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ALL ABOUT ELECTRONICS. So, in this video and the subsequent videos,
we will learn about the diode. And these are the following topics that we
will cover in the series of videos. And in this particular video, we will learn
that what is a diode, and how the V-I characteristics of the diode will look like. And we will also see the equivalent circuit
for the diode. And in the subsequent videos, we will learn
about the device physics of this diode and we will also see the different applications
of the diode. So, now the question is what is a diode. Now, we all know about the resistor. It is the most widely used type of passive
circuit element. And it allows the flow of current in both
directions. Similarly, the diode is a two terminal semiconductor
device which allows the flow of current only in one direction. And if you see the symbol of the diode, it
looks like this. So, here this arrow indicates the direction
in which this diode allows the flow of current. Now, the one terminal of this diode is known
as the anode and the second terminal of the diode is known as the cathode. Now, whether this diode will allow the flow
of current or not, it depends on the polarity of the voltage that is applied between this
anode and cathode. So, if the voltage that is applied between
this anode and cathode is positive, then, in general, we can say that this video will
allow the flow of current in one direction. While the voltage that is applied between
this anode and cathode is negative then it will not allow the flow of current. So, now when this diode connected in particular
circuit and if we want to analyze that circuit, then the circuit can be analyzed by knowing
the voltage and current that is flowing through this diode. And that can be found using the V-I characteristics
of the diode. Now, for the resistor, we know that it is
a linear element. And the relationship between the voltage and
current is linear. So, using the Ohm's law, we can easily find
the voltage and current through this resistor. But unlike the case of a resistor, the diode
is a non-linear element. So, the relationship between the voltage and
current for the diode is non-linear. And moreover that this diode allows the flow
of current only in one direction. And it almost blocks the current in the reverse
direction. Now, if you look at this graph, then it looks
like, the graph is symmetrical in both directions. But don't get confused by the symmetricity,
because here in this graph, both positive and the negative axis has a different scale. For example, on the positive Y-axis, we have
a scale in milli-ampere. While on the negative Y- axis, we have a scale
in micro-ampere. Similarly, on the positive X- axis if you
see, the voltage varies like 0.5 V, 1V, and 1.5V.
While, if you observe on the negative X-axis, the voltage scale varies like -10V, -20V and
-30V. So, because of the different scale, this characteristics
looks symmetrical on both axis. But if we keep the same scale on both axis
then the diode characteristics will look like this. So, as you can see, in the reverse direction
the current that is flowing through the diode is almost negligible. So, using this V-I characteristic of the diode,
we can easily find the voltage and current that is flowing through the diode. And using this we can easily analyze any circuit
which contains this diode. But because of the non-linear characteristics
of the diode curve, it is a bit difficult to find the voltage and current very easily. So, what we can do, we can approximate this
diode characteristic and, using this we can analyze the circuits. So, first of all, what we will do, we will
consider this diode as an ideal diode and for this ideal diode, we will see the V-I
characteristics. And also we will see the equivalent circuit
for this ideal diode. And step by step, we will introduce few more
parameters to this ideal diode. And using this we will approximate the diode
characteristic curve. So, for the ideal diode, whenever the voltage
that is applied between the anode and cathode is positive, then simply it will act as a
closed switch. On the other end, if the voltage that is applied
between this anode and cathode is negative, in that case simply it will act as an open
switch. So, now let's see the V-I characteristic for
this ideal diode. So, in the case-1, when the applied voltage
is positive, on the V-I characteristic we will get the vertical line. And for the case-2, when the applied voltage
is negative then, for that case, we will get the horizontal line on the negative x-direction. So, this is the V-I characteristics for the
ideal diode. So, now whenever the positive voltage is applied
to this diode, then it can be said that the diode has been forward biased. And it will allow the flow of current. While on the other end, whenever the negative
voltage is applied to this diode then it will not allow the flow of current. And simply we can say that it has been reversed
biased. So, we will talk more about this forward and
reverse bias at the later part of the video. So, now let's take one simple example, in
which the diode is connected in series with voltage source and resistor. Now, here we are assuming that the diode is
an ideal diode. It means that whenever the voltage that is
appearing across the anode and cathode of the diode is positive then simply it can be
represented by the close switch. And as you can see over here, the applied
voltage across the diode is 10V. So, simply it can be represented by the close
switch. And the current that is flowing through the100
ohm resistor will be equal to 10V divided by 100 ohms. That is equal to 0.1A.
While on the other end, if we apply the -10V in this circuit, then the applied voltage
across the diode will be -10V. And it will not allow any flow of current. So, simply it will act as an open switch. And the current that is flowing through the
circuit will be equal to 0A. Now, some of you might have a question that
how to find the voltage across this diode. So, that can be found by finding the Thevenin's
equivalent between these two terminals. So, simply you need to remove this diode from
the circuit and you need to find the Thevenin's equivalent voltage between these two terminals. So, if we remove this diode, then the Thevenin's
equivalent voltage which appears between this anode and cathode in this particular case
will be equal to -10V. And because of that, the diode will be nonconducting. On the other end, if 10V is appearing between
these two terminals then diode will simply act as a closed switch. Now, if you see the V-I characteristics of
this ideal diode, then you can observe that even if we apply very small positive voltage
between this anode and cathode, then also this diode will start conducting. So, let's say, even if we apply positive 0.1
V, between this anode and cathode then also this ideal diode should start conducting. But if you see the actual diode, it will start
conducting, only after applied voltage crosses some threshold voltage. And if you see the V-Characteristics, then
it will look like this. So, up to certain threshold voltage, the diode
will not allow any flow of current. And whenever the applied voltage crosses this
threshold voltage, then only, the diode will allow the flow of current. So, this voltage is known as the threshold
voltage or the Cut-in voltage for the diode. Now, like I said before, this diode is a semiconductor
device. And usually, it is made of either silicon
or germanium. So, for silicon, this threshold voltage used
to be in the range of 0.6 to 0.7 V. While for germanium usually, it is around
0.3V. So, actually, the diode will start conducting
once the applied voltage crosses this threshold voltage. And before that, it will not allow any flow
of current. So, this is the first approximation for the
diode curve. So, here in this approximation, we are considering
this diode as an ideal diode but it will only allow the flow of current once the applied
voltage crosses this threshold voltage. And the equivalent circuit will look like
this. So, in case of reversed bias, it will simply
act as an open switch. But whenever the applied voltage crosses this
threshold voltage then only it will allow the flow of current. And simply it will act as a closed switch. so, now let's take the same example that we
have taken earlier. So, in this approximation, we are considering
this diode as an ideal diode except the fact that it will allow the flow of current only
after that applied voltage crosses the threshold voltage. And here we are assuming that the diode that
is used is the silicon diode. So, it has a threshold voltage of 0.7V. So, once the applied voltage crosses this
0.7V barrier, then only it will allow the flow of current. Now, here as the applied voltage is 10V, So
simply this diode will allow the flow of current. And the equivalent circuit will look like
this. So, now if you find the current that is flowing
this 100-ohm resistor, it will be equal to 10 V minus 0.7V divided by 100 ohms.
that is equal to 0.093A. While on the earlier case, when we have considered
this diode as an ideal diode, at that time, we found the value of this current as 0.1A.
So, using this approximation, we can find the more accurate values of voltage and current
int he circuit. Now, in the same approximation, suppose if
we apply the -10V, then, in that case, the circuit will simply act as an open switch. And it will not allow any flow of current. So, the current that is flowing through this
resistor will be equal to 0A. Now, in this first approximation also, we
have considered that diode has zero resistance. And that can be also visible from the V-I
characteristics of this diode. Because if you observe over here, once the
applied voltage crosses this threshold voltage then the voltage that appears across the diode
will remain constant. And current will increase. It means that the diode offers the zero resistance. But in the actual case, every device has some
finite resistance. which will limit the current that is flowing
through that particular device. Now, let's consider the second approximation,
where we are also considering that the diode has some finite resistance. And in that case, if you see the V-I characteristics,
then it will look like this. So, up to the threshold voltage, the diode
will offer an infinite resistance, or simply it will act as an open switch. And then after it will provide some finite
resistance. And that resistance can be found from the
slope of this curve. So, this resistance is also known as the bulk
resistance or the body resistance. So, this resistance is the resistance that
is offered by the semiconductor device out of which this diode has been made. So, now let's see the equivalent circuit for
this second approximation. So, whenever the diode is reversed biased
in that case, simply it will offer the infinite resistance. Or we can say that it will act as an open
switch. But whenever the applied voltage crosses this
threshold voltage, in that case now it will offer some finite resistance of Rb. That is the bulk resistance of this diode. So, this is the equivalent circuit in case
of the second approximation. Now, apart from this bulk resistance, the
diode has one more resistance. And this resistance is present in the diode
because of its internal structure. So, we will talk more about this diode resistance
in the separate video. No, typically the value of this resistance
used to be few ohms. So, in the circuit, if the Thevenin's equivalent
resistance which appears across the diode is much more than the diode resistance, in
that case, the diode resistance can be neglected. But if the Thevenin's equivalent resistance
which appears across the diode is comparable to the diode resistance, in that case, we
also need to consider this diode resistance. So, for example, in this case, let's assume
that the diode has a resistance of 25 ohms. Now, this resistance is comparable to the
100-ohm resistor, so we need to consider this diode resistance. So, if we consider this diode resistance,
then the current that is flowing through this 100-ohm resistor will be equal to 10V minus
0.7V divided by 125 ohms. And if you see the current, it will be equal
to 0.074A. So, because of this diode resistance, if you
see, the actual current that is flowing through this 100-ohm resistor will change. So, using this second approximation, we can
find the more accurate value of the current and voltage that is flowing through the circuit. But in most of the circuits, the Thevenin's
equivalent resistance which appears across the diode used to be much larger than this
diode resistance. And simply we can neglect this diode resistance. So, in this second approximation of this diode,
we have assumed that the diode is non-conducting till the point this applied voltage crosses
the threshold voltage. And then after it offers some finite resistance. So, if you see this type of characteristics
then it is known as the piecewise linear characteristics. Because what we have done, we have segmented
the actual characteristics of this diode into the piecewise linear characteristics. So, up to the threshold voltage, we have assumed
that the diode is non-conducting. And after the threshold voltage, it will offer
some finite resistance. OK, so now let's talk about the actual curve
of this diode. So, in this diode curve, as I said before,
this region is known as the forward region. Because, once the applied voltage crosses
the threshold voltage, then the diode starts conducting. And whenever the applied voltage is less than
the threshold voltage, in that case, the current that is flowing through the diode is almost
negligible. So, now whenever we apply the reverse voltage
to this diode, then that region of operation is known as the reverse region of operation. And in this region, the current that is flowing
through the diode used to be very small, typically in the micro-amperes. And this current is known as the reverse saturation
current of the diode. So, as you can see from the graph, even if
we increase this reverse voltage across the diode, then also the current will only increase
by a marginal amount. But we cannot increase this reverse voltage
indefinitely. Because if we go beyond certain voltage then
the diode will come into the breakdown region. So, this region of operation is known as the
breakdown region and this region of operation should be avoided. Now, some diodes are specifically made to
operate in this particular breakdown region. And this types of diodes are known as the
Zener diode. So, we will talk more about this Zener diode
in the separate video. But for any normal signal diodes, this region
of operation should be avoided. So, if you see the datasheet, you will find
that the maximum breakdown voltage for the diode has been mentioned. So, the applied reverse voltage should be
less than this breakdown voltage. Now, in the forward region of this diode characteristic
curve, if you observe, as the voltage across the diode increases, the current that is flowing
through the diode will increase exponentially. Si, in datasheets you will also find the one
more parameter that is known as the maximum allowable forward current. So the diode current should be less than this
maximum allowable forward current. So, here are the few parameters that you will
find in the datasheet of any diode. So, now in the next video, we will talk more
about the diode resistance. So, I hope in this video, you understood what
is a diode, and the V-I characteristics of the diode. So, if you have any question or suggestion
then do let me know in the comment section below. If you like this video, hit the like button
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