4. Transistor Characteristics Theo ry Support
Electronics - Diodes and Transistors
Transistor Characteristics
1 of 15
Topics covered in this presentation:
Transistor Construction
Transistor Operation
Transistor Characteristics
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Guidelines and tips
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Transistor Characteristics and Biasing Circuits, Lecture notes of Digital & Analog Electronics
The characteristics of bipolar junction transistors (BJTs), including their input and output characteristics, transfer characteristic, and current gain. It also covers the design of biasing circuits to achieve a stable operating point for the BJTs. Examples of common-emitter and common-collector amplifiers are provided.
Typology: Lecture notes
2020/2021
1 / 70
This page cannot be seen from the preview
Don't miss anything!
Related documents
Partial preview of the text
Download Transistor Characteristics and Biasing Circuits and more Lecture notes Digital & Analog Electronics in PDF only on Docsity!
Electronics -
Diodes a n d Transistors
Transistor Characteristics
Topics covered in this presentation:
Transistor Construction
Transistor Operation
Transistor Characteristics
Electronics -
Diodes a n d Transistors
The Transistor
The transistor is a semiconductor device that revolutionised electronics
in the 1950s and 60s.
Although individual transistors have been replaced in many
applications, transistors are still used in power gain applications.
They can be found in driver circuits where the signal from an input
or a process is not powerful enough to drive the output device.
There are many different types of transistors, but in this assignment we
will examine only the bipolar junction transistor.
Today, transistors are used in integrated devices where several
thousand transistors are formed in a single device.
Electronics -
Diodes a n d Transistors
The three layers are called the base, emitter, and collector.
Base (B) - The base is the middle layer
of semiconductor material and controls
the flow of charge carriers between the
emitter and the collector.
Emitter (E) - The emitter region is
heavily doped with charge carriers
that can be emitted into the base.
Collector (C) - The collector is lightly
doped so that it can collect charge
carriers from the base region.
Bipolar Junction Transistor
Electronics -
Diodes a n d Transistors
Bipolar Junction Transistor
Each type of bipolar junction
transistor has its own circuit
symbol showing the base,
emitter and collector.
The arrow on the symbol always
appears on the emitter terminal
and points from P to N.
For an NPN transistor, the arrow
on the emitter will point outwards.
Electronics -
Diodes a n d Transistors
NPN Transistor Biasing
For an NPN transistor, to allow the
electrons in the emitter to move to
the base, the voltage at the base
must typically be 0.6V higher than
the voltage at the emitter.
To allow these electrons to
continue on to the collector, the
collector voltage must be higher
than the base voltage.
Although some of the electrons
will exit the transistor at the base,
the majority of them will continue
onto the collector.
Electronics -
Diodes a n d Transistors
Transistor Characteristics
When selecting a transistor for an application, there are three
main characteristics that must be taken into account. These
are generally plotted as graphs.
Input Characteristic
- Base current (I B
) against base-emitter voltage (V BE
Output Characteristic
- Collector current (I C
) against collector-emitter voltage (V CE
Transfer Characteristic
- Collector current (I C
) against base current (I B
Electronics -
Diodes a n d Transistors
Output Characteristic
The output characteristic is a plot of collector current (I C
) against
collector-emitter voltage (V CE
) for a range of base currents.
The graph shows a series of plots,
one for each base current value.
From the graph it can be seen
that initially the collector current
varies greatly for small change
to the collector emitter voltage.
At this point the transistor is not
very stable.
Electronics -
Diodes a n d Transistors
Output Characteristic
However, above a certain value the collector-emitter voltage has little
effect on the collector current (for a given base current) and the
transistor becomes more stable.
The transistor is generally
operated in this stable region, and
the output characteristic graph
shows what minimum value of
collector-emitter voltage must be
applied to keep it operating in the
stable region.
Electronics -
Diodes a n d Transistors
Transistor Datasheet
Some of the criteria, found on the product data sheet,
to consider when selecting a transistor.
- DC current gain
- Collector-emitter breakdown voltage
- Collector-emitter saturation voltage
- Base-emitter ‘on’ voltage
- Package Type
Electronics -
Diodes a n d Transistors
Transistor as a Switch
If the base-emitter voltage is below
0.6V, the transistor will not conduct,
resulting in no base current and no
collector current.
A base current is required to
obtain a collector current.
By switching the base current on
and off, the collector current can
also be switched on and off.
The transistor can therefore be
used as a switch with the base
current controlling the flow of the
collector current.
4. Bipolar Junction Transistors
- Bipolar Junction Transistors TLT-8016 Basic Analog Circuits 2005/ 1
4.1 Basic Operation of the npn Bipolar Junction Transistor
npn BJT consists of thin p-type layer between
two n-type layers;
Layers: emitter, base, collector;
Two interacting pn junctions: emitter-base and
base-collector;
Emitter region is doped very heavily, compared
with the base region
- Bipolar Junction Transistors TLT-8016 Basic Analog Circuits 2005/ 2
Figure 4.1 The npn BJT.
First - Order Common - Emitter Characteristics
- Bipolar Junction Transistors TLT-8016 Basic Analog Circuits 2005/ 4
i B
- common-emitter current gain.
Typically = 10 .. 1000
Figure 4.4 Common-emitter characteristics of a typical npn BJT.
Amplification by the BJT
In Figure 4.4: If i B
= 30A, i C
= 3mA – 100 times
more
i C
Factors Affecting the Current Gain
- Doping of the emitter area compared with the
base area
- Base region should be thin;
- Geometry of the device
Device Equations
i B
1 i E
T
B ES
i 1 I
exp
(^) v BE
V
i E
i C
i B
i C
i E
- common-base current gain.
Typically = 0.9 .. 0.
i C
i
B
^ T^
C ES
V
v BE
i I exp
i B
i C
s ES
I I
- Bipolar Junction Transistors TLT-8016 Basic Analog Circuits 2005/ 5
T
s
V
v
BE
i
C
I exp
I
s
- scale current
1