Download Chapter 4 - Portal UniMAP

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
4.1 Introduction
A transistor is a three-terminal device whose output
current, voltage, and/or power controlled by its input.
The three terminals are called collector, base, and
emitter.
The arrow on the symbol is important for two reasons:
1. It identifies the component terminals. The
arrow is always drawn on the emitter
terminal. Terminal opposite the emitter is
the collector, and the center terminal is
the base.
2. The arrow always points toward the n-type
materials. If the arrow points toward the
emitter, the transistor is a npn type. If it points
toward the base, the transistor is a pnp type.
(a) n-p-n
(b) p-n-p
Fig.4-1: Standard
transistor symbols
4.2 Bipolar Junction Transistor Structure
Different with diodes having one p-n junction, bipolar junction
transistors (BJT) have three layers and two p-n junctions.
Fig.4-2: Basic BJT construction.
4.3 Transistor Currents
The values of the collector and emitter currents are determined
by the value of the base current. An increase or decrease in
base current (IB) causes a similar change in collector current
(IC) and emitter current (IE).
Fig.4-3: Transistor terminal currents.
According to Kirchhoff’s current law, the current leaving a
component must equal the current entering the component. By
formula,
I E  IC  I B
4-1
Since IB is normally much less than IC, IC and IE are approximately
equal, expressed as follows:
IC  I E
4-2
The value of IC is normally some multiple of the value of IB.
The factor by which current increases from base to collector is
referred to as dc current gain (βDC) of a transistor.
I E  I B ( DC  1)
4-3
If we combine this Eq.4-3 with Eq.4-1, we get
I C   DC I B
4-4
βDC is usually designated as an equivalent hybrid parameter, hFE.
hFE   DC
4-5
The ratio of the dc collector current (IC) to the dc emitter current
(IE) is a dc alpha (αDC).
 DC
IC

IE
4-6
Using the relationships of Eq.4-1 and 4-6, we can calculate base
current as
I B  I E (1   )
4-7
The relationship between alpha and beta:
 DC 
 DC
 DC  1
4-8
4.4 BJT Characteristics and Parameters
Several voltages are normally involved in the transistor operation,
described in Table 4-1. VCC and VBB are dc voltage sources that are
used to bias the transistor.
Fig.4-4: Transistor
currents and voltages.
TABLE 4-1: Transistor voltages
Voltage
Abbreviation
Definition
VCC
Collector-bias voltage source. This is a dc voltage
source applied to the base-collector junction.
Base-base voltage source. This is a dc voltage source
applied to the base-emitter junction.
DC voltage across base-emitter junction.
DC voltage across collector-base junction.
DC voltage from collector to emitter.
VBB
VBE
VCB
VCE
For proper operation, the base-emitter junction is forward-biased
by VBB and conducts just like a diode and has a nominal forward
voltage drop of
VBE  0.7 V
4-9
The collector-base junction is reverse-biased by VCC and blocks
current flow through it’s junction just like a diode.
Since the emitter is at ground (0 V), by Kirchhoff’s voltage law,
the voltage across RB is
VRB  VBB  VBE
4-10
Also, by Ohm’s law,
VRB  I B RB
Substituting for VRB yields
I B RB  VBB  VBE
Solving for IB,
VBB  VBE
IB 
RB
4-11
The voltage at the collector with respect to the grounded emitter
is
VCE  VCC  VRC
Since the drop across RC is
VRC  I C RC
the voltage at the collector with respect to the emitter can be
written as
VCE  VCC  I C RC
4-12
The voltage across the reverse-biased collector-base junction is
VCB  VCE  VBE
4-13
4.4.1 Collector characteristic curves
These curves give a graphical
illustration of the relationship
between collector current (IC)
and VCE with specified amounts
of base current. With greater
increases of VCC , VCE continues
to increase until it reaches
breakdown. When VCE exceeds
0.7 V, the base-collector
junction becomes reversebiased and the transistor goes
into the active or linear
region. In this region, the
current remains from 0.7 V to
the breakdown voltage.
Fig.4-5: Collector characteristic curves.
4.4.2 Cutoff
Cutoff is a non-conducting state of a transistor. This occurs when
the base lead opens and the base current is zero. There is only a
very small amount of collector leakage current , ICEO, caused by
thermally produced carriers. However, it will usually be neglected
so that VCE = VCC.
In the cutoff, neither the
base-emitter nor the basecollector junctions are
forward-biased.
The subscript CEO
represents collector-toemitter with the base open.
Fig.4-6
4.4.3 Saturation
Saturation is the state of a BJT in which the collector current has
reached a maximum and is independent of the base current.
Note that saturation value of IC can be determined by application
of Ohm’s law. When VCE reaches its saturation value (VCE(sat) = 0,
we obtain,
I C sa t
VCC

RC
4-14
Fig.4-7
4.4.4 DC Load Line
DC load line graphically illustrates IC(sat) and cutoff for a transistor.
The bottom of the load line
is at ideal cutoff where IC
= 0 and VCE = VCC.
The top of the load line is
at saturation where IC =
IC(sat) and VCE = VCE(sat).
Along the load line is the
active region of the
transistor’s operation.
Fig.4-8
4.5 The BJT As An Amplifier
Amplification is the process of increasing the amplitude of an electrical
signal. The circuits used to provide the amplification are referred to as
amplifiers.
Fig.4-9: Basic transistor amplifier circuit.
'
If the internal ac emitter resistance designated re is in series with RB, the
ac base voltage can be written as
Vb  I e re'
4-15
The ac collector voltage, Vc, equals the
ac voltage drop across RC.
Vc  I c RC
Since
Ic
4-16
I c  I e , the ac collector voltage is
Vc  I e RC
4-17
The ratio of Vc to Vb is the ac voltage
gain, Av, of the transistor.
Vc I e RC RC
Av 

 '
'
Vb
I e re
re
Because RC is always larger than
input voltage.
4-18
re' , the output voltage is greater than
4.6 The BJT As A Switch
A BJT can be used as a switching device. If the base-emitter junction is
not forward-biased, the BJT is in the cutoff (switched off) and there is
an open circuit between collector and emitter, as shown in Fig. 4-10(a).
If the base-emitter junction and base-collector junction are forwardbiased, the BJT is in the saturation (switched on). There is an short
circuit between collector and emitter, as shown in Fig. 4-10(b).
Fig.4-10: Switching action of an ideal transistor.
Conditions in Cutoff
All of the currents are zero, and VCE is equal to VCC.
VCE ( cut )  VCC
4-19
Conditions in Saturation
The formula for collector saturation current is
I C ( sat) 
VCC  VCE ( sat)
RC
4-20
The minimum value of base current needed to produce saturation is
I B (min) 
I C ( sat)
 DC
4-21
4.7 Troubleshooting
Several faults that can occur in a transistor bias circuit are:
1. Open bias resistors.
2. Open or resistive connections.
3. Shorted connections.
4. Opens or shorts internal to transistor itself.
These faults will cause the current to cease in the collector.
4.7.1 Troubleshooting a Biased Transistor
This figure shows the correct voltage measurements at base and
collector of a basic transistor bias circuit.
Erroneous voltage measurements
are a result of point that is not
“solidly connected”. This called a
floating point. This is typically
indicative of an open.
A simple check at the top of the
collector resistor and at the collector
itself will quickly ascertain if VCC is
present and the transistor is
conducting normally or is in cutoff
(VC = VCC) or saturation (VC ≈ 0).
Fig.4-11: A basic transistor bias
circuit with all voltages referenced
to ground.
4.7.2 Testing a Transistor with a DMM
Several faults that can occur in the circuit and the accompanying
symptoms are illustrated in Fig. 4-12. Symptoms are shown in terms of
measured voltages that are incorrect.
Fig. 4–12: Typical faults and symptoms in the basic
transistor bias circuit.
4.7.2 Testing a Transistor with a DMM
A fast and simple way to check a transistor for open or shorted
junction is by using a digital multimeter. Testing of a transistor
can be viewed as testing of two diode junctions. The testing
shows that forward bias has low resistance and reverse bias has
infinite resistance.
Fig.4-13: A transistor viewed as two diodes.
The diode test function of a multimeter is more reliable than
using an ohmmeter. However, make sure to note whether it is an
npn or pnp and polarize the test leads.
Fig.4-14: Typical DMM test of a properly functioning npn transistor.
4.7.3 Transistor Testers
In addition to the traditional DMMs,
there are also transistor testers.
Some of these have the ability to
test other parameters of the
transistor, such as leakage and gain.
Fig. 4-15: Transistor tester.
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.