After completing this chapter, students should be able to do
the following:• Describe the operation of bipolar junction transistors
(BJTs).• List the types of bipolar transistors and their
characteristics.• List the circuit configurations of transistor amplifiers
and their relative advantages and disadvantages.• Analyze and design bipolar transistor biasing circuits.• Determine the small-signal model parameters of
bipolar transistors.• Analyze and design bipolar transistor amplifiers.• Design a BJT amplifier to meet certain specifications.• Determine the low and high cutoff frequencies of
bipolar transistor amplifiers.
CONTENT:
1.Bipolar Junction Transistors
2.Principles of BJT Operation
3.Input and Output Characteristics
4.BJT Circuit Models
5.The BJT Switch
6.DC Biasing of Bipolar Junction Transistors
7.Common-Emitter Amplifiers
8.Emitter Followers
9.Common-Base Amplifiers
10.Multistage Amplifiers
11.The Darlington Pair Transistor
12.DC Level Shifting and Amplifier
13.Frequency Model and Response
of Bipolar Junction Transistors
14.Frequency Response
of BJT Amplifiers
15.MOSFETs versus BJTs
16.BJT Amplifier Design
Symbols and Their Meanings
1.Bipolar Junction Transistors
■ The emitter and the collector regions are not symmetrical because the impurity-doping
concentrations in the emitter and collector are different and the geometry of these regions
can also differ significantly.■ We use the notation of actual current direction rather than the IEEE notation so that all
currents have positive values. That is, IC, IB, and IE are positive for npn-type transistors,
and they are negative for pnp-type transistors
concentrations in the emitter and collector are different and the geometry of these regions
can also differ significantly.■ We use the notation of actual current direction rather than the IEEE notation so that all
currents have positive values. That is, IC, IB, and IE are positive for npn-type transistors,
and they are negative for pnp-type transistors
2.Principles of BJT Operation
■ A BJT can operate in any of the four operating modes depending on the biasing conditions: saturation, normal active, cutoff, and inverted. For an amplification, the B-E junction is forward biased and
the C-B junction is reverse biased, while for operation in the saturation region, both B-E and C-B
junctions are forward biased.■ The major physical parameters of a BJT are the forward current gain, the forward current ratio, the
saturation current, and the Early voltage.■ The collector voltage affects the width of the space charge or depletion regions and the width of the
depletion region depends on the C-B voltage
the C-B junction is reverse biased, while for operation in the saturation region, both B-E and C-B
junctions are forward biased.■ The major physical parameters of a BJT are the forward current gain, the forward current ratio, the
saturation current, and the Early voltage.■ The collector voltage affects the width of the space charge or depletion regions and the width of the
depletion region depends on the C-B voltage
a/Forward Mode of Operation
The B-E pn junction is forward biased, and the base–collector (B-C) pn junction is reverse biased in the
normal, active bias configuration as shown in Fig. 8.4(a). This configuration is called the forward-activeoperating mode. Using the pn junction theory developed in Sec. 6.5, the description of the device operation is as follows:
normal, active bias configuration as shown in Fig. 8.4(a). This configuration is called the forward-activeoperating mode. Using the pn junction theory developed in Sec. 6.5, the description of the device operation is as follows:
Collector Current :
iC = IC,n = IE,n - IB,n = ISevBE>VT which is 25.8 mV at room temperature
iE = IE,n + IE,p + IB,n = IE,n + IE,p = ISE evBE>VT
where ISE is the saturation current that depends on the emitter saturation current density and is related to
the doping profiles and levels
where ISE is the saturation current that depends on the emitter saturation current density and is related to
the doping profiles and levels
Base Current:
iB = iB1 + iB2 = IE,p + IB,n = ISB evBE>VT
where ISB is the base saturation current.
Forward-Current Ratio :
Forward-Current Gain :
b/Cutoff, Saturation, and Inverse-Active
Modes of Operation
+ In the cutoff mode, the B-E junction is either reverse biased, or zero biased, and the B-C junction is also
reverse biased. That is, VBE has negative voltage or zero, and VCB has a positive voltage. For reversebiased junctions, the minority carrier concentrations are ideally zero at each depletion edge. The potentialbarrier heights of both the B-E and B-C junctions are increased, so there is essentially no charge flow.
+In the saturation mode, both junctions are forward biased. The B-E potential barrier is smaller than
the potential barrier of the B-C junction. There is a gradient in the minority carrier concentration in thebase to induce the collector current. Since both junctions are forward biased, the minority carrier concentrations are greater than the thermal equilibrium values at the depletion region edges. There will be
a net flow of electrons from the emitter to the collector.
+In the inverse-active mode, the B-E junction is reverse biased, and the B-C junction is forward biased. It is a mirror image of the forward-active mode. The potential barrier height of the B-E junction will increase while the potential barrier height of the B-C junction will decrease. Electrons from the collector will diffuse across the B-C junction into the base and then diffuse into the emitter. The bipolar transistor is not a symmetrical device and the characteristics will therefore be different from those of the active-mode operation. The B-C area is normally much larger than the B-E area, and as a result, not all of the injected electrons will be collected by the emitter. The relative doping concentrations in the base and collector are also different compared with those of the base and emitter. Therefore, we expect a significantly different characteristic between the forward-active and inverse-active modes of operation. The transistor is not normally operated in this mode
3.Input and Output Characteristics
Each of the three terminals of a transistor may be classified as an input terminal, an output terminal, or a common terminal. There are three possible configurations: (1) common emitter (CE), in which
the emitter is the common terminal; (2) common collector (CC) or emitter follower, in which the collector is the common terminal; and (3) common base (CB), in which the base is the common terminal.
■ Each of the three terminals of a transistor may be classified as an input terminal, an output terminal, or a common terminal. There are three possible configurations: (1) common
emitter (CE), in which the emitter is the common terminal; (2) common collector (CC) or
emitter follower, in which the collector is the common terminal; and (3) common base
(CB), in which the base is the common terminal.■ The output characteristic of a BJT can be divided into three regions: (1) a cutoff region
in which the transistor is off, (2) an active region in which the transistor exhibits a high
output resistance and has a current amplification, and (3) a saturation region in which the
transistor offers a low resistance.
emitter (CE), in which the emitter is the common terminal; (2) common collector (CC) or
emitter follower, in which the collector is the common terminal; and (3) common base
(CB), in which the base is the common terminal.■ The output characteristic of a BJT can be divided into three regions: (1) a cutoff region
in which the transistor is off, (2) an active region in which the transistor exhibits a high
output resistance and has a current amplification, and (3) a saturation region in which the
transistor offers a low resistance.
4.BJT Circuit Models
a/Linear DC Model
b/Small-Signal AC Model
c/Small-Signal Hybrid Model
d/Small-Signal Analysis
5.The BJT Switch
6.DC Biasing of Bipolar Junction Transistors
7.Common-Emitter Amplifiers
8.Emitter Followers
9.Common-Base Amplifiers
10.Multistage Amplifiers
11.The Darlington Pair Transistor
12.DC Level Shifting and Amplifier
13.Frequency Model and Response
of Bipolar Junction Transistors
14.Frequency Response
of BJT Amplifiers
15.MOSFETs versus BJTs
16.BJT Amplifier Design
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