Chapter 3. Bipolar Junction
Transistors
Outline:
 Fundamental of Transistor
 Common-Base Configuration
 Common-Emitter Configuration
 Common-Collector Configuration
Bipolar Junction Transistors
Introduction
 The transistor is a three-layer semiconductor
device consisting of two n- and one p-type
layers of material or two p- and one n-type
layers of material.
 The former is called an npn transistor, and
the latter is called a pnp transistor.
Bipolar Junction Transistors
The terminals have been indicated by E for
emitter, C for collector and B for Base.
They have different thickness and degrees of
doping.
This three-terminal device is often referred to
as bipolar junction transistor.
The term bipolar reflects the fact that holes
and electrons involve in the current flow.
Bipolar Junction Transistors
Figure: Types of Transistor
Bipolar Junction Transistors
Advantages over Vacuum Tubes
Semiconductor devices:
 of smaller size, lightweight, rugged, efficient
 no warm-up period
 no heater requirement
 lower operating voltages.
Bipolar Junction Transistors
Transistor Operation
The biasing of the two types of transistor has
been illustrated in the figures.
The operation of npn and pnp transistors are
same if the roles played by the electrons and
holes are interchanged.
One p-n junction of a transistor is reversebiased, whereas the other is forward-biased.
Bipolar Junction Transistors
Just like the role played by external potential
to diode, the applied voltages influence the
depletion regions of the transistors.
Thus the flow of holes and electrons has been
manipulated.
So for both types of transistors, we obtain:
IE = IC +IB
Bipolar Junction Transistors
IE
IB
IC
Figure: Biasing of pnp transistor
Bipolar Junction Transistors
IE
IB
IC
Figure: Biasing of npn transistor
Bipolar Junction Transistors
Common-Base Configuration
The common-base configuration with pnp
and npn transistors are shown in the figures.
The term common-base is derived from the
fact that the base is common to both the input
and output sides of the configuration.
The arrow in the symbol defines the direction
of emitter current through the device.
Bipolar Junction Transistors
The applied biasing are such as to establish
current in the direction indicated for each
branch.
That is, direction of IE is the same as the
polarity of VEE and IC to VCC .
Also, the equation IE = IC + IB still holds.
Bipolar Junction Transistors
IE
IC
IB
IE
IC
IB
Figure: Common-base configuration of
pnp transistor
Bipolar Junction Transistors
IC
IE
IB
IE
IC
IB
Figure: Common-base configuration of
npn transistor
Bipolar Junction Transistors
Input characteristics
The driving point or input parameters are
shown in the figure.
An input current (IE) is a function of an
input voltage (VBE) for various of output
voltage (VCB ).
This closely resembles the characteristics of
a diode.
Bipolar Junction Transistors
As an approximation, the change due to
changes in VCB can be ignored.
The characteristics can be shown in orange
curve.
If piecewise-linear approach is applied, the
blue curve is obtain.
Furthermore, ignoring the slop of the curve
and the resistance results the red curve.
Bipolar Junction Transistors
It is this red curve that is used in the dc
analysis of transistors.
Once a transistor is in “on” state, the b-e
voltage is assumed to be 0.7V.
And the emitter current may be at any level
as controlled by the external network.
Bipolar Junction Transistors
Figure: Input characteristics for common-base
transistor
Bipolar Junction Transistors
Figure: Equivalent model for b-e junction
Bipolar Junction Transistors
Output characteristics
The output set relates an output current (IC)
to an output voltage (VCB) for various of
level of input current (IE ).
There are three regions of interest:
 Active region
In the active region, the b-e junction is
forward-biased, whereas the c-b junction
is reverse-biased.
Bipolar Junction Transistors
The active region is the region normally
employed for linear amplifier.
Also, in this region,
I C  IE
 Cutoff region
The cutoff region is defined as that region
where the collector current is 0A.
Bipolar Junction Transistors
In the cutoff region, the b-e and c-b junctions
of a transistor are both reverse-biased.
 Saturation region:
It is defined as that region of the
characteristics to the left of VCB= 0 V.
In saturation region, the b-e and c-b
junctions of a transistor are both forwardbiased.
Bipolar Junction Transistors
Active Region
Saturation
Region
Cutoff
Region
Figure: Output characteristics for common-base
transistor
Bipolar Junction Transistors
Alpha (α)
In the dc mode, the levels of IC and IE at the
operation point are related by:
αdc = IC / IE
Normally, α  1.
For practical devices, α is typically from 0.9
to 0.998.
Bipolar Junction Transistors
Common-Emitter Configuration
The common-emitter configuration with npn
and pnp transistors are shown in the figures.
The term common-emitter is derived from the
fact that the emitter is reference to both the
input and output terminals.
The current relations are still applicable, i.e.,
IE = IC + IB and IC =α IE
Bipolar Junction Transistors
Figure: Common-emitter configuration
of npn transistor
Bipolar Junction Transistors
Figure: Common-emitter configuration
of pnp transistor
Bipolar Junction Transistors
Input characteristics
An input current (IB) is a function of an
input voltage (VBE) for various of output
voltage (VCE ).
The characteristics of the input or baseemitter circuit is shown in the figure.
The magnitude of IB is in μA and not as
horizontal as IE in common-base circuit.
Bipolar Junction Transistors
Figure: Input characteristics for common-emitter
transistor
Bipolar Junction Transistors
Output characteristics
The output set relates an output current (IC)
to an output voltage (VCE) for various of
level of input current (IB ).
There are three portions as shown:
 Active region
The active region, located at upper-right
quadrant, has the greatest linearity.
Bipolar Junction Transistors
The curve for IB are nearly straight and
equally spaced.
In active region, the b-e junction is
forward-biased, whereas the c-b junction
is reverse-biased.
The active region can be employed for
voltage, current or power amplification.
Bipolar Junction Transistors
 Cutoff region
The region below IB = 0μA is defined as
cutoff region.
For linear amplification, cutoff region
should be avoided.
 Saturation region:
The small portion near the ordinate, is
the saturation region, which should be
avoided for linear application.
Bipolar Junction Transistors
Active Region
Saturation
Region
Cutoff
Region
Figure: Output characteristics for common-emitter
transistor
Bipolar Junction Transistors
Beta (β)
In the dc mode, the levels of IC and IB at the
operation point are related by:
βdc = IC / IB
Normally, β ranges from 50 to 400.
For ac situations, β is defined as
I C
 ac 
I B V
CE  constant
Bipolar Junction Transistors
Biasing
The proper biasing is essential to place the
device in the active region.
A common-emitter amplifier of a pnp
transistor is shown in the figure.
1. The first step is to indicate the direction
of IE as established by the arrow in the
transistor symbol.
Bipolar Junction Transistors
2. The other current , IB and IC , are
introduced, satisfying IC + IB = IE .
3. The supplies are introduced with
polarities that will support the resulting
directions of IB and IC .
4. If the transistor is a npn transistor, all
the current and polarities would be
reversed.
Bipolar Junction Transistors
Figure: Biasing for common-emitter pnp transistor
Bipolar Junction Transistors
Common-Collector Configuration
The common-collector configuration with
npn and pnp transistors are shown in the
figures.
It is used primarily for impedance-matching
purpose since it has a high input impedance
and low output impedance.
Bipolar Junction Transistors
The load resistor is connected from emitter
to ground.
The collector is tied to ground and the
circuit resembles common-emitter circuit.
The output set relates an output current (IE)
to an output voltage (VCE) for various of
level of input current (IB ).
Bipolar Junction Transistors
This is almost the same as the output
characteristics of common-emitter circuit,
which are the relations between IC and VCE
for various of level of input current IB.
Since that: I E  IC .
The input characteristic of commonemitter are sufficient for requirement of
common-collector circuit.
Bipolar Junction Transistors
Figure: Common-collector configuration of
npn transistor
Bipolar Junction Transistors
Figure: Common- collector configuration
of pnp transistor
Bipolar Junction Transistors
Figure: Common-collector circuit used for
impedance-matching purpose
Bipolar Junction Transistors
Summary of Chapter 3
• Three-terminal devices, transistor
• Three types of configurations:
common-base, common-emitter and
common-collector.
• Proper biasing of the three configurations.
• Input and output characteristics of the
three configurations.
Bipolar Junction Transistors