INTRODUCTION TO BIPOLAR
JUNCTION TRANSISTORS
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Transistors are three port devices used in most
integrated circuits, such as amplifiers
Transistors are active components (recall that
resistors, capacitors and inductors are passive)
There are two types of transistor:
1. The unipolar or field effect transistor (FET), its
operation is due to the flow of majority carriers only
(either electrons or holes)
2. The bipolar junction transistor, its operation
depends on the flow of majority and minority carriers
The bipolar junction transistor is one of the most
important and widely used semiconductor devices
Its principal applications are as an amplifier or as a
switch
Generally transistors are highly efficient, robust,
incredibly reliable and usually inexpensive
10. Bipolar Junction
Transistor
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GENERAL CONSTRUCTION
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A bipolar junction transistor consists of two pn junctions
in close proximity to each other
Two arrangements are possible – according to whether
the middle region is n- or p-type material
npn or pnp
Leads are attached to the three regions, known as
emitter, base, and collector
Emitter
Collector
p
n
p
Base
Emitter
Collector
n
p
n
Base
10. Bipolar Junction
Transistor
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THEORY OF OPERATION (1)
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The majority of current in a pnp transistor is carried by
holes which, being positively charged particles, move in
the same direction as conventional current flow
In an npn transistor, the majority of current is carried by
electrons
The theory of operation of an npn transistor is in all
respects the same as that of a pnp device
The theory of operation for pnp transistors will be
considered here
Transistor circuit symbols are shown below
The arrow on the emitter indicates the direction of
conventional current flow under normal bias conditions
Collector
npn
base
Emitter
Collector
base
pnp
Emitter
10. Bipolar Junction
Transistor
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THEORY OF OPERATION (2)
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An amplifier is a device that boosts the power of a
signal, whilst keeping its waveform the same
Transistors are used in amplification circuits
In normal use as an amplifier, the pn junction between
collector and base is reversed biased
The junction between emitter and base is forward
biased
The operation of the transistor is also dependent on the
width of the material used to construct the base region
The effect or reducing the base width is to increase the
collector current, IC, whilst correspondingly reducing the
base current, IB
The emitter current, IE, is highly dependent on the
base-emitter voltage, VBE
The collector current, IC, is more or less independent of
the collector-base voltage VCB
10. Bipolar Junction
Transistor
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THEORY OF OPERATION (3)
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A transistor may be thought of as an electronic tap
that is able to control a large flow of electrons with only
small variations of the handle
The handle in the case of a transistor is called the base
The in and out pipes are called the emitter and
collector
Voltage changes at the base of the transistor result in
changes to the flow of current through the transistor
In practical terms, a small base current, IB, flows and
turns on a much larger collector current, IC
IC is zero until IB flows
A junction transistor is thus a current operated device
10. Bipolar Junction
Transistor
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THE TRANSISTOR AS AN
AMPLIFIER (1)
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There are three ways to connect a transistor for
amplification purposes in practice
In each case one terminal is connected to signal
common (ground) – though not always directly
As a result, the input and output signals are taken
between common and the other two terminals
The three amplifier configurations are: common
emitter, common collector and common base
Here we shall only consider the common emitter
Typical circuit (npn) and values shown below
10. Bipolar Junction
Transistor
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THE TRANSISTOR AS AN
AMPLIFIER (2)
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Collector typically fed from approx. 12V DC supply
Fairly high resistor, RC, placed in series (4.7kΩ)
A further DC supply is also connected between base
and emitter – in series with input signal Vin
DC base bias supply is arranged to make IC about 1mA
The collector-emitter voltage, VCE, is then the collector
supply voltage less the voltage drop across RC
For silicon transistors, the base bias voltage is about
0.6V, with IB about 10µA
10. Bipolar Junction
Transistor
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TYPICAL OPERATION OF
JUNCTION TRANSISTOR
AMPLIFIER
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Typically the collector current, IC, is 100 to 1000 times
greater than the base current IB, depending on the type
of transistor used
If we take the base current IB as an input and the
collector current IC as an output, then we can say that
the transistor amplifies current, i.e. it is a current
amplifier
The current gain is denoted by hFE
F indicates forward current and E denotes the
connection type which is common emitter
hFE = IC / IB
For the circuit shown on the previous slide, IC = 1mA
and IB = 10µA
Therefore hFE = 1mA/10µA = 100
The input current is amplified by a 100 times in this
example
Also note that since the current flowing out of the
transistor must equal the current flowing into it
(Kirchhoff’s Current Law) then IB + IC = IE
Also IC >> IB and IE ≈ IC
10. Bipolar Junction
Transistor
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CONVENTIONAL AMPLIFIER
CIRCUIT SET UP
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Above shows the most common configuration for a
stable operating amplifier
VCC is the power supply voltage, i.e. the circuit needs
power in order to perform its function (amplification)
Such circuits have to be biased in order to give the
required value of IC
In order to help achieve this, a voltage VBB, is set up
using a voltage divider, which has an effect on the size
of IC
10. Bipolar Junction
Transistor
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IMPORTANT AMPLIFIER CIRCUIT
RELATIONSHIPS
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VBB = VBE + VRE
In practice VRE > VBE. For silicon transistors, VRE ≈ 2V
R1 and R2 chosen so that base-emitter junction is
forward biased – resistors form a potential divider
For good stability, R2 < 10RE
Typical values for current gain hFE are around 100
As an approximation VBB = (VCCR2)/(R1 + R2)
VRE = IERE
Since IB << IC and IC ≈ IE, then IC ≈ VRE/RE
Since VRE = VBB - VBE
Then
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IC =
RE
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 VCC R2
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− VBE 
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 R1 + R2

VBE is approximately 0.7V
RC is chosen so that VCE is VCC/2 to ensure that the
amplifier output does not exceed the boundary limits of
-VCC/2 to VCC/2
VCE = VCC – ICRC - ICRE
10. Bipolar Junction
Transistor
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WORKED AMPLIFIER EXAMPLE
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Given that hFE = 150, VBE = 0.7V and VCC = 15V, find
R1, R2, RC and RE (i.e. bias the amplifier circuit) so
that IC is 1mA.
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hFE = IC / IB → IB = 1×10-3/150 = 6.67×10-6A
IE ≈ IC ≈ 1×10-3A
For silicon transistors VRE ≈ 2V
VBB = VBE + VRE = 2.7V
VRE = IERE → RE = 2kΩ
R2 < 10RE → R2 = 20kΩ
VBB = (VCCR2)/(R1 + R2) → R1 = 91.1kΩ
VCE = VCC/2 = 7.5V
VCE = VCC – ICRC – ICRE → RC = 9.5kΩ
10. Bipolar Junction
Transistor
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APPLYING AN AC INPUT SIGNAL
TO THE AMPLIFIER
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Coupling capacitors C1 and C2 are added.
C1 protects the base from being short circuited by the
source.
C2 blocks any DC component at the output.
The AC input signal, VS, is superimposed on the base
bias voltage, so that the new base emitter voltage
becomes vbe = VBE + VS
The output voltage Vout = VCC/2 + AVS
A is the overall gain of the amplifier
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Transistor
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TRANSISTOR AS A SWITCH
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So far we have looked at transistors as amplifying
devices. However another important application is their
use as a switch.
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Advantages
Very high speed of operation – present day transistor
can operate at up to 109 times a second
Very high reliability
Electronically controlled operation
Low cost
Small size
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Disadvantages
The switch is not a true open circuit in the OFF
condition – a small but finite current still flows
The switch is not a true short circuit in the ON condition
– there is a small but finite voltage drop across it of
about 0.1V
10. Bipolar Junction
Transistor
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PRACTICALITY OF TRANSISTOR
SWITCHING OPERATION
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In many practical cases, the disadvantages are of little
significance
In many applications the use if transistor switches
provides great improvement in operation over
alternative methods such as relays and other
mechanical switches
One of the most important applications is in the control
of logic levels in digital circuits
Modern digital computers rely almost exclusively for
their operation on the use of transistor switches
The operation of a transistor as a switch is the basics of
switching (digital) circuits
In digital circuits, the outputs and inputs involve only
two levels of voltage – HIGH or LOW – i.e. two state
circuits
HIGH logic level is referred to as logic 1 level, which in
voltage terms is 5V or near the supply voltage value
LOW is referred to as logic 0
Transistors are cheap, reliable, there are no moving
parts with almost indefinite life and can switch millions
of times a second
A perfect switch would have zero resistance when ON
and infinite resistance when OFF with no power
consumption
10. Bipolar Junction
Transistor
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TRANSISTOR SWITCHING CIRCUIT
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When transistor is switched on IC = VCC/RC
We already know that IC = hFE * IB
Substituting: IB = VCC/hFERC
This value of IB is the minimum for satisfactory
switching
Also, IB = (VBB – VBE)/RB
However, in practice, VBB >> VBE
So IB ≈ VBB/RB
10. Bipolar Junction
Transistor
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