Download Ch 6

MALVINO & BATES
Electronic
PRINCIPLES
SEVENTH EDITION
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Chapter
Bipolar Junction Transistors
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6
Topics Covered in Chapter 6
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Unbiased transistor
Biased transistor
Transistor currents
The CE connection
The base curve
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Topics Covered in Chapter 6
(Continued)
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Collector curves
Transistor approximations
Reading data sheets
Surface mount transistors
Troubleshooting
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Unbiased transistor
• Three doped regions: emitter, base, and
collector
• Two pn junctions: emitter-base and basecollector
• NPN or PNP
• Silicon or germanium
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The bipolar junction transistor has 3 doped regions.
N
COLLECTOR (medium doping)
P
BASE (light doping)
N
EMITTER (heavy doping)
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Biased transistor
• Forward bias the emitter diode
• Reverse bias the collector diode
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In a properly biased NPN transistor, the emitter electrons
diffuse into the base and then go on to the collector.
RC
N
VCE
RB
P
VBB
VBE
N
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VCC
Transistor currents
• The ratio of collector current to base
current is current gain (βdc or hFE)
• Current gain is typically 100 to 300
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IC
IC
IB
IB
IE
IE
Conventional flow
IC @ IE
I E = IC + I B
adc =
Electron flow
IC
IE
bdc =
IB << IC
IC
IB
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The CE connection
• The emitter is grounded or common
• The base-emitter acts like a diode
• The base-collector acts like a current
source that is equal to βdc times the base
current
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The common emitter connection has two loops:
the base loop and the collector loop.
RC
collector
loop
V
RB
CE
VCC
base loop
VBB
VBE
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Subscript notation
• When the subscripts are the same, the
voltage represents a source (VCC).
• When the subscripts are different, the
voltage is between two points (VCE).
• Single subscripts are used for node
voltages with ground serving as the
reference (VC).
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Base curve
• Graph similar to that of a diode
• Diode approximations are used for
analysis (typically - ideal or second)
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The base circuit is usually analyzed with the
same approximation used for diodes.
VBB - VBE
IB =
RC
RB
VCE
VCC
RB
VBB
VBE
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A graph of IC versus VCE
(Note that each new value of IB presents a new curve.)
100 mA
14
12
10
IC in mA 8
6
4
2
80 mA
60 mA
40 mA
20 mA
0 2 4 6
8 10 12 14 16 18
0 mA
VCE in Volts
This set of curves is also called a family of curves.
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Regions of operation
• Active - - - used for linear amplification
• Cutoff - - - used in switching applications
• Saturation - - - used in switching
applications
• Breakdown - - - can destroy the
transistor and should be avoided
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Transistor circuit approximations
• First: treat the base-emitter diode as ideal and
use bIB to determine IC. Use for
troubleshooting.
• Second: correct for VBE and use bIB to
determine IC.
• Third (and higher): correct for bulk resistance
and other effects. Usually accomplished by
computer simulation. Use for design work.
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The second approximation:
VBE = 0.7 V
bdcIB
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VCE
VBB - VBE
IB =
IB =
RB
5 V - 0.7 V
= 43 mA
100 kW
RC
100 kW
VCC
RB
VBB
5V
VBE = 0.7 V
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IC = bdc IB
IC = 100 x 43 mA = 4.3 mA
100 kW
RC
bdc = 100
RB
IB = 43 mA
VBB
5V
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VCC
VRC = IC x RC
VRC = 4.3 mA x 1 kW = 4.3 V
100 kW
1 kW
IC = 4.3 mA
12 V
RB
IB = 43 mA
VBB
RC
5V
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VCC
VCE = VCC - VRC
IC = 4.3 mA
VCE = 12 V - 4.3 V = 7.7 V
100 kW
1 kW
VCE
12 V
RB
IB = 43 mA
VBB
RC
5V
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VCC
Reading transistor data sheets
• Maximum ratings on voltage, current,
and power
• Power transistors dissipate more than 1
watt
• Temperature can change the value of a
transistor’s characteristics
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Typical Breakdown Ratings
• VCBO = 60 V
• VCEO = 40 V
• VEBO = 6 V
• Note: these are reverse breakdown
ratings with one transistor leg open (e.g.
VCBO is voltage collector to base with
emitter open)
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A graphic view of collector breakdown
14
12
10
IC in mA 8
6
4
2
0
50
VCE in Volts
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Typical Maximum Ratings
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IC =
PD =
PD =
PD =
200 mA dc
o
250 mW (for TA = 60 C)
350 mW (for TA = 25 oC)
1W
(for TC = 60 oC)
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Data sheet hFE
“On Characteristics”
IC in mA
hFE(min)
0.1
1
10
50
100
40
70
100
60
30
hFE(max)
___
___
300
___
___
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Surface-mount transistors
• A variety of package styles (threeterminal gull-wing is typical)
• Some SMTs can dissipate 1 watt or more
• Some SMTs house multiple transistors
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Troubleshooting
• Look for gross voltage errors.
• First approximation and mental
estimates will usually suffice.
• Resistors generally don’t short but circuit
boards can.
• Circuit boards can and do open.
• Junctions can and do short.
• Junctions can and do open.
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