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7 trans

  1. 1. JUNCTION TRANSISTOR : COMMON EMITTERCHARACTERISTICSRAVITEJ UPPU1. Aim1. Connect NPN common emitter transistor circuit to determine experimentallyand plot the family of collector( VCE (potential between the collector and emitterends of transistor) vs Ic (collector current)) characteristic curves for the CommonEmitter configuration.2. To measure the effects on IC on varying IB(base current).3. To determine beta (β)(current gain in common emitter).2. TheoryFirst, let try to describe a Junction Transistor. It is a three-terminal device con-structed of doped semiconductor material.Junction transistor’s operation involvesboth electrons and holes.Although a small part of the transistor current is dueto the flow of majority carriers, most of the transistor current is due to the flowof minority carriers and so Junction Transistors are classified as ’minority-carrier’devices.An NPN transistor can be considered as two diodes with a shared anode region.In typical operation, the emitterbase junction is forward biased and the basecollec-tor junction is reverse biased. The electrons in the base are called minority carriersbecause the base is doped p-type which would make holes the majority carrier inthe base.The base region of the transistor must be made thin, so that carriers candiffuse across it in much less time than the semiconductor’s minority carrier life-time, to minimize the percentage of carriers that recombine before reaching thecollectorbase junction.The grounded emitter or common emitter configuration is pretty frequentlyused.In the case of the common emitter, the input signal is applied between baseand emitter the output taken from the collector to emitter i.e the emitter is com-mon to both input and output circuits. The current gain in common emitter isdefined as the change in collector current effected by a change in base current withcollector voltage maintained at a constant value i.e β = ∆IC∆IB.1
  2. 2. 2 RAVITEJ UPPU3. ProcedureWe connect the circuit as follows :In the experiment, VBB is 1.5V and VCC is a variable DC supply which we varyfrom 0 to 6V Now, we vary VCC and measure IC for a fixed value of IB. TabulateVCE and IC. Now vary IB and repeat the same steps. Take such readings forabout 5 values of IB. From this, we can plot the VCE versus IC curves and we canalso calculate the β, the current gain, by finding the difference between the basecurrent and Collector current for a fixed VCE.These can also be tabulated/4. Observations and ResultsVC Collector Voltage (V ) Ic Collector Current (µA) VC (V ) IC (µA)IB = 0µA IB = 30µA0 0 0 00.95 1 0.03 5382.05 2 0.05 8722.88 3 0.11 20373.81 4 0.21 23434.88 5 0.46 24645.81 6 1.02 25136.43 7 2.88 26693.81 27515.81 29426.00 2999IB = 10µA IB = 40µA0 0 0 00.02 980.03 1330.05 196 0.05 14550.11 267 0.11 32000.21 280 0.21 48000.46 282 0.46 49001.02 285 1.02 51002.23 290 2.88 55003.81 296 3.81 58005.81 302 5.81 65006.00 304 6.00 6600
  3. 3. JUNCTION TRANSISTOR : COMMON EMITTER CHARACTERISTICS 3IB = 20µA IB = 50µA0 0 0 00.01 30 0.03 9580.05 551 0.05 20860.11 1024 0.11 53000.21 1160 0.21 70000.46 1176 0.46 72001.02 1188 1.02 74002.88 1244 2.88 82003.81 1267 3.81 87005.81 1321 5.81 99006.00 1333 6.00 10400The graph of the above data with VC versus Ic looks likeThe curves rises very steeply to a higher value of IC for higher IB.Now, let us try to find β, the current gain at VC = 3.81V .S.No ∆IB(reference IB = 10µA) ∆IC µA β1 10 971 97.12 20 2455 122.753 30 5504 183.474 40 8404 210.10
  4. 4. 4 RAVITEJ UPPUThe plot looks like this for the base current versus collector current at V = 3.81VSo the range of β for this voltage is as given in the previous table and is depictedin the graph below

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