the testwithout loading and consequent distortion of theoutPut.is a conventional unijunction relaxation oscillator. Clthrough R2 and R3 unt i l the emi t ter of Qlapproximately 8 to 10 volts. Bl then fires, conduct-heavily from emitter to base I and passing a sharp nega-spike through Cl to the base of 02 q2 is a constant-source which is gated on by the negative spikes' C2identical current pulses from the collector of B2each negative spike produced by Ql , and therefore C2

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vc E v)Fig. 2. Readingsobtainable from characteristic curve display.(1 ) A.c. beta : Atc /Alu - (15-7.5 mA)/(2OO-1OOpA) : 75 (a tE " : 4 V , 1 " : 1 2 m A ) .(2 ) D.c.beta : lc / la - 16 mA/2OOpA - 80 (a t E" - 10 V' l" : 15 641.(3 ) lceo leakage : 1 mA at 11 V. Collector cutoff current, lcBois approximately equal to 1666/beta.(4 ) Collector-emitterbreakdown voltage : 16 V (a t lu - 2OO pA).(5 ) Co f ector dynamic output resistance : AE/AI - 6 Y/2.5 mA :24OOohms (a t Ec - 7 V, lc : 25 mA). The commonly given para-meter h6g : Al lA E - 2. 5 mA/6 V : 416 micromhos.(6 ) Saturation voltage Vsslsa,T) : O. 5 V at l" : 20 mA.charges by equal voltage steps at every spike. p3 resets thesta ircase generator by discharging C2 when th e voltageacross it reaches the firing voltage of the trnijunction.

Dl and fI2 are clamping diodes which shift the d.c. levelofthe base-driving staircase voltage either positive or neE{a-tive as rcquired by the test transistor. R9 provicles controlover the d.c. level shift so that one of the staircase stepsmay be adjusted to a level which will give zero base current(zero gate voltage in the case of an FET test).

F i g . 5 . E x p a n d e dd e t a i l o f s a t u r a t i o nre g io n o f 2 N3 5 6 9 . Sc a le sa re O .1 V /d iv .h o r i z . , 1 mA/d iv . v e r t . ; b - 1 O p A ls te p .. 3 . Re la t io n s h ip f b a s e -d r i v in g ta i r -c o l l e c to r v o l ta g e . Up p e r t r a c e ,volts/division; lower trace 1O volts/div.

Display or 2N4O4 germanium "p'n'p"W a v e fo rm s c a le s a re 2 Y /d iv .mA/d iv . v e r t . ; u : 1 O p A ls te p .

Fig. 4. Display for 2N3569 sil icon "n-p-n"transistor. Waveform scales are 2Y/div.h o r i z . . 2 mA/d iv . v e r t . ; 6 : 1 O p A ls te p '

F ig .7 . D is p la y o r 2 N3 O5 3 s i l i c o n "n -p -n "p o we r t ra n s is to r . Th e s c a le s a re 2 Y /d iv 'horiz., 20 mA/div. vert.; 16 - O.5 mA/step'

F ig .8 . D is p la y o r 2 N3 8 1 9 "n " - c h a n n e l FET.Sc a le s a re 2 Y /d iv .h o r i z . , 2 mA, /d i v . v e r t ;Esate : O.5 V/step; 2nd line from top Es : O.53

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lnside iewof curve racershowing hoconstruction otails.Rll, Rl2, and Rl3 are provided in series with the collec-tor of the test transistor to prevent damage due to excessivepower dissipation. Rl3 also serves as a current-sensing resis-ior, providing 0.1 volt to the vertical amplifier of the oscil-loscope for each milliampere of collector current.The parts for the adapter retail cost the author about $35'although few of them are critical and many substitutionscan bJ made to take advantage of components already onhand. For example, nearly any unijunction transistor will dofor Ql and Q3, since differences in parameters can be off-set 6y adjustment of trimmer potentiometers R4 and R6.q2, 04, and @ should be small-signal silicon types, butthere are undoubtedly hundreds of types which will workas well as the particular ones used in this unit. Similarly,diodes Dl through D10 may be nearly any silicon junctiontype.Resistors Rl4 through R26 are all standard-value compo-nents selectively picked from a junkbox for the values not-ed on the diagram (a high "4.7k" was used for the Sk-resis-tor, for example). Capacitors CB, C4, and C5 must be sever-al hundred pF each, but there is nothing special about theparticular values shown. Capacitor C2 is perhaps the most

critical componen! it must have a leakage resistance of 1054

megohms or more if a clean CRT display is to be obtained.The curve tracer is housed in a standard aluminum boxwith rnost of the electronic circuitry built on a 3" X5' pieceof perforated breadboarding stock. Resistors R4 and R6 areminiature trimmers mounted directly on the board. All oth-er variable resistors and switches are mounted on the frontpanel. Resistors Rl4 through R26 are connected directlybetween corresponding pins of the two decks of 53. In wir-ing the transformer secondaries, it is important that the twoseries windings be connected in-phase as shown in Fig. l.Reversal of one of the secondaries would result in the volt-age canceling instead of adding as desired.

Calibration and TestOnce a thorough check of the wiring has been made, theadapter is ready for test and calibration. The power supplyshould be checked first; 14 volts should be measured acrossf i lter capacitor C5 and a drop of approximately 0.5 voltshould appear across series resistor R10. Readings whichdiffer significantly from these norms indicate a serious mal-function which should be located before further tests areconducted.Next, a calibrated oscilloscope should be connected to theemitter of p3. If all is well, a staircase waveform wil! .=-observed. R4 is then adjusted so that each step of thr: r-;.r-

case represents a l-volt increase. R6 is adjusted to providethe desired number of steps (this will be the same as thenumber of trace lines in the family of curves displayed orthe screen).It is recommended that a low-power silicon n-p-n transis-tor be used as a test transistor to gain some familiarity withthe curve tracer. Set the function switch Sl to position I(NPN), the power switch 52 to .l W, the Fc control R27 atmidrange, the 1rlStep switch to 10 pA, the Position controlfi9 full counterclockwise, the Stability control R2 near thecenter of its range, and the 1t - O-Stepswitch Sb to Step ' Itis a good idea to make these initial settings with the a.c.switch (S4) off to avoid inadvertently applying damaginvoltages to the test transistor while switching ranges.The & output of the adapter is connected to the externhorizontal input of the scope and the 16output is connectto the vertical amplifier. If the scope is calibrated, the hon'zontal scale can be read directly in volts, while on the verti-cal scale each volt represents l0-mA collector current. Ifthe scope does not have both normal and inverting inputs,you may have to settle for a display which is either upsiidown or backwards from the usual position, but the sanr.information can be obtained in any case.With 54 "on," a characteristic-curve display should be ob-tained on the scope. The stability control is then adjusted tominimize the flicker of the display. The step adjust is set sothat the first display line corresponds to tbe single line ob-tained when 55 is set to /u : 0. A summary of the readingswhich can be taken from the display is given in Fig. 2.Junction and insulated-gate field-effect transistcrs may betested by connecting the emitter, base, and collector leads

to the source, gate, and drain, respectively, of the FET. Ifthere is a fourth "substrate" lead, it should be connectedwith the source. 53 must be set to one of the last threepositions to provide the necessary gate-voltage steps. Again,the step-position control must be adjusted so that one of thedisplay lines corresponds to the single line obtained when,$ is set to 1o : 9.Transistor base-emitter input characteristics, as well ascharacteristics for rectifier and zener diodes, photocells,and other twolead devices can be obtained by using the"emitter" and "collector" leads only to supply a sweepingvoltage. 52 should be left on the .l-W position for such ap-plications unless the device under test is capable of han-dling currents of 10 mA and above.Typical curves produced by the instrument are shown inFigs. 3 through 8. r'ELECTRONICS WORLD

Close-up f the front panel of the transistor curve tracer.