1. What is a BJT? What are its two general purposes?
BJT is a three semiconductor device which can act as a conductor or insulator based on the
applied input signal. The two general purpose of this transistor is that it can be used as a switch
in the digital electronics or it can be used as an amplifier in the analog electronics.
2. How are BJTs constructed? What does "weight of doping" have to do with BJTs?
BJT is constructed with three doped regions, known as the emitter, the base, and the collector
and based on the doping of these three regions, it is known as either NPN or PNP transistor.
The internal construction of the BJT and its weight of doping is the emitter is heavily doped
because its function is to supply the electron, the base is lightly doped, and the collector is
moderately doped. The width of these three regions is the base region is much narrower than
the two regions, and the width of the collector region is wider than the other two regions
because the job of the collector region is to collect the electrons supplied by the emitter.
3. What are the two types of BJTs? How does one differ with the other?
The two types of BJT are the NPN and the PNP transistor. In the NPN transistor, both the
emitter and the collector are doped with the n-type impurity, and the base is doped with the
p-type impurity. On the other hand, in the PNP transistor, the base is doped with n -type
impurity and the emitter and the collectors are dope with the p-type impurity.
4. What are the terminals of a BJT called? What is the mathematical relation of the currents
through ALL terminals?
The terminals of BJT are the base, collector and emitter. The relation of base current, emitter
current, and collector current is established though mathematical equation IE = (B+I)IB.
5. Define alpha and beta constants as used in BJTs.
Alpha is the ratio of the collector current to the emitter current and alpha defines what fraction of
the emitter current is flowing through the collector terminal. Beta is the current gain of the BJT
and typically the value of beta varies from 50 to 400 for the different transistors.
6. What are the three regions of operations of a BJT? What are the conditions and
corresponding effects for one to conclude that the BJT is operating in a particular region?
The three regions of operations of BJT are the active region, cut-off region and the saturation
region. In the active region of operation the base-emitter Junction is forward biased and the
base-collector Junction is reversed biased and that means the collector voltage should be
greater than the base voltage and the base voltage should be greater than the emitter voltage.
In the cut-off region both base-emitter and base-collector are in reversed biased and
therefore, the condition in this region is that both emitter and collector voltage should be
greater than the base voltage. While in saturation region both base-emitter and basecollector are in reversed biased and that means the condition in this region is that base
voltage should be greater than both emitter and collector voltage.
7. Write the mathematical symbol convention for the following:
a.) current flowing through the emitter
b.) dc voltage supply near the collector
c.) voltage drop measured from base to emitter
d.) resistance near the base
8. What are the three basic circuit configurations of a BJT?
The three basic configurations of a BJT are (a) common emitter (CE), (b) common base
(CB), (c) common collector (CC).
now depending on the biasing the BJT can be operated in three regions the active region the
cutoff region in the saturation region so in case of the active region of operation the emitter
base Junction is forward biased and the base collector Junction is reversed biased so let's say
the voltage at the emitter is ve the voltage at the base is VB and the voltage at the collector is
VC then to forward bias this base emitter Junction the voltage of base should be greater than
the emitter and similarly to reverse bias this collector base Junction the voltage at the collector
should be greater than base that means to operate the BJT in the active region we can say
that the collector voltage should be greater than base voltage and the base voltage should be
greater than emitter voltage so once this condition is satisfied then the BJT will operate in the
active region similarly in the cutoff both base-emitter junction in the base corrected junctions
are reversed biased so to operate the BJT in this region the emitter voltage should be greater
than base voltage and at the same time the collector voltage should also be greater than this
voltage similarly in case of the saturation region of operation both base emitter and the base
collector junction of the BJT are forward biased that means in this region of operation the base
voltage VB he is greater than emitter voltage and at the same time his base voltage is also
greater than collector voltage so these are the three regions of operation in case of the BJT
apart from that there is a one more region of operation which is known as the reverse active
region of operation so in this region of operation instead of base emitter junction here the base
collector Junction is forward biased and the base emitter Junction is reverse biased but in this
region of operation the game provided by the BJT s very less and you do that this region of
operation is usually our datum similarly if we talk about the PNP transistor then in case of the
active region of operation this base emitter Junction is forward biased and the collector base
Junction is reverse biased but in this case now the emitter voltage is greater than the base
voltage and similarly the base voltage is greater than collector voltage so we can say that in
case of the PNP transistor to operate it in the active region the emitter voltage should be
greater than base voltage and the base voltage should be greater than the collector voltage
and similarly these plane P transistor can also be used in the different regions so whenever the
BJT is used for the amplification then it is used in this active region and whenever it is used as
a switch and it is used in the saturation in the cutoff region and in the upcoming videos we will
see in detail about these different regions of operation now if we talk about the symbol then
this is the symbol of the NPN transistor so these three terminals are the base collector and the
emitter and here these arrow indicates the direction of the current during the active region of
operation so in case of the NPN transistor the current will flow from the base towards the
emitter on the other end if you see the symbol of the PNP transistor and it is similar to the
NPN transistor but here the direction of the arrow will get reversed so now the current will flow
from the emitter towards the base region now as I said when the BJT is used for the
amplification of the signal then it is operated in the active region and there are different ways to
configure it so in case of the common emitter configuration the emitter terminal is common
between the input and the output that means in this configuration the input signal is applied
between the base and the emitter and the output is measured between the collector and the
emitter terminal similarly in case of the common collector configuration the collector terminal
he is common between the input and the output side and likewise in the common base
configuration the base terminal is common between the input and the output side so each
configuration has its own advantage and disadvantage and we will see all these configurations
in detail in the upcoming videos but in short depending on the requirement and the application
the beginning can be configured in the any of these three configurations alright so now let's
understand the working of the BJT whenever it is operated in the active region and here we will
take the example of the NPN transistor now before we understand the working let me just clear
the notations which is used for the supply voltages so as you can see were here for the base
and the collector supplies double subscription is used that means here this V VB is a source
voltage for the base and the VCC is the source voltage which is connected to the collector
terminal and this voltage VB defines the difference between the voltage at the base and the
emitter terminal so this voltage VB can be defined as voltage VB minus ve so if the voltage at
the base terminal is let say VB and the voltage at the emitter terminal is V then the work VB
can be defined as voltage VB minus V and here this voltage VB and V are measured with
respect to the ground terminal similarly this voltage VC can be defined as the collector voltage
VC minus V so instead of VB if we write voltage V EB then it can be written as voltage ve
minus VB and this voltage will be negative of the VB so throughout our discussion on the BJT
we will use these notations so here these base voltage and the collector voltage is applied in
such a way that the base emitter Junction will get forward biased and the base collector
Junction will get reversed bias that means over here the BJT is biased in such a way that this
voltage VB is positive and this voltage VC B is also positive now if you notice over here this is
the PN Junction so whenever this PN Junction is forward bias and the typical photo voltage
drop across this diode is in the range of 0.6 to 0.7 volt that means whenever we forward bias
these base emitter Junction then the typical voltage drop between these base and the emitter
Junction will be roughly around 0.7 volt now once we apply this biasing voltages and the
electrons from the emitter will be pushed towards the base terminal because as I said earlier
the emitter is the heavily doped that means the emitter has the large number of electrons as
the majority carriers and once we have that is biasing voltage then this negative voltage will
push the electrons towards the base region so due to that the electrons will starts moving
towards the base region now once the electrons enters this base region and there are two
paths for them one is they can flow towards the positive terminal on the left and the second is
they can flow into the collector region but most of the electrons will enter into the collector
region because if you see over here the base is lightly doped that means a number of holes in
this base region is very small compared to the electrons which are coming from the emitter
region that means the free electrons which has come into this base region will see the longer
lifetime and the second reason is the width of this base region is very thin that means most of
the electrons will be able to escape this phase region and they can go into the collector region
that means in this base region only few electrons will recombine with these holes and they will
been attracted towards the positive terminal of this vvb and the remaining electrons will enter
into the collector region now if you notice over here once the electrons from the emitter enters
into the base region then they will become the minority charge carriers and if you see over
here the base collector Junction is reversed biased so due to the applied electric field at the
collector terminals these minority charge carriers are the electrons will get activated towards
the collector terminal so once the electrons enters into this collector region then they will get
attracted by the positive terminal of this VCC so if you see the direction of the flow of electron
and from the emitter the electrons will flow in this direction and most of the electrons which is
emitted by the emitter will get collected by the collector region and very small amount of
electrons only will flow in this direction and if we see the direction of the holes then it will be
exactly opposite to the flow of electrons and in fact the conventional current will also flow in
the same direction that means the base current IB will flow in this direction while the collector
and the emitter current will flow in this direction so now let us establish the relationship
between all these currents so if you apply the KCl then we can say that this current IB plus IC
that is equal to emitter current that means the emitter current is the summation of this base
current and the collector current as I said only fraction of electrons are able to go in this
direction that means the base current will be very small one can say that this collector current
IC is approximately equal to ie and exactly it can be defined as collector current IC is equal to
alpha times I and this alpha defines what fraction of the emitter current is flowing through the
collector terminal now if he put this value of IC in this expression then we can say that his base
current IB plus alpha times I is equal to I that means the base current IB is equal to one minus
alpha times emitter current and once again if he put the value of I in terms of the collector
current then we can say that the base current IB is equal to one minus alpha times hi-c
divided by alpha that means the base current IB is equal to one minus alpha divided by alpha
times collector current or we can say that the collector current IC is equal to alpha divided by
one minus alpha times base current and let's say this is equal to beta that means the collector
current IC is equal to beta times IB so this beta is known as the current gain of the BJT and
typically the value of beta varies from 50 to 400 for the different transistors so from this we can
say that IB plus IC that is equal to emitter current that means base current IB plus beta times
IB is equal to I that means the emitter current I can be given as beta plus 1 times IB so this is
the between the base current emitter current and the collector current now if you notice over
here in this common emitter configuration this base current is the current on the input side
while the collector current is the current on the output side and these two currents are related
by this expression that means by controlling this base current on the input side it is possible to
control the collector current and that is why these bipolar Junction transistors are known as
the current control device that means just by controlling the input current on the base side it is
possible to control the output current on the other end if you see the other type of transistor
that is the field effect transistor it is the voltage control device that means in that case by
controlling the input voltage it is possible to control the output current also if you notice over
here in this configuration of the BJT the output collector current gets amplified by the factor of
B term and if we connect the resistor between the collector and the emitter terminal and it is
possible to amplify the input signal that means after biasing mobility in this configuration if we
apply the AC signal at the input then it is possible to amplify that signal and we will discuss
about it in the detail in the upcoming videos now during our discussion we haven't considered
the current due to the minority charge carriers in this collector region because if you notice
over here these this collector Junction is reversed bias so for a moment if we remove this
emitter connection and then the current which is flowing through the collector is only due to the
minority charge carriers and let's say this current is equal to IC o---- so this current is similar to
the reverse saturation current which we have seen in the page action diode so the total
collector current IC t will be equal to the IC plus IC o---- where this IC o---- is the current due
to the money ready charge carriers and typically this current is in the range of micro amperes
while this collector current IC is in the range of milliampere so this is all about the different
types of currents in the BJT so in the upcoming videos we will see that different configurations
of the BJT as well as the input and output characteristic of the BJT and we will also see how
the BJT can be biased using the different techniques but I hope in this video you got the brief
overview about the BJT so if you have any question or suggestion do let me know here in the
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