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500 450 400 Base current, mA E C 350 300 250 200 150 100 50 0 0 01 02 03 04 05 06 07 Base-emitter voltage, V 08 09 1
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B IBB
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iB + vBE _
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Figure 98 BE junction open-collector curve
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50 45 Saturation region Active region 40 35 30 25 20 15 10 5 00 1 2 3 4 Cut off region
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B IBB
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IB + VBE _
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+ VCC _
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VCE _ E
275 A 250 A 225 A 200 A 175 A 150 A 125 A 100 A 75 A 50 A 25 A
Figure 99(a) Ideal test circuit to determine the i-v characteristic of a BJT
Collector current, mA
5 6 7 8 9 10 11 12 13 14 15 Collector-emitter voltage, V
Figure 99(b) The collector-emitter output characteristics of a BJT
3 The saturation region, in which both junctions are forward-biased 4 The breakdown region, which determines the physical limit of operation of the device From the curves of Figure 99(b), we note that as vCE is increased, the collector current increases rapidly, until it reaches a nearly constant value; this condition holds until the collector junction breakdown voltage, BVCEO , is reached (for the purposes of this book, we shall not concern ourselves with the phenomenon of breakdown, except in noting that there are maximum allowable voltages and currents in a transistor) If we were to repeat the same measurement for a set of different values of iB , the corresponding value of iC would change accordingly; hence, the family of collector characteristic curves Determining the Operating Region of a BJT Before we discuss common circuit models for the BJT, it will be useful to consider the problem of determining the operating region of the transistor A few simple
9
Transistor Fundamentals
RC 1 k C RB 40 k B V3 VCC VBB 4V V1 E 12 V V2
voltage measurements permit a quick determination of the state of a transistor placed in a circuit Consider, for example, the BJT described by the curves of Figure 99 when it is placed in the circuit of Figure 910 In this gure, voltmeters are used to measure the value of the collector, emitter, and base voltages Can these simple measurements identify the operating region of the transistor Assume that the measurements reveal the following conditions: VB = V1 = 2 V VE = V2 = 13 V VC = V3 = 8 V
RE 500
What can be said about the operating region of the transistor The rst observation is that knowing VB and VE permits determination of VBE : VB VE = 07 V Thus, we know that the BE junction is forward-biased Another quick calculation permits determination of the relationship between base and collector current: the base current is equal to IB = VBB VB 4 2 = = 50 A RB 40,000 VCC VC 12 8 = = 4 mA RC 1,000
Figure 910 Determination of the operation region of a BJT
while the collector current is IC =
Thus, the current ampli cation (or gain) factor for the transistor is IC = = 80 IB Such a value for the current gain suggests that the transistor is in the linear active region, because substantial current ampli cation is taking place (typical values of current gain range from 20 to 200) Finally, the collector-to-emitter voltage, VCE , is found to be: VCE = VC VE = 8 13 = 67 V At this point, you should be able to locate the operating point of the transistor on the curves of Figures 98 and 99 The currents IB and IC and the voltage VCE uniquely determine the state of the transistor in the IC -VCE and IB -VBE characteristic curves What would happen if the transistor were not in the linear active region The following examples answer this question and provide further insight into the operation of the bipolar transistor
EXAMPLE 92 Determining the Operating Region of a BJT
Problem
Determine the operating region of the BJT in the circuit of Figure 910 when the base voltage source, VBB , is short-circuited
Solution
Known Quantities: Base and collector supply voltages; base, emitter, and collector
resistance values
Find: Operating region of the transistor
Part II
Electronics
RC = 1 k ; RE = 500
Schematics, Diagrams, Circuits, and Given Data: VBB = 0; VCC = 12 V; RB = 40 k ;
Analysis: Since VBB = 0, the base will be at zero volts, and therefore the base-emitter junction is reverse-biased and the base current is zero Thus the emitter current will also be nearly zero From equation 91 we conclude that the collector current must also be zero Checking these observations against Figure 99(b) leads to the conclusion that the transistor is in the cutoff state In these cases the three voltmeters of Figure 910 will read zero for VB and VE and +12 V for VC , since there is no voltage drop across RC Comments: In general, if the base supply voltage is not suf cient to forward-bias the
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