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Module No.3
=
Transistors
Hi friends.In this module, y=
ou
will get the information about transistor, their types, configurations,
application circuits, characteristics etc. This information will help you =
to
know better about the solid-state technology. Technical details, operation,
applications of special purpose devices .
J. Bardeen and W. H. Brattain of bell telephone laboratories, U.S.A=
. in
1948, invent bipolar junction transistor. It is the development of electro=
nics
field that is replacing vacuum tubes by solid-state technology. Transistor=
is
nothing but a third doped element that is added to a crystal diode to form=
two
p-n junctions.
=
In
this Unit you will learn about Bipolar Junctions Transistor configurations,
Load line, Input – Output Characteristics, Amplification factor and
other technical aspects.
Transistors are developed in “P” and
“N” type semi- ducting
materials from the crystal with adding corresponding impurities. Transistors consists of two
p-n junctions formed =
by
sandwich pattern of either p-type or
n-type materials between a pair of opposite type semiconductor materials.
Advantages of Transistor over vacuum tubes:
=
According
to this theory of sandwiching the layers, there are two basic types of transistors i.e. N-P-N =
type
transistor, and P-N-P=
type
transistor.
The three terminals taken out from each section of (i.e. P-N-P or N-P-N) trans=
istors
are named according to
their actual actions.
![](3D"slide0007_image002.gif")
It is taken from the section of transistor from which injection of
electrons or holes is achieved. The name of this terminal itself indicates
that this terminal emits maximum number of electrons or holes. This termin=
al
is always kept is forward bias.
It is taken from another section of transistor, which provides base
support to transistor i.e. base is the section situated at the middle
position. It is of opposite type. It means for p-type emitter the base is
n-type and vice-versa.
It is taken from the third section of the transistor, which does the
process of collection of holes or electrons.
![](3D"slide0009_image004.gif")
The forward bias veb is applied to base-emitter junctio=
n of
the transistor and reverse bias vcb is applied to base-collector
junction.
=
Transistor
consists three layers of semi conducting material i.e. N-P-N. Forward bias=
(vbe=
)
is applied in between the emitters and base of the transistor.
The forward bias veb is applied across base-emitter jun=
ction
of the transistor and reverse bias vcb is applied across the base-collect=
or
junction.
This transistor consists of three layers as p-type,
n-type and again p-type semiconductor materials. P-region consists of holes
are majority carriers.
=
Transistor
is a three terminal device i.e. emitter base and collector. In any circuit,
there is a necessity of four terminals i.e. two for i.e. put and other two=
for
output.. Therefore transistor is connected in the circuit by means of three
ways:
Current amplification factor (a<=
/span>):
=
The
ratio of change in collector current to the change in emitter current at
constant collector-base voltage (vcb) is called as ‘current
amplification factor.
=
It
is the graphical representation of input currents and voltages. In cb mode the input is given to the
base-emitter junction=
.
Input resistance (i.e. resistance at
input stage of cb mode) is very small.
It is the ratio of change in base-emitter voltage (v=
be=
)
to the resulting change in emitter current (ie) at constant
collector-base voltage (vcb).
![](3D"slide0012_image006.gif")
=
The
ratio of change in emitter current (Die) to the change in base current at
constant (D=
ib<=
/span>)
is known as current amplification factor in common collector (cc mode)
=
;
![](3D"slide0013_image008.gif")
In transistor circuit, load line is used to
determine the collector current for various collector –emitter volta=
ges.
This load line analysis is quite easy and frequently used in analysis of
transistor applications.
The zero signal values of collector current
“ic” and collector-emitter voltage “vce” are
known as operating point. This point is also called silent point or Q-poin=
t.
![](3D"slide0014_image010.gif")
<=
/span>It
is the process of raising the strength of weak signal without any change in
its general shape. In =
order
to get faithful amplification, following conditions must be satisfied:
=
;
<=
/span>The
fulfillment of these conditions for faithful amplification is known as
“transistor biasing”. The proper flow zero signal
collectors’ current and the maintenance of proper collector-emitter
voltage during the passage of signal are known as “transistor
biasing”. The circuit, which provides biasing for the transistor, is
known as “biasing circuit.
During such changes, operating point also changes. The
process of making operating point independent of temperature changes or
variations in transistor parameter are known as stabilization.
Stabilization of the operating point of transistor is necessary due=
to:
The collector current is influenced by the rapid changes in collect=
or
leakage current because of temperature variations. During the biasing of
transistor, the zero signal collectors’ current is 1ma.
The self-destruction of an unstabilised transistor is
called thermal runway. Thermal runway, it is essential to stabilize the
operating point of the transistor by causing decrease in base current
automatically. Designing the suitable circuit does it.
The values of base-emitter voltage (vbe=
)
and base current amplification factor (b) are not same for any two transistors of sa=
me
type.
It
is the rate of change in collector current (ic) with respect to the collec=
tor
leakage current (ico) at the constant value of base current (ib<=
/span>)
and base current amplification factor (b).
1) =
Establish
the operating point at the center of the active region characteristics (faithful amplification).=
3;
In order to get requirements, various biasing
circuits are used as described below:
This biasing circuit uses only one resistor ‘rb<=
/span>”.
It is connected in bet=
ween
base and positive end of supply for the transistor.
In this type of circuit, ze=
ro
base signals are provided by the &nb=
sp;
“vccR=
21;.
It flows through resistor “rb”.
![](3D"slide0019_image012.gif")
In this type circuit, the resistor “re” is connec=
ted
in the emitter circuit during the temperature changes, this circuit makes
stabilization of operating point.
With increase in temperature, collector current and =
consequently emitter curren=
t will
increase. It increases voltage
drop across resistor “re”
In this type resistor “rb” is connected in between base
and collector.
In this circuit, the required zero signal base current is determine=
d by
collector-base voltage “vcb”.
1) This circuit provides negative feedback, which reduces
the gain of the amplifier.
In
this method, two resistors “r1” and “r2<=
/span>”
are connected across the supply voltage “vcc”
<=
/span>As
name indicates, voltage deviation is formed by two resistors “r1<=
/span>”
and “r2”. With selecting values of resistors, the operating p=
oint
of the transistor can be made independent of b (beta).
This is most widely method of providing bias and
stabilization to a transistor.
![](3D"slide0021_image014.gif")
<=
/span>These
amplifiers are most efficient among the three modes of transistor connecti=
ons.
CE mode amplifiers are mostly applicable in 95% of all transistor
applications. The main properties of CE mode are:
.During the positive half cycle of the a.c. Sig=
nal,
the base emitter junction is in forward bias and also vbb keeps base-emitter junction in for=
ward
bias. During negative half cycle of the a.c. Signal the opposite voltage t=
o vbb=
is present. It decreases forward b=
ias
voltage.
=
The
action of CE transistor amplifier is analyzed by two ways i.e. D.C. Analys=
is
and an A.C. Analysis. In the
transistor amplifier both the D.C. and A.C. Conditions are considered beca=
use
of their effect in the working of amplifier circuits.
<=
/span>In
this type, CE amplifier circuit is analyzed under the D.C. Conditions. It
means no A.C. Signal is applied at the input.
=
In
this type, CE amplifier circuit is analyzed under the A.C. Conditions only.
The capacitors are generally to bypass the A.C. Signal.
![](3D"slide0023_image016.gif")
Output is taken across collector terminal is CE mode
connection and hence it is necessary to analyze the collector current in
following conditions:
Only when positive cycle is applied at input terminal, the forward =
bias
is increased.
<=
/span>When
negative cycle of A.C. Signal is applied at input terminal, the forward bi=
as
is decreased.
![](3D"slide0025_image018.gif")
Transistor amplifier and its performance is che=
cked
by knowing the important terms as stated below:
It is the ratio of small ch=
ange
in base-emitter voltage to the resulting change in base current at the
constant collector-emitter voltage.
It is the ratio of change in collector emitter voltage to the resulting change in col=
lector
current at constant b=
ase
current. The output resistance is vary high because the output junction is always reverse biased
=
Output resistance of transistor amplifier is in several hundred kilo-ohms.
Effective
collector load:
It
the total load on the A.C. Collector current.
To study the collector load, it is necessary to output stage of the CE mode transistor
amplifier.
![](3D"slide0027_image020.gif")
<=
/span>We
consider the CE mode transistor amplifier circuit. In transistor amplifier,
maximum power transfer to the output device is achieved by matching the
impedance of power source and impedance of load. If the input and output
impedance are not equal, we cannot get good results from an amplifier.
=
;
<=
/span>Output
stage of the transistor amplifier is having transformer as coupling device=
.
This unit will give you deta=
ils
regarding Junction Field Effect Transistors (JFET). They give high input
impedance and low noise level. Hence they are better than the BJTs The FET=
is
three terminal and unidirectional semiconductor device in which current
conduction is by one type of carrier either by electrons or holes. JFET are
widely used in many application circuits
Field Effect Transistor is a majority-carrier device=
in
which the resistance of a current path, or channel, from source to drain
electrodes is modulated by the voltage applied to the gate.
Drain and source terminals are taken out from n-type
substrate and the gate terminal is taken out by making short two p-type
substrates. For n-channel FET, electrons are the majority carriers and hol=
es
are the minority carriers
For p-channel FET, holes are the majority carriers and electrons are
the minority carriers.
=
In
symbols, drain and source terminals are symmetrical because both terminals=
are
taken out from the same substrate and hence they are interchangeable.
=
<=
/span>Switch
s1 is connected in the gate circuit o=
f FET.
When it is at off condition, the gate terminal is not having any kind of
potential.
When switch ‘s1” is closed, reverse bias is applied acr=
oss
gate and source junction.
![](3D"slide0030_image022.gif")
<=
/span>In
curve plotted in between drain current (Ib) and drain-source voltage (vds=
)
is called output characteristics of the FET.
These are useful to determine the performance of the FET circuit. T=
here
are three important parameters of the FET:
ii) =
Trans
conductance
iii) Amplification
factor.
It is the ratio of change in drain-source voltage (Dvds=
)
to the change in drain current (Did) at constant gate-source voltage.
=
It
is the ratio of change in drain current (Did) to the change in gate-source voltage (=3DDvds=
)
at constant drain-sou=
rce
voltage.
It is the ratio of change in drain-source voltage (<=
span
style=3D'font-family:Symbol;mso-char-type:symbol'>Dvds=
)
to the change in
gate-source volgage (Dvgs=
)
at the constant value=
of
drain current.
It has a very high input impedance
(Input impedance is about 100mW). It permits high degree of isolation between the input a=
nd
output circuit.
The operation of FET depends upon the current carrie=
s. They do not cross-junctions.
There fore the inherent noise of
valves and junction transistors is not present in JFET’s.
Junction field effect transistor raises the strength of weak signal=
and
thus acts as an amplifier.
The basic circuit of the CS amplifier using JFET.
P-channel FET is used for the amplifier circuit. In this amplifier, gate is
kept at positive with respect to source.
![](3D"slide0033_image024.gif")
![](3D"slide0033_image026.gif")
. In this unit we will concentrate on the
semiconductor devices that are used in the special purpose circuits i.e. U=
ni
– Junction Transistor (UJT), Silicon Controlled Rectifier (SCR), Diac
and Traic. These devices are used in the control circuits, power electroni=
cs
and even in power rectifier circuits.
Basic Characteristics of a UJT:
The basic characteristics of UJT are considered with th= e parameters. These parameters are:
1 Vbb(max): The
maximum inter base voltage that can be
applied to the UJT.The basic characteristics of UJT are considered with th= e parameters. These parameters are:
2. Rbb:
3. n: The intrinsic standoff ratio which defines Vp.
4. Ip: Th= e peak point emitter current.
Silicon Controlled Rectifier (SCR)<=
b>
It is four layer and three terminal semiconductor device. The gate =
current
controls its operation.
If we ground both the cathode and the gate, and
apply a positive voltage to the anode, no current will flow through this
device. This is in keeping with the basic four-layer diode.
SCR
is used in the number of applications i.e. in speed control of Motors, pow=
er
control and in rectifier circuits etc.
The Diac is a bi-directional trigger diode, which is designed to trigger S=
CR.
Basically the Diac does not conduct until the break over voltage is reache=
d.
The main characteristics of the Diac are studied with the following terms:=
1. Break over volt= age 2.Voltage symmet
3. Break back voltag 4.Break over current
5. Power dissipation&#= 13;
1. Break over volt= age 2.Voltage symmet
3. Break back voltag
5. Power dissipation&#= 13;
Triac
It is a two terminal semiconductor device. It is used for controlling current
in either direction.
Characteristics of the Triac are
considered in to the terms given below:
1. Forward and reverse break ov= er voltage.
2. Maximum current
3. Minimum holding current
4. Gate voltage and gate current trigger requirements.
5. Switching speed
1. Forward and reverse break ov= er voltage.
2. Maximum current
3. Minimum holding current
4. Gate voltage and gate current trigger requirements.
5. Switching speed
![](3D"slide0037_image028.jpg")
1. =
TRIACs
are usually seen in simple, low-power applications. They are used in house=
hold
dimmer switches.
A simple lamp dimmer circuit with complete the
phase-shifting resistor-capacitor network, which is necessary for after-pe=
ak
firing.
![](3D"slide0038_image030.jpg")