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Chapter 4
Bipolar Junction Transistors
Objectives
 Describe the basic structure of the bipolar junction
transistor (BJT)
 Explain and analyze basic transistor bias and
operation
 Discuss the parameters and characteristics of a
transistor and how they apply to transistor circuits
 Discuss how a transistor can be used as an
amplifier or a switch
 Troubleshoot various failures typical of transistor
circuits
Introduction
A transistor is a device that can be used to either
switch flow on/off or “control flow” device.
Let’s first consider its operation in a simpler view as a
flow switching device.
+
1 - open
ollector
II
-
ollector
I
2 - closed
-
mitter
+
mitter
On/Off Flow
Introduction
Let’s first consider its operation in a simpler view as a
current controlling
device.
I
Variable Flow
+ ollector
I
I
- mitter
- ollector
I
+ mitter
Flow
Transistor Structure
With diodes there is one p-n junction. With bipolar
junction transistors (BJT), there are three layers
forming two p-n junctions. Transistors can be either
pnp or npn type.
BJT Construction
Physical construction of a BJT is deposited layers of “doped “
N and P semiconductor materials.
Note: Epitaxial films may be grown from gaseous or liquid precursors. Because the substrate acts as a seed
crystal, the deposited film takes on a lattice structure and orientation identical to those of the substrate. This is
different from other thin-film deposition methods which deposit polycrystalline or amorphous films, even on
single-crystal substrates. If a film is deposited on a substrate of the same composition, the process is called
homoepitaxy; otherwise it is called heteroepitaxy.
Basic Transistor Operation
Look at this one circuit as two separate circuits:
• The base-emitter (left side) circuit and
• The collector-emitter (right side) circuit.
Note that the emitter leg serves as a conductor for both circuits.
• The amount of current flow in the base-emitter circuit controls the
amount of current that flows in the collector circuit.
• Small changes in the transistor base-emitter current yields a large
change in collector-current.
I E = I C + IB
http://www.bcae1.com/transres.htm
Basic Transistor Operation
Below, is the proper bias configuration for the npn and the pnp
transistor. Note that base-emitter junction is forward-biased,
while the base-collector junction is reverse-biased (known as
forward-reverse bias). For the npn, the forward-bias BE, narrows
the BE junction depletion zone, while the reverse-bias BC
depletion zone grows larger. Let’s examine this more closely
(next slide).
I C = ß IB
Forward-Reverse bias of a bipolar transistor.
Basic Transistor Operation - Conventional Flow
Looking at a forward-reverse bias NPN
1.
Forward-bias BE narrows “Depletion
Zone” (see forward-biased diode).
2.
Reverse-bias BC widens “Depletion
Zone”.
3.
Emitter (doped) region is “rich” with
free (conduction-band) electrons.
4.
These “conduction-band” electrons
easily migrate across the BE junction
to become “minority carriers” in the
base.
5.
Since there are few “holes, only a few
recombine to form a small “valence
electron current” thru RB to BE supply.
6.
The “free electrons” are attracted
across the BC junction into the
Collector and thru to the +C supply as
Ic (collector current).
7.
The PNP transistor follows a similar
logic.
NPN
Transistor
Collector
“valence
electron”
current
5
6
2
4
3
Base
1
Emitter
Transistor Currents (Conventional Flow)
IE = IC + I
B
Note relative current sizes.
Regions of operation
•
•
•
•
•
Applied voltages Mode
E < B < C Forward active
E < B > C Saturation
E > B < C Cut-off
Forward active: base higher than emitter, collector higher than base (in this
mode the collector current is proportional to base current by βF).
• Saturation: base higher than emitter, but collector is not higher than base.
• Cut-Off: base lower than emitter, but collector is higher than base. It means
the transistor
• is not letting conventional current to go through collector to emitter.
• In terms of junction biasing: ('reverse biased base–collector junction' means
Vbc < 0 for NPN, opposite for PNP)
• Forward-active (or simply, active): The base–emitter junction is forward
biased and the base–collector junction is reverse biased. Most bipolar
transistors are designed to afford the greatest common-emitter current gain,
βF, in forward-active mode. If this is the case, the collector–emitter current
is approximately proportional to the base current, but many times larger,
for small base current variations.
• Saturation: With both junctions forward-biased, a BJT is in saturation
mode and facilitates high current conduction from the emitter to the
collector. This mode corresponds to a logical "on", or a closed switch.
• Cutoff: In cutoff, biasing conditions opposite of saturation (both junctions
reverse biased) are present. There is very little current, which corresponds
to a logical "off", or an open switch.
• There are basically three possible ways to connect a Bipolar
Transistor within an electronic
• circuit with each method of connection responding differently
to its input signal as the static characteristics of the transistor
vary with each circuit arrangement.
• 1. Common Base Configuration - has Voltage Gain but no
Current Gain.
• 2. Common Emitter Configuration - has both Current and
Voltage Gain.
• 3. Common Collector Configuration - has Current Gain but no
Voltage Gain
Transistors Characteristics and Parameters
For proper operation, the base-emitter junction is forwardbiased by VBB and conducts just like a diode. IB is set.
The collector-base junction is reverse biased by VCC and
blocks current flow through it’s junction just like a diode.
Current flow
through the baseemitter junction will
help establish the
path for current flow
from the collector to
emitter.
IC + IB results in IE
BC
Reverse-biased
BE
Forward-biased
Transistor Characteristics and Parameters
Analysis of this transistor circuit to predict the dc voltages and
currents requires use of Ohm’s law, Kirchhoff’s voltage law and
the ß for the transistor.
Determine the base BIAS current.
Using Kirchhoff’s voltage law, subtract the 0.7VBE from VBB and
the remaining voltage is dropped across RB. Determining the
current for the base with this information is a matter of applying
of Ohm’s law. VRB/RB = IB (ohm’s law)
The collector DC current is
determined by multiplying
the base current by beta.
ICdc =  IBdc
or DC = IC/IB
Note:
0.7 VBE will be used in
most analysis examples.
Transistor Characteristics and Parameters
What we ultimately
determine by use of
Kirchhoff’s voltage law
for series circuits is that;
In the base circuit, VBB is
distributed across the
base-emitter junction and
RB in the base circuit.
In the collector circuit we
determine that VCC is
distributed proportionally
across RC and the
transistor (VCE).
Transistor Characteristics and Parameters
• Small ΔIDCbase-emitter yields Large ΔIDCcollector-emitter
• This relationship of current change (collector to base)
(ΔIDCcollector-base) is called beta. (  )
DC = IC /IB
(Current Gain)

Data sheets will refer to hybrid (h) parameters
hFE =
DC (typical hFE values 20 – 200)
• The relationship of current change
(collector to emitter)
(ΔIDCcollector-emitter) is called alpha.
(∝)
∝DC = IC / IE
(Typical
is always >1.
∝ values 0.95 – 0.99+)
See Ex. 4-1
∝
Transistor Characteristics and Parameters
There are three key dc voltages and three key dc currents to
be considered. Note that these measurements are important
for troubleshooting.
Relationships of Key Parameters:
IB: dc base current
IE: dc emitter current
IC: dc collector current
VBE: dc voltage across
VBE = 0.7 V.
IB = (VBB – VBE)/RB
VCE = VCC – ICRC
VCB = VCE - VBE
base-emitter junction
VCB: dc voltage across
collector-base junction
VCE: dc voltage from
collector to emitter
See Ex 4-2
Collector Characteristic Curves
Collector characteristic curves give a graphical illustration of the
relationship of: Ic (collector current), and VCE (Voltage-Collector to Emitter)
with specified amounts of IB (base current).
With greater increases of VCC , VCE continues to increase until it reaches
breakdown, but the current remains about the same in the linear region
from .7V to the breakdown voltage.
IC
Ic
IB
Ic
VCE
Vcc
VCE
Active Region
See Ex. 4-2
IBase
Cutoff
region
VCE
Transistor Characteristics Curves - Review
1. Vcc = 0
IB is thru the base & emitter.
Transistor is in Saturation
2. VCC , VCE and Ic with increase
Vcc
3. VCE exceeds 0.7V
Base/Collector junction is
reverse-biased
Transistor operates
3
in Linear region
4.
“breakdown” or
2
Avalanche region.
……. Bad!!
See Ex. 4-3
1
2
4
Transistor Characteristics and Parameters
With no IB this transistor is in the cutoff region and
just as the name implies there is practically no current
flow in the collector part of the circuit. With the
transistor in a cutoff state the full VCC can be measured
across the collector and emitter (VCE). Rc≈0V.
Transistor Characteristics and Parameters
Current flow in the collector part of the circuit is, determined by IB
multiplied by .
However, there is a limit to how much current can flow in the
collector circuit regardless of additional increases in IB.
Once this maximum is reached, the transistor is said to be in
saturation. Note that saturation can be determined by application
of Ohm’s law.
IC(sat) =VCC/RC
The measured voltage across the now “shorted” collector and
emitter is VCE = 0V.
Transistor Characteristics and Parameters
The dc load line graphically illustrates IC(sat) and cutoff for a
transistor.
NOTE:
This will be the
area of operation
for all out transistor
applications.
See Ex.4-4
Transistor Characteristics and Parameters
More about β
The beta for a transistor is not always constant. Temperature
and collector current both affect β. Variances in the manufacture
of the transistor may also result in variable β’s.
There are also maximum power ratings to consider.
The data sheet below, provides information on these characteristics.
Note: ΔβDC for various temperatures.
Note: ΔβDC for ΔIC
See End of Chap. Q.4-20 & 21
Transistor Characteristics and Parameters
Maximum Power Ratings
•
Maximun ratings are available from datesheets and on-line.
•
Typical max. ratings address:
•
A primary limitation is; Ic = PD(max) / VCE rearranged: VCE = PD(max)/IC
VCE max, VCB max, VEB max, IC max & Power.
• Power: P= EI, P= I2R, P=E2/R
Assuming PD(MAX) is 500mW:
VCEmax is 20V, IC(MAX) is 50mA.
IcMAX
PDMAX
The graph shows the transistor can’t be
operated in the “shaded“ areas.
VCE(MAX)
See Ex. 4-5 & 4-6
Transistor Characteristics and Parameters
Power Derating/Data Sheets
Power Derating – PDmax
3 factors decide Power Deratings: see the previous slide.
ICmax , VCEmax & PDmax -------- IC = PD(max) / VCE
See the Datasheet for Deratings
See Ex. 4-7
Transistor Datasheets
Provide:
• Physical details
• Absolute Maximum ratings:
Voltage, Current, Temperature & Power
• Thermal characteristics
• Electrical Characteristics – hfe, Cutoff, Breakdown voltage, Gain, Capacitance,
Switching speeds, etc.
See Ex. 4-8
BJT (Transistor) Amplifier
Amplification is the process of linearly increasing the amplitude of
an electrical signal.
Some designation standards and conventions:
DC Currents:
IC IE IB
DC Voltages:
VBE VCB VCE VB VC VE
External DC Resistance: RE RC RB
AC Currents:
Ic
Ie
Ib
AC Voltages:
Vbe Vcb Vce Vb Vc Ve
External AC Resistances: Re Rc Rb
Internal Transistor Resistances: r ’ r ’e
Transistor Switch
A transistor when used as a switch is simply being biased so that it
is in cutoff (switched off) or saturation (switched on). Remember
that the VCE in cutoff is VCC and 0 V in saturation.
VCE(cutoff) = VCC
See Ex 4-10
IC(sat) = VCC – VCE(sat)
βDC
IB(min) = Ic (sat)
βDC
Transistor Switch
Application: Transistor Switch
Base of transistor is “pulsed”
with a “squarewave”.
“Off” periods provide no base
biasing….transistor remains
OFF.
“On” periods provide DC bias,
switching the transistor “ON”
IB is max. Ic is max.
LED lights!!
See Ex 4-11
Transistor Types & Packaging
2N3904
2N3906
Same transistor type in metal packaging
Transistor Types & Packaging
RF Transistors
High-density, multi-transistor packaging
Power transistors
Troubleshooting
Troubleshooting a live transistor circuit requires us to
be familiar with known good voltages, but some
general rules do apply. Certainly a solid fundamental
understanding of Ohm’s law and Kirchhoff’s voltage
and current laws is imperative. With live circuits it is
most practical to troubleshoot with voltage
measurements.
Troubleshooting
Opens in the external resistors or connections of the base or the
circuit collector circuit would cause current to cease in the collector
and the voltage measurements would indicate this.
Internal opens within the transistor
itself could also cause transistor
operation to cease.
Erroneous voltage measurements that
are typically low are a result of point
that is not “solidly connected”. This
called a floating point. This is typically
indicative of an open.
More in-depth discussion of typical
failures are discussed within the
textbook.
Troubleshooting
Testing a transistor can be viewed more simply if you view it
as testing two diode junctions. Forward bias having low
resistance and reverse bias having infinite resistance.
Troubleshooting
The diode test function of a multimeter is more reliable than
using an ohmmeter. Make sure to note whether it is an npn or
pnp and polarize the test leads accordingly.
Troubleshooting – Bias Testing
Summary
 The bipolar junction transistor (BJT) is constructed of
three regions: base, collector, and emitter.
 The BJT has two pn junctions, the base-emitter
junction and the base-collector junction.
 The two types of transistors are pnp and npn.
 For the BJT to operate as an amplifier, the base-emitter
junction is forward-biased and the collector-base junction is
reverse-biased.
 Of the three currents IB is very small in comparison to IE
and IC.
 Beta is the current gain of a transistor. This the ratio of
IC/IB.
Summary
 A transistor can be operated as an electronics switch.
 When the transistor is off it is in cutoff condition (no
current).
 When the transistor is on, it is in saturation condition
(maximum current).
 Beta can vary with temperature and also varies from
transistor to transistor.