PRESENTED BY: MD. NASIR IBNE REZA. Student ID: 090914.
Discipline : E.C.E. Session : 2008- 09KHULNA UNIVERSITY Electronics
an d Communication Engineering Discipline Khulna 9208
WELCOME TO MY PRESENTATION ON BIPOLAR JUNCTION TRANSISTOR
TOPICS OF MY PRESENTATION:PRESENTATION: IntroductionTRANSISTOR
OPERATIONCOMMON BASE CONFIGARATION
Common-Emitter configuration
Current Amplification Factor ( ) Current gain Factor (
)Relationship between E andFIXED-BIAS CIRCUIT Forward Bias of Base
Emitter
Collector Emitter Loop Q point Load line analysisEMITTER BIAS
CIRCUIT
Base Emitter LoopCollector Emitter Loop
BIPOLAR JUNCTION TRANSISTOR
:-
Bipolar junction transistor(bjt) transistor(bjt) is a special
kind of transistor which has three trerminal. The three terminals
are emitter ,base and collector.Bjt is of two kinds.They
are:1.P-N-P transistor:A p-n-p transistor is made of two p-type
diodes and one n-type diode .The n-type diode is kept between
p-type diodes as like as a sandwitch. 2.N-P-N transistor:A n-p-n
transistor is made of two n-type diodes and one p-type diode.The
p-type diode is kept between n-type diodes as like as a sandwitch.
The three terminals of the BJT are called the Base (B), the
Collector (C) and the Emitter (E).
THE OPERATION OF A TRANSISTOR:TRANSISTOR:The bipolar junction
transistor is of two kinds,but their processing of operation are
mostly same.For discussing the operation of a transistor there the
operation of a p-n-p transistor is discussed below:pbelow:-
Fig.2:The operation process of a p-n-p transistor.
A p-n-p transistor is made of two pieces of p-type diodes and
one pieces of n-type diode in middle of them.Among them the emitter
is heavily doped,collecter is doped mediumly and the base is
lightly doped.There are two junctions in a transistor :
(a).Emitter-base junction:is made by the emitter and
base.(b).Collector-base junction:is made by the collector and base.
In fig.2. we can see that the emitter-base junction is in forward
bias.For this the holes in emitter will go to the base .Among them
some of the hole will be filled up by electron. And the
collector-base junction is in reverse bias.For this collector will
attract the hole in base which came from the emitter .thus the
collector will get a huge flow of hole .And the emitter will get a
huge flow of current. There is another flow of current created by
minority carriers electrons.If Ie ,Ib and Ic be the emitter ,base
and collector current respectively then
Ie= Ic + Ib.We can also write
I= Imajority + Iminority.
REGIONS OF OPERATION:-
The emitter-base junction is forward biased and the
base-collector junction is reverse biased. Most bipolar transistors
are designed to afford the greatest common-emitter current gain, f
in forward-active mode. If this is the case, the collectoremitter
current is approximately proportional to the base current, but many
times larger, for small base current variations.
Forward-active (or simply, active):
Reverse-active (or inverse-active or inverted):By reversing the
biasing conditions of the forward-active region, a bipolar
transistor goes into reverse-active mode. In this mode, the emitter
and collector regions switch roles.
Saturation:-
With both junctions forward-biased, a BJT is in saturation mode
and facilitates high current conduction from the emitter to the
collector. This mode corresponds to a logical "on", or a closed
switch
Cutoff:In cutoff, biasing conditions opposite of saturation
(both junctions reverse biased) are present.There is very little
current flow, which corresponds to a logical "off", or an open
switch.The bipolar transistor is widely used for amplification and
processing of various kind of signals.In case of a bjt the output
depend on the current cotroller. We can distinguish three ways in
which the transistor may be configured: (a) with emitter current
controlling collector current, (b) with base current controlling
collector current, (c) with base current controlling emitter
current. It is significant that in all these modes of use,
operation of the transistor is given in terms of input and output
current. This is an inevitable consequence of the physics of the
bipolar transistor: such transistors are current-controlled
devices: by contrast field-effect transistors are voltagecontrolled
devices. Corresponding to the three modes of operation listed above
there are three fundamental transistor amplifying circuits: these
are shown in Fig. 3.01. At signal frequencies the impedance of the
collector voltage supply is assumed negligibly small and thus we
can say for circuit (a) that the input is applied between emitter
and base and that the output is effectively generated between
collector and base. Thus the base connection is common to the input
and output circuits: this amplifier is therefore known as the
common-base type.
THE CONFIGURATION OF TRANSISTOR:-
Fig.3.01.:-Three types of transistor config.:(a)common
base,(b)common emitter,(c)common collector configuratoin
In (b) the input is again applied between base and emitter but
the output is effectively generated between collector and emitter.
This is therefore the common-emitter amplifier, probably the most
used of all transistor amplifying circuits In (c) the input is
effectively between base and collector, the output being generated
between emitter and collector. This is the commoncollectorcircuit
but it is better known as the emitter follower
Alpha:In a common-base amplifier the ratio of a small change in
collector current ic to the small change in emitter current ie
which gives rise to it is known as the current amplification
factor. It is measured with short circuited output. Thus we have
(1.01) Here is current amplification .As ic is very nearly equal to
ie. Thus is nearly equal to unity and is seldom less than 0.95. In
approximate calculations is often taken as unity.
Beta:In a common-emitter amplifier the ratio of a small change
in collector current ic to the small change in base current ib
which gives rise to it is represented by which is called current
gain factor . It is also measured with short-circuited output and
indicates the maximum possible current gain of the transistor. Thus
(1.02) The values of
lie between 20 and 500
THE RELATION BETWEEN
AND :We know
ie = ic + ib
(1.03) and thus
From Eqns1.01, 1.02 and 1.03 we can deduce a relationship
between
This is the relationship between
and
.
THE CHARACTERISTICS GRAPH OF TRANSISTOR:TRANSISTOR:THE
CHARACTERISTICS GRAPH OF COMMON BASE TRANSISTOR:
Graph.1.01:The characteristics graph of common base
transistor
THE CHARECTERISTICS GRAPH OF COMMON EMITTER TRANSISTOR:-
Gra h1. 2:Th charact ristics gra h f c
itt r tra sist r
OPERATING POINT:For transistors amplifires the resulting dc
current and voltage establish an operating point on the
characteristics curve that define the region will be employed for
amplification of the applied signal.Since the operating point is a
fixed point on the characteristics curve ,it is also called
quiescent point in short term Q-point.There a characteristics
curves with Q-point in different conditions has given below:Various
operating points within the limits of operation of a transistorIC(m
) PCmax ICmax 18 15 12 Saturation 9 6 3 C B IB=60 u IB=50 u IB=40 u
IB=30 u IB=20 u IB=10 u IB=0 u VCE(V) VCEsat 10 20 Cutoff 30 40
VCEmax
Graph.2.01:Characteristics curve of bjt.
In the above graph ,when ib=0,then the transistor is in cutoff
region where it is not in condition of operation.In that condition
the Q-point is C.In the Q-point B the transistor is in best mode
for operation.Here the saturation area is also pointed on the
graph.
FIXED BIAS CIRCUIT:-
Applying KVL in the base-emitter loop, Vcc-IBRB-VBE=0
IB=(VCC-VBE)/RB IC= IB= (VCC-VBE)/RB Applying KVL in the
collector-emitter loop, Vcc-IcRc-VCE=0
VCE=Vcc-IcRc
VOLTAGE DIVIDER BIAS:-
Thevenin s Equivalent Circuit for the base-emitter loop
RTH
VR2=ETH
BASE EMITTER LOOP:-
COLLECTOR EMITTER LOOP:-
BIAS STABLIZATION:Where,
If RTh
