Hey friends, welcome to the YouTube channel
ALL ABOUT ELECTRONICS. So, in this video, we will learn about the
basics of the transistor biasing and we will also learn the different terminologies like
Q-point and the load line in the context of the transistor biasing. So, first of all, let's understand, what is
biasing and why we need to bias the transistor. Now, as you are aware, one of the most common
applications of the BJT is to use it as an amplifier. where if we apply the time-varying signal
as an input then it amplifies the input signal. But this BJT won't amplify the input signal
until we apply the DC power supply. And in fact, the energy supplied by this DC supply
is used to amplify the input signal. So, the process of applying this DC voltage
source to the BJT is known as the biasing. And if you have followed the previous videos,
we had seen that the common emitter configuration of the BJT is frequently used for the signal
amplification. So, for understanding the transistor biasing,
we will use the common emitter configuration. Now, whenever the BJT is used as an amplifier,
then it is biased in a such a way, that base-emitter junction gets forward biased and the collector-base
junction gets reverse biased. And once we apply the DC voltage sources to
the BJT then the current and the voltage will establish in the circuit. And as we had seen in the previous video,
this voltage Vbe and the Ib, are the parameters on the input side, while this voltage Vce
and Ic, are the parameters of the output side. And after biasing the BJT in the active region,
if we apply the sine wave as an input then we should get the amplified signal at the
output. But whether we will get the proper amplified
output or not that depends on how well the BJT is biased in the active region. So, let's understand it through the output
characteristics. So, let's say the BJT is biased in a such
a way that the Vce is equal to 5V and the collector current Ic is equal to 10 mA. And this point on the collector curve is known
as the operating point or the Q-point of the BJT. And the reason it is known as the operating
point because it tells us the operating voltage and the current of the given transistor. So, this particular operating point, tells
us that the transistor is biased in a such a way that, the base current Ib is equal to
30 uA and the voltage Vce and Ic are 5V and 10 mA respectively. So, basically, this point on the collector
curve, tells us the operating condition of the transistor. Now, as I said earlier, the operating point
should be in the active region. But in the active region, it could be set
anywhere. So, this point could be in the center, or
it could be over here, or maybe it could be over here also. So, whenever the operating point is over here,
and on top of it, whenever we apply the AC signal, then some portion of the amplified
signal will get clipped. Because as you can see, this voltage Vce cannot
go below zero volt. Similarly, whenever it is operated at this
point, then also some portion of the current will get clipped. Because the collector current can't go below
zero amperes. And due to that, some portion of the voltage
Vce will also get clipped. That means whenever the operating point is
near saturation or the cut-off region, then it may lead to the non-linear distortion in
the output waveform. Also if you notice over here, when the BJT
is operating in this region, then it is operating near the breakdown region. And the operation of the BJT in this region
should be avoided. On the other end, when the operating point
is in the center of the collector curves, then it is possible to amplify the input signal
without any kind of distortion. And also in this region, the gain of the BJT
is almost constant. That means for the small-signal amplification,
the biasing point or the operating point should be in the center region of the collector curves. Now, another important aspect of biasing is
the stability of the operating point. Because with temperature, the operating point
may get changed. Because with temperature, the device parameters
like the transistor current gain and the reverse saturation current will get changed. So, the designed biasing circuit should provide
temperature stability such that even if there is any change in the temperature, then there
is a minimum change in the operating point. And the stability of the operating point is
defined by the term stability factor. which indicates the change in the operating
point with the change in the temperature. So, in this video, as well as in the upcoming
videos, we will see the different biasing techniques for the BJT and we will compare
the stability of the different biasing configurations. So, first of all, let's start with the very
simple fixed-bias configuration. So, this is the circuit of the fixed bias
configuration. Or if I redraw the same circuit, then it can
be redrawn like this. So, as you can see over here, the AC input
signal is applied between the base and the emitter terminal, while the output is taken
across this collector and the emitter terminal. Now, here as we are interested in the DC analysis
of the circuit, so we can replace all the capacitors with the open circuit. And then after if you see the equivalent circuit,
then it will look like this. Now, the reason it is known as the fixed bias
configuration, because for the given power supplies if we fix the value of this base
resistor then this base current Ib will get fixed. And here, if we mark the voltages then this
voltage Vbe is the voltage on the input side while the voltage Vce is the voltage on the
output side. So, first of all, let's find out the expression
for this base current Ib. So, if you notice over here, the voltage at
this node is equal to Vbe. So, from this, we can say that this base current
Ib is equal to this voltage Vbb, minus Vbe, divided by Rb. Now, here the typical value of Vbe is equal
to 0.7V. So, for the given supply voltage, once we
set the value of Rb, then the this base current Ib will get fixed. And once we know the value of this base current,
then we can find the value of this collector current and the voltage Vce. Now, we know that this collector current Ic
can be given as beta times Ib. So, form the base current Ib, we can find
the value of this collector current. Now, if we apply the KVL in this output side,
then we can write the expression as Vcc, minus the voltage across this resistor Rc, let's
say that is equal to minus Vrc, minus Vce, that is equal to 0. Or from this, we can say that this voltage
Vce = Vcc - Vrc. That is equal to Vcc - (Ic x Rc)
So, in this way, we can also get the value of the Vce. So, here this voltage Vce and the collector
current Ic defines the operating point of the transistor. Now, from the equations, if you observe, if
the value of Rb and Rc changes or the value of Vcc changes then the operating point will
also change. So, now let's find out, how these network
parameters define the possible values of the operating point. Now, from this expression, we can write, this
collector current Ic is equal to voltage Vcc, minus Vce, divided by Rc. So, this collector current Ic will be maximum
whenever this voltage Vce is equal to zero. That means the maximum value of the collector
current will be equal to voltage Vcc divided by Rc. That means this collector current will be
maximum whenever the voltage Vce is equal to zero. Similarly, whenever this collector current
Ic is zero, at that time, the value of Vce will be maximum. So, we can say that the voltage Vce (max)
is equal to Vcc. So, from this graph, we can say that, whenever
this collector current Ic is zero at that time, the value of Vce is maximum. So, if we join these two points, then we will
get the possible values of the operating point for the given values of Vcc and Rc. So, this line is also known as the load line. Because it is defined by the value of this
resistor Rc. Now, here if the value of Ib is changed by
varying this Rb, then Q-point will also change. So, for the given value of Vcc and Rc, if
the value of Ib is increased, then the Q-point will also move upwards. Similarly, by keeping the Vcc fix, if we change
the value of Rc, then the load line will also change. So, here three different load lines are shown
for the different values of the Rc. And as you can see, for the fixed value of
the base current, if the load line changes then the operating point will also shifts
towards the left- hand side. And similarly, by keeping the value of Rc
fix, if we change the value of Vcc, then the load line will look like this. So, here the different load lines are shown
for the different value of Vcc. But let's say, for the given value of Vcc
and Rc, we are getting this load line. And the operating point is also set over here. Now, as I said earlier, as the temperature
changes, then the operating point will also change. Because with temperature, the value of beta
or the current gain of the transistor will also change. Or in case, if we replace the transistor,
then also the value of beta will change. Because if you take similar transistors then
also they have different values of beta. So, now let's take one simple example, and
let's understand, how the variation in the beta can affect the operating point. So, let's say for one transistor, the nominal
value of the beta is equal to 100. And due to the change in the temperature or
due to the replacement of the transistor, this beta varies from 50 to 200. And assuming this voltage Vbe is not changing
with the temperature, this base current Ib is set to 3o micro-ampere. Now, here let's say, the value of Rc is equal
to 1.5 kilo-ohms. And the value of Vcc is equal to 10V. So, whenever the beta is equal to 100 at that
time, this collector current Ic will be equal to 100 *30 uA. That is equal to 3 mA. And for that, if we calculate the value of
Vce, then it will be equal to 10V - (1.5* 3 mA)
That is equal to 5.5 V. So, at that time, the operating point would
be over here. That means the Vce is equal to 5.5V and the
collector current Ic is equal to 3 mA. Now, let's say we have replaced the transistor
and due to that now the new value of the beta is equal to 50. So, in that case, this collector current Ic
is equal to 50 * 30 uA. That is equal to 1.5 mA. And at that time, if we calculate the value
of Vce, then it will be equal to 10V - (1.5 * 1.5 mA)
That is equal to 7.75V. That means whenever, the value of beta becomes
50, at that time, the operating point would be somewhere around here. Because at that time the value of Vce is equal
to 7.75 V and the value of Ic is equal to 1.5 mA. And similarly, let's also see whenever the
beta becomes 200. So, at that time, the collector current Ic
will be equal to (200)*(30 uA). That is equal to 6 mA. And at that time, the value of Vce will be
equal to 10V - (1.5 * 6 mA) That is equal to 1V. So, at that time, the operating point would
be somewhere around here. Because at that time, this collector current
Ic is equal to 6 mA and the voltage Vce is equal to 1V. So, this shows the maximum possible change
in the operating point due to the change in the temperature or due to the change in the
transistor. That means in this fixed bias configuration,
even if we set the fixed value of the base current, then also the operating point may
vary due to the change in the external parameters. And in the upcoming videos, we will see some
other biasing configurations which can provide a relatively stable operating point. And also we will solve some examples on this
fixed bias configuration on our second channel. So I hope in this video, you understood the
basics of the transistor biasing as well as the fixed bias configuration. So, if you have any question or suggestion,
do let me know here in the comment section below. If you like this video, hit the like button
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