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Chapter
28
Bipolar Junction Transistors
Topics Covered in Chapter 28
28-1: Transistor Construction
28-2: Proper Transistor Biasing
28-3: Operating Regions
28-4: Transistor Ratings
28-5: Checking a Transistor with an Ohmmeter
28-6: Transistor Biasing
© 2007 The McGraw-Hill Companies, Inc. All rights reserved.
28-1: Transistor Construction
 A transistor has three doped regions, as shown in Fig.
28-1 (next slide).
 Fig. 28-1 (a) shows an npn transistor, and a pnp is
shown in (b).
 For both types, the base is a narrow region
sandwiched between the larger collector and emitter
regions.
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© 2007 The McGraw-Hill Companies, Inc. All rights reserved.
28-1: Transistor Construction
 The emitter region is
heavily doped and its job is
to emit carriers into the
base.
 The base region is very
thin and lightly doped.
 Most of the current
carriers injected into the
base pass on to the
collector.
 The collector region is
moderately doped and is
the largest of all three
regions.
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Fig. 28-1
28-2: Proper Transistor Biasing
 For a transistor to function properly as an amplifier, the
emitter-base junction must be forward-biased and the
collector-base junction must be reverse-biased.
 The common connection for the voltage sources are at
the base lead of the transistor.
 The emitter-base supply voltage is designated VEE and
the collector-base supply voltage is designated VCC.
28-2: Proper Transistor Biasing
 Fig. 28-4 shows transistor biasing
for the common-base connection.
 Proper biasing for an npn transistor
is shown in (a).
 The EB junction is forward-biased
by the emitter supply voltage, VEE.
 VCC reverse-biases the CB junction.
 Fig. 28-4 (b) illustrates currents in a
transistor.
Fig. 28-4
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28-3: Operating Regions
 Collector current IC is controlled solely
by the base current, IB.
 By varying IB, a transistor can be made
to operate in any one of the following
regions
 Saturation
 Breakdown
 Cutoff
 Active
Fig. 28-6: Common-emitter connection (a)
circuit. (b) Graph of IC versus VCE for different
base current values.
28-3: Operating Regions
 Fig. 28-7 shows the dc equivalent circuit of a transistor operating in the
active region.
 The base-emitter junction acts like a forward-biased diode with current, IB.
 Usually, the second approximation of a diode is used.
 If the transistor is silicon, assume that VBE equals 0.7 V.
Fig. 28-7
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28-4: Transistor Ratings
 A transistor, like any other device, has limitations on its
operations.
 These limitations are specified in the manufacturer’s
data sheet.
 Maximum ratings are given for
 Collector-base voltage
 Collector-emitter voltage
 Emitter-base voltage
 Collector current
 Power dissipation
28-5: Checking a Transistor
with an Ohmmeter
 An analog ohmmeter can be used to check a transistor because the
emitter-base and collector-base junctions are p-n junctions.
 This is illustrated in Fig. 28-8 where the npn transistor is replaced by its
diode equivalent circuit.
Fig. 28-8
28-5: Checking a Transistor
with an Ohmmeter
 To check the base-emitter junction of an npn transistor, first connect the
ohmmeter as shown in Fig. 28-9 (a) and then reverse the ohmmeter leads as
shown in (b).
 For a good p-n junction made of silicon, the ratio RR/RF should be equal to
or greater than 1000:1.
Fig. 28-9
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28-5: Checking a Transistor
with an Ohmmeter
 To check the collector-base junction, first connect the ohmmeter as shown
in Fig. 28-10 (a) and then reverse the ohmmeter leads as shown in (b).
 For a good p-n junction made of silicon, the ratio RR/RF should be equal to
or greater than 1000:1.
 Although not shown, the resistance measured between the collector and
emitter should read high or infinite for both connections of the meter leads.
Fig. 28-10
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28-6: Transistor Biasing
 For a transistor to function properly as an amplifier, an
external dc supply voltage must be applied to produce
the desired collector current.
 Bias is defined as a control voltage or current.
 Transistors must be biased correctly to produce the
desired circuit voltages and currents.
 The most common techniques used in biasing are
 Base
 Voltage-divider
 Emitter
28-6: Transistor Biasing
 Fig. 28-12 (a) shows the simplest
way to bias a transistor, called
base bias.
 VBB is the base supply voltage,
which is used to forward-bias the
base-emitter junction.
 RB is used to provide the desired
value of base current.
 VCC is the collector supply
voltage, which provides the
reverse-bias voltage required for
the collector-base junction.
 The collector resistor, RC,
provides the desired voltage in the
collector circuit.
Fig. 28-12
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28-6: Transistor Biasing
 The dc load line is a graph
that allows us to determine all
possible combinations of IC
and VCE for a given amplifier.
 For every value of collector
current, IC, the corresponding
value of VCE can be found by
examining the dc load line.
 A sample dc load line is
shown in Fig. 28-14.
Fig. 28-14
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28-6: Transistor Biasing
Fig. 28-15 illustrates a dc load line
showing the end points IC (sat) and
VCE (off), as well as the Q point
values ICQ and VCEQ.
Fig. 28-15
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28-6: Transistor Biasing
 The most popular way to bias a
transistor is with voltage-divider bias.
 The advantage of voltage-divider bias
lies in its stability.
 An example of voltage-divider bias is
shown in Fig. 28-18.
Fig. 28-18
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28-6: Transistor Biasing
 Fig. 28-19 shows the dc
load line for voltage-divider
biased transistor circuit in Fig.
28-18.
 End points and Q points are
IC (sat) = 12.09 mA
VCE (off) = 15 V
 ICQ = 7 mA
 VCEQ = 6.32 V
Fig. 28-19
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28-6: Transistor Biasing
 If both positive and negative power
supplies are available, emitter bias
provides a solid Q point that fluctuates
very little with temperature variation
and transistor replacement.
 An example of emitter bias is shown
in Fig. 28-23.
Fig. 28-23
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