Logic Gates - 2 Video Lecture | Physics for Grade 12

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38 now we will focus on digital
electronics where we will talk about
signals which are in the form of zeros
and ones that means the signal is either
zero or it is high so when we talk about
voltage in terms of digital signal so
how will we say we say that voltage can
either be 0 or 1 so voltage can either
below or hi I am not bothered about any
other value of voltage but when we are
talking about voltage as an analog
signal I am bothered about every value
of voltage right now voltage is 0.02
volts next moment it is 0.025 volts next
moment it is 0.03 volts so at every
point the voltage has got some value
right
so the basic building blocks of digital
electronics is the launching cage so
what our logic gates logic gates and
nothing but they are a kind of circuit
which will tell you a specific pattern I
mean that is why they are known as logic
gates they will use a logical
relationship between the input and the
output so they are also nothing but
digital circuits then when you give some
input you get some specific output so
what is the output that you will get
that depends upon a logical relationship
between the inputs right you will
understand this as we go ahead so what
where are the applications of logic is
where do we actually see or use logic
gates we use it in digital watches
digital calculator these are something
which is very common with you you would
have seen all these things so we use
logic games in such things in your
computers so now let us see what is a
logic gate it is a digital circuit that
follows a certain logical relationship
between input and output so there are
many types of logic gates so there are
some basically
some day remain five types of logic
gates now using those five types of
logic gates you can design a huge number
of seconds right so in this lesson as of
now we will only talk about the those
five types of logic gates just the basic
logic gates and that's it then in your
higher classes you will see how those
logic gates are used for making
complicated circuits so what are the
common logic gates naught or and NAND
and not so these are the four logic
gates so we will discuss each of them
one by one so let us start now each gate
has single or multiple inputs they can
have one input they can have many inputs
and they will give one output and the
output will depend on a relationship
between the inputs truth table okay this
is an important term which I need to
introduce what is truth table truth
table is a table which will indicate all
possible input combinations and their
respective outputs now you will
understand it better as we go to the
next slide and start discussing each of
the gates one by one so as I told you
that the output which we will get that
will depend on a logical relationship
between the inputs so this truth table
is nothing but a table which will tell
us all possible input combinations and
their respective outputs so the logic
gate will operate in accordance with the
truth table so let us start with the not
gate not gate has one input and one
output as the name suggests not that
means if the input is high the output
will not be high so the output will be
low if the input is low the output will
not be more so the output will be high
so if the outer output is high when the
input is low and vice versa so this not
gate is often termed as inverter because
it inverts the input so let us look at
the symbol of a not gate a not gate
would look somewhat like this so this is
your input and this is what your output
so how do we define the logical
relationship between output and input
output is inverse of input so this is
how this is the logical relationship
which governs the not gate what does the
truth table of a naught not get so let
us prepare the truth table so the truth
table will have all possible inputs in
this case there is only one input and it
will also have the output so what are
the oil possible values of input here 0
and 1 so an input is 0 your output is 1
where input is 1 your output is 0 so
this is known as the truth table and
this is your not get now let us look at
our gate gotas or gate it has multiple
inputs and one output output is high
when any of the inputs is high let us
suppose you have two Ingram's if one of
them is high your output is going to be
high so that means output is high when a
or B is high right that is either a or b
is high so what is the symbol of or gate
check so the or gate will look like this
so you have two inputs let us say a and
B and you get output Y so the logical
relationship is y is equal to a plus B
so if you see this relationship will
satisfy this condition so we will see
that when we draw the truth table so in
this case the truth table will consists
of all possible inputs and the outputs
so what are the possible combinations of
inputs both are low a is 0 B is 1 a is 1
B is 0 both a and B are high so these
are the possible input combinations now
let us see what are the outputs that
we'll get when a is 0 B is 0 what will
be Y it will be 0 when a is 0 B is 1
what we'll do
12 in 1 when y is 1 a a is 1 and B is 0
it will take 1 when both are 1 it will
be 1 don't tell that 1 plus 1 will be 2
because right now we are dealing with
binary operations and in binary
operation 1 plus 1 is 1
that means high plus high is high it is
something like that so this will be the
truth table for or gate now the third
one that is and it this also has
multiple inputs but one output here the
output is high only when all inputs are
high rest all of the time output will be
low so in this case the symbol of an and
gate is somewhat like this so this is
your input a this is your input B and
this is your output so the logical
relation which drives this gate is y is
equal to a B so here also we will plot
the truth table this is a this is V and
this is y so what are the possible input
combinations 0 0 0 1 1 0 1 1 so when
both are 0 your output will be 0 0 1
will also be 0 1 0 will also be 0 1 1
then B 1 now we will talk about NAND
gate what is NAND gate NAND gate also
has multiple inputs and 1 output it is a
combination of not gate and anted so not
plus and becomes NAND so how would be
the symbol the symbol look like it is
like first you draw an iron gate for not
gate you just put this knot so this
represents the north gate so this is how
the Ninth Gate is representing this is a
this is B and this is y so what is the
logical relationship Y is equal to a B
this is called and gate but now that
whatever output will come
from that and gate that will get
inverted so this is the logical
expression which drives the NAND gate so
how will the truth table of an iron gate
look like it will be somewhat like this
0 0 0 1 1 0 1 1 these are the possible
combinations when both are 0 what will
happen a B will be 0 invert of 0 will be
1 now 0 1 a B will be 0 but invert will
be 1 similarly 1 0 1 into 0 will be 0
but inverted will become 1 1 into 1 will
be 1 but when inverted it will become 0
so this is the truth table of the NAND
gate and then the last one that is nor
gate which is as on the same lines as
NAND gate it has got multiple inputs and
one output it is a combination of not
gate and or gate so in this case the
symbols I think you can guess at how it
would be or gate with a not so this is
how your nor gate will look like this is
a this is B and this is right so the
logical relationship would be y is equal
to a plus B but so this drives your
nodded so how would be how the truth
table would look like like a senses a
this is V and this is y so it would be
something like this 0 0 0 1 1 0 1 1 so 0
plus 0 is 0 but when inverted it becomes
1 0 plus 1 is 1 but when inverted it
becomes 0 1 plus 0 is 1 then invert it
becomes 0
1 plus 1 is again 1 when invert it again
becomes 0 so these are the common gates
or the common logic gates now we have
something about Universal gates out of
these 5 gates NAND and nor are called as
Universal gates NAND gate and nor gate
are known as Universal gates by because
we can derive all other gates using
and nor gates I mean we can make a
combination of NAND gates work as an ant
gate we can make a combination of NAND
gates work as an or gate we can make a
combination of NAND gate Boyle Kasim not
yet so that means these are the two
gates whose combination can work as all
their other types of gates so that is
why these two gates are known as
universal gates so now we will see how
NAND gate can function as or and as well
as or so let us first look at that how
NAND gates are the how and its acts act
as Universal gates so NAND gate is y is
equal to a B by right so first we will
see how NAND gate can act as a not gate
now if you Club the two inputs of the
NAND gate into one what would you get if
you give only one input to the nine Kate
what would be the output the output
would be nothing but Y is equal to a bar
that so that means the NAND gate can be
used as a not get now let us look at and
how land it can act as a and get let us
suppose this is mine and it when this
NAND gate is made to pass through
another NAND gate that means when two
nine gates are connected they act as an
D let us see how this is a B when it
comes out from here the output is AD by
when this output comes out of here the
output becomes a be bad but so that is
any so that means it access and get now
howl and it can act as or get in that
case do you have to use three NAND gates
if you use three nine gates in this way
let us suppose this input a goes from
here input B goes from here so what you
get you get a bar here you get B bar
and then these two are made to pass
through another NAND gate so what is
your output the output is a bad be bad
bad but this would be your output now
this output is same as a plus P how is
it you can see it from the truth table
this is a this is B this is why
so if zero one one zero one one zero
zero so for this truth table you see
that what you get for this a bar b bar
that means when a is 0 d is 1 this
becomes 1 this becomes 0 so this comes
out to be 1 similarly for all these when
you try it out you see that you get it
in this way which is similar to the
truth table for a plus B so that is how
we can see that NAND gate can form not
gate it can form and gate and it can
also form or get similarly let us look
at the nor gates what is non gate it is
y is equal to a plus B bar so here also
let us see how it acts as a not it or
not if the same funda instead of two
inputs if you make only one input pass
through it so what will be your output
it would be a by that is a not gate now
let us look at or gate since it is nor
gate so output from nor gate when made
to pass through another nor get you get
a or gate say this is a this is B so
what will be this output a plus B bar
when it passes through this you get y is
equal to a plus B and the last one that
is and gate in this you would need P or
gates so if this is a this is B you will
get a bar here you will get B bar here
and then you make both of these parts
true
and either nor get so what you get you
get tea-bagged plus be bad back so this
comes out to be a V how do we get this
this is also in accordance with the
truth table so you can verify with
yourself that when you put all these
values to this expression you get the
truth table which is similar to that of
and get right so this was all about the
launching gates now this the most
important application of the logic gates
is seen in the integrated circuits or
which you call as ICS what are
integrated circuits integrated circuits
are nothing but the entire circuit
fabricated on a single semiconductor
chip normally you also make circuits in
your physics lab right so those circuits
are generally bulky and large but these
semiconductor chips they are very
extremely small in size almost of and
their size and in the range of
millimeters it's very small and on that
you have an entire circuit fabricated so
they are quite integrated circuits so
the chip dimensions are as small as 1
millimeter by 1 millimeter or even
smaller than that so as you go for your
higher studies as I mentioned you will
see that there are so many varieties of
in ICS which have been designed with the
help of these logic gates now broadly
the integrated circuits are classified
as analog Isis and digital Isis analog
Isis are those which process analog
signals and digital ICS are those which
process digital signals I am just
telling you for the sake of it this
these are not within your curriculum and
you will study all this thing in detail
when you go ahead with term
semiconductor electronics so with this I
will conclude this lesson and we will
try to see some problems on launching
gates thank you please visit exam few
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