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Chapter 4
BIPOLAR JUNCTION
TRANSISTORS (BJTs)
INTRODUCTION
 What is transistor?
 A three-terminal device whose output current,
voltage and/or power are controlled by its input.
 Commonly used in audio application as an
amplifier, in switching application as a switch and
in power supply voltage and current regulator
circuit.
 2 basic transistor types: BJT and FET
 These two transistor differ in their operating
characteristic and their internal construction.
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
LECTURE OUTLINE
1. BJT structure
2. Basic BJT operations
3. BJT Characteristics and Parameters
4. BJT as an amplifier
5. BJT as a switch
6. Troubleshooting
Summary
1. BJT STRUCTURE
1. BJT STRUCTURE
 The BJT is constructed with three doped semiconductor regions
separated by two pn junctions.
 The three region are called emitter (E),base (B) and collector (C)
 The BJT have 2 types:
1. Two n region separate by a p region – called npn
2. Two p region separated by a n region – called pnp
 The pn junction joining the base region and the emitter region is
called the base-emiter junction
 The pn junction joining the base region and the collector region is
call base-collector junction
 The base region is lightly doped and very thin compared to the
heavily doped emitter and the moderately doped collector region
1. BJT STRUCTURE
1. BJT STRUCTURE
 BJT schematic symbol
 The arrow on schematic symbol is important
because:
 Identify the component terminal
The arrow is always drawn on the emitter terminal.
The terminal opposite emitter is collector and the
center terminal is base.
 The arrow always points toward n-type material
If the arrow point toward base, transistor is pnp type.
If it points toward emitter, transistor is npn type.
1. BJT STRUCTURE
 Transistor terminal current
1. BJT STRUCTURE
Transistor Currents:
 The directions of the currents in npn transistor and pnp transistor are
shown in the figure.
 The emitter current (IE) is the sum of the collector current (IC) and the
base current (IB)
I E  I B  IC
 IB << IE or IC
 The capital letter – dc value
 Transistor is a current-controlled device - the value of collector and
emitter currents are determined by the value of base current.
 An increase or decrease in value of IB causes similar change in values
of IC and IE.
Current gain (β)  factor
by which current increases
C
DC B from base of transistor to
its collector.
I  I
1. BJT STRUCTURE
 Transistor Voltages:
 VCC – collector supply voltage. This is a power supply voltage
applied directly to collector of transistor.
 VBB – base supply voltage. this is dc voltage used to bias base
of transistor.
 VEE – emitter supply voltage. dc biasing voltage and in many
cases, VEE is simply a ground connection.
1. BJT STRUCTURE
 Transistor Voltages:
 VC – dc voltage measured from collector terminal of
component to ground
 VB – dc voltage measured from base terminal to ground.
 VE – dc voltage measured from emitter terminal to ground.
1. BJT STRUCTURE
 Transistor Voltages:
 VCE – dc voltage measured from collector to emitter terminal
of transistor.
 VBE – dc voltage measured from base to emitter terminal of
transistor.
 VCB – dc voltage measured from collector to base terminal of
transistor.
2. BJT OPERATION
2. BJT OPERATION
 To operate the transistor properly, the two pn
junction must be correctly biased with external dc
voltages.
 The figure shown the proper bias arrangement for
both npn and pnp transistor for active operation as
an amplifier.
2. BJT OPERATION
 Transistor is made of 3 separate semiconductor
materials that joined together to form two pn
junction.
 Point at which emitter and base are joined forms a
single pn junction  base-emitter junction
 Collector-base junction  point where base and
collector meet.
2. BJT OPERATION
 Cutoff region
 Both transistor
junctions are reverse
biased.
 With large depletion
region between C-B
and E-B, very small
amount of reverse
current, ICEO passes
from emitter to
collector and can be
neglected.
 So, VCE = VCC
2. BJT OPERATION
 Saturation region
 Both transistor junctions are
forward-biased.
 IC reaches its maximum value
as determined by VCC and total
resistance in C-E circuit.
 IC is independently from
relationship of β and IB.
 VBE is approximately 0.7V and
VCE < VBE.
VCC
IC 
RC  RE
2. BJT OPERATION
 Active region

BE junction is forward biased
and the BC junction is reverse
biased.
 All terminal currents have
some measurable value.
 The magnitude of IC depends
on the values of β and IB.
 VCE is approximately near to
0.7V and VCE falls in ranges
VBE<VCE<VCC.
3. BJT CHARACTERISTICS &
PARAMETERS
3. BJT CHARACTERISTICS & PARAMETERS
DC Beta ( DC ) and DC Alpha ( DC ):
 The ratio of the dc collector current (IC) to the dc base current (IB) is the
dc beta
(  DC ) = dc current gain of transistor
 Range value :
20<
 DC <200
 Usually designed as an equivalent hybrid (h) parameter,
transistor data sheet – hFE   DC
 DC
hFE
on
IC

IB
 The ratio of the dc collector current (IC) to the dc emitter current (IE) is
the dc alpha (  DC ) – less used parameter in transistor circuits
 Range value-> 0.95<
 DC <0.99 or greater , but << 1 (Ic< IE )
 DC
IC

IE
3. BJT CHARACTERISTICS & PARAMETERS
Current and Voltage Analysis:
 The current and voltage can be identified as follow:
 Current:
dc base current,
IB
dc emitter current, I E
dc collector current, I C
forward-biased the
base-emitter junction
Voltage:
dc voltage at base with respect to emitter,
VBE
dc voltage at collector with respect to base, VCB
dc voltage at collector with respect to emitter, VCE
reverse-biased the
base-collector junction
Transistor current & voltage
3. BJT CHARACTERISTICS & PARAMETERS
Current and Voltage Analysis:
 When the BE junction is forward-biased, like a forward biased
diode and the voltage drop is V  0.7V
BE
 Since the emitter is at ground (0V), by Kirchhoff’s voltage law, the
voltage across RB is: VRB  VBB  VBE …….(1)
 Also, by Ohm’s law:
 From (1) ->(2) :
VRB  I B RB ……..(2)
VBB  VBE  I B RB
 Therefore, the dc base current is:
VBB  VBE
IB 
RB
3. BJT CHARACTERISTICS & PARAMETERS
Current and Voltage Analysis:
 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 dc voltage at the collector with respect to the emitter is:
VCE  VCC  I C RC
where
I C   DC I B
 The dc voltage at the collector with respect to the base is:
VCB  VCE  VBE
Example 1
 Determine IB, IC, IE, VCE and VCB in the circuit
below. The transistor has a βDC=150.
Solution Example 1
 When BE junction is FB, act as normal diode. So,
VBE=0.7V.
VBB  VBE 5  0.7
 The base current,
IB 

 430A
RB
10k
 Collector current,
I C   DC I B  150(430A)  64.5mA
 Emitter current,
V  V  I R  10V  (64.5mA )(100)  3.55V
CE
CC
C
C
I E  I C  I B  64.5mA  430A  64.9mA
 Solve for VCE and VCB.
VCB  VCE  VBE  3.55  0.7  2.85V
3. BJT CHARACTERISTICS & PARAMETERS
Collector Characteristic Curve:
 Using a circuit as shown in below, we can generate a set of
collector characteristic curve that show how the collector current,
Ic varies with the VCE voltage for specified values of base current,
IB.
variable voltage
3. BJT CHARACTERISTICS & PARAMETERS
Collector characteristic curve:
3. BJT CHARACTERISTICS & PARAMETERS
Collector Characteristic Curve:
 Assume that VBB is set to produce a certain value of IB and VCC is zero.
 At this condition, BE junction and BC junction are forward biased
because the base is approximately 0.7V while the emitter and the
collector are zero.
 IB is through the BE junction because of the low impedance path to
ground, therefore IC is zero.
 When both junctions are forward biased – transistor operate in
saturation region.
 As VCC increase, VCE is increase gradually, IC increase – indicated by
point A to B.
 IC increase as VCC is increased because VCE remains less than 0.7V due
to the forward biased BC junction.
 When VCE exceeds 0.7V, the BC becomes reverse biased and the
transistor goes into the active or linear region of its operation.
3. BJT CHARACTERISTICS & PARAMETERS
Collector Characteristic Curve:
 Once BC junction is RB, IC levels off and remains constant for given
value of IB and VCE continues to increase.
 Actually IC increases slightly as VCE increase due to widening of the BC
depletion region
 This result in fewer holes for recombination in the base region which
effectively caused a slight increase in I C   DC I B indicated in point
B and C.
 When VCE reached a sufficiently high voltage, the reverse biased BC
junction goes into breakdown.
 The collector current increase rapidly – as indicated at the right point C
 The transistor cannot operate in the breakdown region.
 When IB=0, the transistor is in the cutoff region although there is a very
small collector leakage current as indicated – exaggerated on the graph
for purpose of illustration.
3. BJT CHARACTERISTICS & PARAMETERS
DC Load Line:
 Cutoff and saturation can be illustrated in relation to
the collector characteristic curves by the use of a load line.
 DC load line drawn on the connecting cutoff and saturation point.
 The bottom of load line is ideal
cutoff where IC=0 & VCE=VCC.
 The top of load line is saturation
where IC=IC(sat) & VCE =VCE(sat)
 In between cutoff and saturation
is the active region of transistor’s
operation.
Example 2
 Determine whether or not the transistor in figure
below is in saturation. Assume VCE(sat) = 0.2V
Solution Example 2
 First, determine IC(sat),
I C ( sat ) 
VCC  VCE ( sat )
RC
10  0.2

 9.8mA
1.0k
 Now, see if IB is large enough to produce IC(sat),
VBB  VBE 3  0.7

 0.23mA
RB
10k
  DC I B  50(0.23)  11.5mA
IB 
IC
 With specific βDC, this base current is capable of
producing IC greater than IC(sat). Thus, transistor is
saturated and IC = 11.5mA is never reached. If
further increase IB, IC remains at its saturation value.
3. BJT CHARACTERISTICS & PARAMETERS
More About beta,  DC , hFE:
-Important parameter for BJT
-Varies both IC & temperature
-Keeping the junction temperature
constant, IC
cause  DC
-Further increase in IC beyond this
max. point cause  DC to decrease
Maximum Transistor Ratings:
-Specified on manufacturer’s data sheet
-Given for VCE,VBE,VBC,IC & power dissipation
-The product of VCE and IC must not exceed the max. power dissipation
-Both VCE and IC cannot be max. at the same time.
IC 
PD (max)
VCE
3. BJT CHARACTERISTICS & PARAMETERS
Derating PD (max)
:
-Specified at 25°C, for higher temp, PD(max) is less.
-Data sheet often give derating factor for determining PD (max) at > 25°C
-Example: derating factor of 2mW/°C indicates that the max. power
dissipation is reduced 2mW for each degree increase in temperature.
Data Sheets
Data sheets give manufacturer’s specifications for maximum operating
conditions, thermal, and electrical characteristics. For example, an
electrical characteristic is βDC, which is given as hFE. The 2N3904
shows a range of β’s on the data sheet from 100 to 300 for IC = 10
mA.
Characteristic
ON Characteristics
DC current g ain
( IC = 0.1 mA dc, VCE = 1.0 V dc)
Symbol
2N3903
2N3904
hFE
Min
Max
20
40
–
–
( IC = 1.0 mA dc, VCE = 1.0 V dc)
2N3903
2N3904
35
70
–
–
( IC = 10 mA dc, VCE = 1.0 V dc)
2N3903
2N3904
50
100
150
300
( IC = 50 mA dc, VCE = 1.0 V dc)
2N3903
2N3904
30
60
–
–
( IC = 100 mA dc, VCE = 1.0 V dc)
2N3903
2N3904
15
30
–
–
Unit
–
4. BJT AS AN AMPLIFIER
4. BJT AS AN AMPLIFIER
• Transistor amplify current
because I C   DC I B
• IB is very small, so IC ≈ IE.
• Amplification of a small ac
voltage by placing the ac
signal source in the base
circuit.
• Vin is superimposed on the
DC bias voltage VBB by
connecting them in series
with base resistor RB.
• Small changes in the base
current circuit causes large
changes in collector current
circuit.
4. BJT AS AN AMPLIFIER
Voltage gain:
r' e  internal ac emitter resistance
•Ac emitter current is Ie ≈ Ic = Vb / r’e.
•Ac collector voltage, Vc equals ac voltage drop across Rc.
•Since I c  Ie , ac collector voltage is
Vc  IeRC
Vc  IcRC
•Vb is considered as ac input voltage where Vb=Vin - IbRB. Vc as the transistor
ac output voltage. The ratio of Vc to Vb is ac voltage gain, Av of the circuit.
Vc
AV 
Vb
•Substituting IeRC for Vc and Ier’e for Vb, yields:
Vc IeRC
AV 

Vb Ier ' e
RC
AV 
r' e
5. BJT AS A SWITCH
5. BJT AS A SWITCH
A transistor when used as a switch is simply being biased so
that it is in:
1. cutoff (switched off)
2. saturation (switched on)
5. BJT AS A SWITCH
Conditions in Cutoff
VCE ( cutoff )  VCC
Neglect leakage current and all currents
are zero. BE junction is reverse biased.
Conditions in Saturation
VCC  VCE ( sat)
IC ( sat) 
RC
IB (min) 
IC ( sat)
DC
Since VCE(sat) is very small compared to
VCC, it can be neglected.
Example 3
a)
b)
c)
For the transistor circuit in below figure, what is
VCE when VIN=0v?
What minimum value of IB is required to saturate
this transistor if βDC is 200?
Calculate the maximum value of RB when VIN=5V.
Solution Example 3
a)
b)
When VIN=0V, the transistor is in cutoff (act as
open switch), so VCE(cutoff)=VCC = 10V.
Since VCE(sat) is neglected (assumed 0V),
I C ( sat )
VCC
10V


 10mA
RC
1.0k
I B (min) 
I C ( sat )
10mA

 50 A
200
 DC
This is the value of IB necessary to drive transistor to
point of saturation.
c) When transistor is ON, VBE=0.7V. The voltage across
RB is
VRB=VIN – VBE = 5 – 0.7 = 4.3V
By Ohm’s Law, the maximum value of RB is:
RB (max)
V RB
4.3


 86k
I B (min) 50
6. TROUBLESHOOTING
6. 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.
6. Troubleshooting
Possible faults are open bias resistors, open or resistive connections,
shorted connections and open or short internal to the transistor itself.
Voltage measurements that are typically
low are caused by a point that not
“electrically connected to ground”. This
called a floating point. This is typically
indicative of an open.
More in-depth discussion of typical
failures are discussed within the
textbook.
Correct voltage measurement
6. 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 high resistance.
6. 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.
6. Troubleshooting
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. Curve tracers give us even
more detailed information about a
transistors characteristics.
Summary
 The bipolar junction transistor (BJT) is constructed of
three regions: base, collector, and emitter.
 The BJT has two p-n 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 (transistor in active region).
 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.