Common Emitter Transistor Characteristics Video Lecture - NEET

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please make sure that you have watched
all the video store part 23 before going
ahead with part 24 so let us look at the
common emitter transistor
characteristics so now we will talk
about some of the characteristics of the
transistor just now I told you right
that whenever we talk about connecting a
transistor in a circuit that means we
need to see where the input is applied
to the transistor and from there do we
get the output so transistor
characteristics basically consists of
the input characteristics of the
transistor as well as the output
characteristics of the transistor so we
will discuss both the input
characteristics and the output
characteristics here you can see that
this is a common emitter transistor the
emitter is grounded it is connected to
the base via a resistance which is the
input resistance and a power supply
similarly the collector is connect again
connected to the emitter through a load
resistance and again a power supply so
this power supply is loaded by VCC this
is V V V because this power supply is
connected to base since this is
connected to collector so it does fight
this is easy right so here we will see
where is my input circuit so in this
case where is my input so when I talk
about input input is applied here so
this is my input circuit so this base
emitter forms the input circuit when I
talk about the output circuit so the
emitter collector forms the output
circuit right now in order to know the
transistor characteristics we will
separately study the input
characteristics as well as output
characteristics in order to study the
input characteristics we will focus only
on the input circuit that means we will
see the variation of the input current
with the input voltage what is input
current input
I be Regency variation of IB with vbe
what is vbe I told you before also it is
nothing but the potential difference
between the base and the collector for
example here what have we taken is it a
PNP transistor or NPN transistor by
looking at this arrow you can see that
it is an NPN transistor right so the
potential difference between this region
this emitter and this base this is
nothing but VV e similarly the potential
difference between these two that is
raised and connected is nothing but V BC
but in this case there is no link
between base and collector in this case
the link is between base and emitter so
the potential difference between base
and emitter is v-b-e all right so here
we will observe when we study the input
characteristics we will see the
variation of current IB with vbe and in
this case we will keep the output
current or the output voltage constant
so that means in this case we will keep
BCE fixed because right now we are not
bothered about the output so we are only
focusing on input so we will keep the
output voltage fixed now similarly when
we study the output characteristics what
we will do we will see the variation of
the output current with the output
voltage that means we will see the
variation of IC with vc e and in this
that case we will see that keep the
input parameter fixed that means we will
keep the input current that is IB as
fixed so this is how we will study the
variation of input current with input
voltage and output current with output
voltage and we will see that how the
output current is influenced by the
input current so what we will see we
will conclude that I see
that is the collector current is
controlled by IB that is the base
current right so now let us look at the
input characteristics and the output
characteristics in detail in the next
slide now also note one more thing here
that I told you that IB is very small
right so you can see that IB is measured
in micro amperes because it is extremely
small whereas IC is large so it is
measured in milli amperes so these are
some small things which I have mentioned
before and right now you can see them
here right so now you understand what
are we going to study in transistor
characteristics in transistor
characteristics we want to study how the
current varies with voltage so we are
basically going to study the VI
characteristics of the transistor so let
us talk about the input characteristics
now the input characteristics of
transistor was studied so why did we
study the input characteristics we
studied only that input circuit that
means we studied the variation of the
input current with the input voltage so
what was our input current input current
was nothing but IB and the voltage was
vbe that means the potential difference
between the base and the emitter right
so what it was observe we observe that
initially there was no change in current
with increase in the voltage but at a
particular value of voltage the current
showed us sharp increase now this
measurement was taken for a fixed value
of VCE so in input characteristics we
studied the variation of the input
current with respect to the input
voltage
keeping the output voltage that is VCE
constant right so we got this kind of a
lot now it is seen that this particular
voltage what is this voltage this is
nothing but the cut in voltage when we
had drawn the VI characteristics of the
semiconductor diode we got this kind of
a cut a similar kind of a curved leg so
this voltage is called cutting voltage
and its value is somewhere around 0.6
volt for silicon right so that means the
value of vbe is always somewhere around
0.6 volts for silicon so this was the
input characteristics of the transistor
that when we study the input circuit we
see that the input current increases
with the input voltage once the input
voltage reaches the cutting voltage now
let us look at the output
characteristics now the output
characteristics is very important and
it's great and it plays a very crucial
role in the working of a transistor so
what do we study in the output
characteristics and output
characteristics we study the variation
of the output current that is the
collector current with respect to the
output voltage that is the collector to
emitter voltage and this time we keep
the input parameter fixed that means
this time we keep the base current
constant and what do we see we see that
with the increase in VCE the collector
current IC also increases up to a
certain point beyond which the current
becomes constant now we also see that as
we keep increasing the value of the base
current the collector current also
increases the pattern of variation of
collector current with respect to VCE
remains the same but the value of the
collector current increases you see here
here the base current was 10 micro
amperes then it was increased to 20 then
30 then 40 and so on and as a result of
this the collector current also
increased right so this was the
observation from the output
characteristic so this showed how the
input current varies with the input
voltage and how the output current
varies with the output voltage but this
also concluded that IC changes with IB
I see is controlled by IB that means if
base current increases collector current
will also increase if this current
decreases collector current will also
decrease so this is a very important
property of transistor thank you please
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