As per the name the meaning of
the transistor is a device which simply transfers resistance. When there is a transfer of
resistance it is obvious that there will be a transfer of the flow of the
current also from one part to another part of the device. It is the advanced
version of the triode. Before the transistor we have a device called a triode
who has three terminals. The problem with the triode is its larger size as well
as the wastage of energy in the form of the heat. To solve this problem is this
transistor is designed.
There are broadly two types of
transistors called PNP transistor as well as NPN transistor. Every transistor
will have three parts called emitter, base and collector. The purpose of the
emitter is to emit the flow of the current and hence it shall be having lots
of free charges available for the purpose of passing and initiating the current
through it. To make it possible it is highly dooped. Doping is a phenomenon of
adding impurities and here we are going to add impurities a little bit more therefore
it can have better availability of the free charges through it.
The base is a part of a
transistor who has the job of just passing the current from emitter to the
third part of the transistor. It has to just neutralize the depletion regions
and hence it is just very lightly doped.
Collector is the third part of
the transistor who has to collect the current that is coming from emitter
through the base and further positive the circuit. Though we are having three parts of the transistor, current any circuit cannot be driven by itself because
of the internal opposition that the electron flow has and hence it demands
batteries.
To initiate the flow of the current through a transistor at the input
level we need one battery and at the output level also to pass the current further
in the circuit in a continuous manner we need one more battery. Thus the input
battery demands two terminals to connect with its positive and negative plates
as well as output battery also demands two more terminals. The problem with the
transistor is it is having only three terminals therefore we have to make one
terminal common for both input and output.
Depending on which terminal we
are going to take as common, transistors can be possibly connected in three
different ways. They are common base configuration, common emitter
configuration and common collector configuration. In any of these processes the
current that is emitted from the emitter is taken over a little bit by the
base and the entire remaining current is
passed over to the collector.
To show the transistors in the
symbolical way three lines are drawn where the emitter is going to have an
arrow mark . In the case of a PNP transistor and NPN transistor,the emitter shall show the direction of the flow of the current through its
arrow. We know conventionally that the flow of the current is always shown from
the positive to the negative and hence the arrow mark is also shown from P type
to N type.
Working of a PNP transistor in
common base configuration:
It is explained earlier the
transistor demands the voltage inputs that both input level as well as the
output level. The voltage that is connected the input shall be always in the
forward bias.Then only current can through the emitter
from the emitter towards the base and then to collector. Similarly the voltage that is
connected the output level shall be always on the reverse bias therefore the
collector current will be further enabled to flow in the circuit and it can
reach back the emitter. Whatever may be the kind of the transistor and whatever
may be the kind of the connection that you are going to make these two
conditions shall be always satisfied transistor to work in a effective
manner.
Depending on the type of the
connection that we are going to choose and depending on choosing the terminal
which is common for both input and output transistor can be connected in three
possible ways. In each of the type of
the connection who is the input and who is the output and what happens to the
current and output level is as described below.
When you connect the transistor
in a common base configuration being the base is common for both input and output
the current amplification factor is the ratio of the current that is generated
through the collector to that of the current that is emitted at the emitter. As the
collector current is always a little bit less than that of the base current,amplification factor is always less than one. That means in this kind of the
configuration and the connection the transistor is not actually going to act
like an amplifier.
When you connect the transistor
in a common emitter configuration ,the current amplification factor is equal to
the ratio of the change in the collector current to that of the base current.
As the collector current is much more than that of the base current, this
application factor is always much greater than that of one. Thus a transistor
connected in a common emitter configuration always gives you a better current
amplification and this kind of connection is always preferred in so many
electric circuit is to get the amplification phenomena when the
transistor is being used as a device.
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