CHAPTER 6
Bipolar Junction Transistors
(BJTs)
Figure 6.1 A simplified structure of the npn
transistor and pnp transistor.
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Table 6.1: BJT modes of Operation
Mode
EBJ
CBJ
Cutoff
Reverse
Reverse
Active
Forward
Reverse
Saturation
Forward
Forward
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Operation of the npn Transistor in the Active Mode
Figure 6.3 Current flow in an npn transistor biased to operate in the
active mode. (Reverse current components due to drift of thermally
generated minority carriers are not shown.)
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iC  I S e vBE / VT
iB 
iC

I S vBE / VT
i B  ( )e

iE  iC  iB
iC  iE


 1


1
= common-emitter current gain
Figure 6.5 Large-signal equivalent-circuit models of the npn BJT
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operating in the forward active mode.
Operation in the Saturation Mode
 forced
iC

iB
saturation
VCB  0.4V    VBC  0.4V
VCEsat  VBE  VBC
VCEsat  0.1 to 0.3V
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Model npn BJT Operation in the Saturation Mode
Figure 6.9 Modeling the operation of an npn transistor in saturation by
augmenting the model of Fig. 6.5(c) with a forward conducting diode
DC. Note that the current through DC increases iB and reduces iC.
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Figure 6.10 Current flow in a pnp transistor biased to
operate in the active mode.
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Figure 6.11 Two large-signal models for the pnp
transistor operating in the active mode.
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BJT
Current-Voltage
Characteristics
Figure 6.13 Voltage polarities
and current flow in transistors
biased in the active mode.
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Table 6.2: Summary of the BJT Current-Voltage
Relationship in the Active Mode
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Example 6.2.
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Graphical Representation of Transistor Characteristics
Effect of temperature on the iC-vBE,
at a constant emitter current, vBE
changes by -2mV/0C
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VCB=-0.4V
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iC  I S e
r0 
v BE / VT
vCE
(1 
)
VA
V A  VCE
IC
VA
r0 
I 'C
I C'  I S eVBE / VT
Figure 6.18 Large-signal equivalent-circuit models of an npn BJT operating
in the active mode in the common-emitter configuration with the output
resistance ro included.
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An alternative form of the
Common-Emitter
Characteristics
Figure 6.19 Common-emitter characteristics. (a) Basic CE circuit; note that in (b)
the horizontal scale is expanded around the origin to show the saturation region in
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some detail. A much greater expansion
of the saturation
region is shown in (c).
IC(max)
VCE(max)
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An Alternative form of the
Common-Emitter
Characteristics
Figure 6.20 A simplified equivalent-circuit model of the saturated transistor.
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BJT Circuits at DC
Cutoff: EBJ: reversed bias, CBJ: reversed bias
NPN
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PNP
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Active: EBJ: forward bias, CBJ: reversed bias
NPN
PNP
Saturation: EBJ: forward bias, CBJ: forward bias
NPN
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PNP
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Example 6.3.
Find VBB to have:
1. VCE=5V (active mode)
2. Edge of saturation,
VCE=0.3V
3. Deep saturation,
VCE=0.2V, force=10
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Example 6.4
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Example 6.5
Figure 6.23 Analysis of the
circuit for Example 6.5. Note that
the circled numbers indicate the
order of the analysis steps.
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Example 6.6
Figure 6.24 Example 6.6: (a) circuit; (b) analysis, with the order
of the analysis steps indicated by circled numbers.
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Example 6.7
Figure 6.25 Example 6.7: (a) circuit; (b) analysis, with the
steps indicated by circled numbers.
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Example 6.8
Figure 6.26 Example 6.8: (a) circuit; (b) analysis, with the steps
indicated by the circled numbers.
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Example 6.9
Figure 6.27 Example 6.9: (a) circuit; (b)
analysis with steps numbered.
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Example 6.11
Figure 6.29 Circuits for Example 6.11.
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Figure 6.30 Example 6.12: (a) circuit; (b) analysis
with the steps numbered.
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