Electricity Electronics Bipolr Trnsistors MEASURE THE HARATERISTI URVES RELEVANT TO AN NPN TRANSISTOR Mesure the input chrcteristic, i.e. the bse current IB s function of the bse emitter voltge UBE. Mesure the trnsfer chrcteristic, i.e. the collector current I s function of the bse current IB for fixed collector-emitter voltge UE. Mesure the trnsfer chrcteristic, i.e. the collector current I s function of the collector-emitter voltge for fixed bse current IB. UE82 6/6 UD Fig. : Experiment set-up (Exmple: input chrcteristic). GENERAL PRINIPLES A bipolr trnsistor is n electronic component composed of three lternting p-doped nd n-doped semiconductor lyers clled the bse B, the collector nd the emitter E. The bse is between the collector nd emitter nd is used to control the trnsistor. In principle bipolr trnsistor resembles two diodes fcing opposite directions nd shring n node or cthode. Bipolrity rises from the fct tht the two vrieties of doping llow for both electrons nd holes to contribute to the trnsport of chrge. / 5
UE82 B SIENTIFI PHYSIS EXPERIMENT I, U Tble : Four operting modes of n npn trnsistor UBE UB Operting mode > < Norml mode I B B E U E > > Sturtion < > Inverse mode < < Off stte U BE I E B N P N E Fig. 2: Design of n npn trnsistor in principle, including ccompnying circuit symbol plus indictions of voltge nd current Tble 2: Four chrcteristics of n npn trnsistor in norml mode. Nme Input chrcteristic Dependency IB(UBE) Prmeter Trnsfer chrcteristic I(IB) UE = const. Output chrcteristic I(UE) IB = const. Feedbck chrcteristic UBE(UE) IB = const. Depending on the sequence of the lyers, the trnsistor my either be termed npn or pnp (Fig. 2). Bipolr trnsistors re operted s qudripoles in three bsic circuits, distinguished by the rrngement of the terminls nd clled common emitter, common collector nd common bse. The nmes indicte which of the terminls is common to both the input nd the output. Only npn trnsistors re considered in the following tretment. There re four operting modes for n npn trnsistor, depending on whether the bse-emitter or bse-collector junctions re ligned in conducting or forwrd-bis direction (UBE, UB > ) or non-conducting or reverse bis (UBE, UB < ) direction (see Tble ). In forwrd-bis mode, electrons from the emitter migrte into the bse cross the trnsistor s forwrdbised bse-emitter junction (UBE > ) while holes from the bse move into the emitter. Since the emitter hs much higher doping thn the bse, more electrons will migrte thn holes, which minimises recombintion between the two. Becuse the width of the bse is shorter thn the diffusion length of the electrons, which count s minority crriers within the bse itself, the electrons diffuse through the bse into the depletion lyer between the bse nd the collector before drifting further towrds the collector itself. This is becuse the depletion lyer only forms brrier for mjority crriers. This results in trnsfer current IT from the emitter into the collector, which is the mjor contributor to the collector current I in forwrd-bis mode. The trnsistor cn therefore be regrded s voltge controlled current source whereby the I t the output cn be controlled by the voltge UBE t the input. Electrons which recombine in the bse emerge from there in bse current IB which gurntees constnt trnsfer current IT, thereby ensuring tht the trnsistor remins stble. A smll input current IB cn therefore control much greter output current I (I IT), which gives rise to current mplifiction. The response of bipolr trnsistor is described by four chrcteristics, the input chrcteristic, the trnsfer or bse chrcteristic, the output chrcteristic nd the feedbck chrcteristic (see Tble 2). This experiment involves mesuring, by wy of exmple, input, trnsfer nd output chrcteristics for n npn trnsistor nd plotting them s grph. LIST OF EQUIPMENT Plug-In Bord for omponents 292 (U25) Set of Jumpers, P2W9 2985 (U9) Resistor, kω, 2 W, P2W9 296 (U24) Resistor, 47 kω,.5 W, P2W9 Potentiometer, 22 Ω, W, P4W5 Potentiometer, kω, W, P4W5 NPN Trnsistor, BD 7, P4W5 2926 (U4) 294 (U42) 296 (U44) 2974 (U82) A/D Power Supply, 2 V / A @2V 2776 (U76-2) or A/D Power Supply, 2 V / A @5V 2775 (U76-5) Escol Anlogue Multimeter 526 (U8557) Set of 5 Experiment Leds, 75 cm, mm² 284 (U8) 2 / 5
292 Plug in bord 292 Plug in bord 292 Plug in bord d e c f b g k 2 W k 2 W k 2 W b b c c d d 22 W e g 22 W e g 47 2 W f f UE82 B SIENTIFI PHYSIS EXPERIMENT SET-UP AND EXPERIMENT PROEDURE Notes: In ll of these circuits, k resistor cts s protective resistor nd must be plugged in t ll times. Only turn on the power supply nd turn up the voltge once the circuits hve been fully ssembled. For ll experiments, set the voltge on the power supply to 5 V. Select the vribles which need to be mesured on the nlogue multimeters (voltge, current) nd choose suitble mesuring rnges for them. Be creful to get the polrities the right wy round. Input chrcteristic Set up the circuit s shown in Fig.. The two nlogue multimeters in the circuit re for the purpose of mesuring the bse-emitter voltge UBE nd the bse current IB. Adjust the k potentiometer in such wy tht the bse-emitter voltge is V. Use the potentiometer to slowly increse the bse-emitter voltge in suitble steps. For ech of these steps, mesure the bse current nd enter the vlues into Tble. ESOLA =...2 V...2 V / A Fig. : Sketch of circuit for recording input chrcteristic. BD 7 ESOLA = Trnsfer chrcteristic Set up the circuit s shown in Fig. 4. The three nlogue multimeters in the circuit re for the purpose of checking the collector-emitter voltge UE while mesuring the bse current IB nd collector current I. Red off the collector-emitter voltge nd mke note of it. Adjust the k potentiometer in such wy tht the bse current is s low s possible. Use the potentiometer to slowly increse the bse current in suitble steps. For ech of these steps, mesure the collector current nd enter the vlues into Tble 4. ESOLA =...2 V...2 V / A Fig. 4: Sketch of circuit for recording trnsfer chrcteristic. BD 7 ESOLA = ESOLA = Output chrcteristic Set up the circuit s shown in Fig. 5. Replce the k potentiometer with 47 k resistor. Also insert 22 potentiometer just before the collector. The three nlogue multimeters in the circuit re for the purpose of checking the bse current IB while mesuring the collector-emitter voltge UE nd collector current I. Red off the bse current nd mke note of it. Adjust the 22 potentiometer in such wy tht the collector-emitter voltge is s low s possible. Use the potentiometer to slowly increse the collectoremitter voltge in suitble steps. For ech of these steps, mesure the collector current I nd enter the vlues into Tble 5. ESOLA =...2 V...2 V / A 22 W Fig. 5: Sketch of circuit for recording output chrcteristic. BD 7 ESOLA = ESOLA = / 5
UE82 B SIENTIFI PHYSIS EXPERIMENT SAMPLE MEASUREMENT Tble : Input chrcteristic mesurements of UBE nd IB. UBE / mv IB /.. 2.. 4. 5. 6. 66. 69. 72.6 74. 75.5 76 2. 77 2.6 78.4 Tble 4: Trnsfer chrcteristic mesurements of IB nd I, UE = 5.2 V. IB / I / Tble 5: Output chrcteristic mesurements of UE nd I, IB = 4.2. UE / mv I / 5 5 7 6 9 2 26 5 4 7 8 9 4 22 44 26 48 5 52 4 54 56 56 7 58 89 6.. 2.2 4. 8.4.5.6 5.7 8.8 2.9 2. 26. 28.2. 2.4 4.5 6.6 8.7 4.8 4.9 425 2. 44 4 / 5
UE82 B SIENTIFI PHYSIS EXPERIMENT EVALUATION The input chrcteristic (Fig. 6), s expected, is the sme s the forwrd-bis chrcteristic of silicon diode. A semiconductor diode strts to conduct in the forwrd-bis direction once voltge threshold hs been reched. To determine wht this threshold is from our mesurements, the shrply rising prt of the input chrcteristic is extrpolted bck to the x-xis nd then the voltge US where it crosses the xis is red off: () US 72 mv.72 V. This vlue is well in greement with the typicl vlue for silicon,.7 V. The trnsfer chrcteristic (Fig. 7) is lmost liner, lthough the grdient decreses slightly once the collector current is greter thn I. The current gin is clculted using the following formul: I (2) B I B Its verge vlue is bout 24. The mximum vlue for it under defined test conditions is specified to be 25. The output chrcteristic (Fig. 8) rises shrply s UE increses until the voltge is bout 2 mv but then grdully flttens out until it is nerly horizontl. The power dissiption is clculted s follows: I / 4 2 2 I B / Fig. 7: Trnsfer chrcteristic for UE = 5.2 V () P UE I Where the curve is roughly horizontl, this corresponds to bout.5 W. The bsolute mximum vlue specified is 8 W. I B / I / 6 5 4 2 2 2 4 6 8 U BE / mv 2 4 6 8 U E / mv Fig. 8: Output chrcteristic for IB = 4.2 Fig. 6: Input chrcteristic B Scientific GmbH, Rudorffweg 8, 2 Hmburg, Germny, www.bscientific.com opyright 26 B Scientific GmbH