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Transistors are easily one of the most
game-changing inventions of the 20th
century. They've made it possible to
build lightning fast, ultra compact, and
energyefficient circuits, all while
keeping costs incredibly low. In fact,
if you're watching this on a phone,
tablet, or computer, you're staring into
a microscopic force of transistors,
billions of them, crammed into the tiny
chips powering your screen right now. In
this video, we're diving deep into the
fascinating world of transistors. We'll
start with the absolute basics and build
our way up, covering everything you need
to know to truly understand how these
tiny devices work and why they're so powerful.
powerful.
A transistor is a solid state device
with three terminals and not a single
moving part. That's a big reason why
it's so fast, incredibly reliable, and
built to last. When you look at a
circuit diagram, you'll often see these
symbols representing different types of
transistors. They all follow the same
basic shape, but with a few key
differences depending on the type. Don't
worry, we'll walk through exactly what
each symbol means in the sections ahead.
At their core, transistors work like
electronic switches. Just like flipping
a switch on your wall turns a light on
or off, a transistor can do the same
job, but without needing a human hand.
There's no clicking or moving parts
involved. Instead, transistors respond
to electrical signals that lets them
switch things on and off automatically,
silently, and at lightning speed. So,
say you want a light bulb to blink in a
pattern, you don't need to sit there
flipping a switch. Just upload a set of
instructions to a microcontroller, and
the transistor handles all the switching
for you. Perfectly timed and completely hands-free.
Now, you might be thinking, "Wait, don't
relays do the same thing?" They can turn
circuits on and off, too, using an
electromagnet to move internal contacts.
That's true, but relays come with a few
big drawbacks. First off, they're power
hungry. It usually takes around 100
milliamps to energize a relay coil. A
transistor, it can do the same job with
less than 10 milliamps. And then there's
speed. Relays are mechanical. They have
parts that physically move. That makes
them slow. If you need to switch
something on and off a thousand times
per second, a relay just can't keep up.
It takes time for the coil to magnetize
and release. Transistors, on the other
hand, don't move. No parts, no lag. They
can switch on and off millions of times
per second. No sweat. Now, let's talk
about the two main families of
transistors, BJTs and FETSS. BJT stands
for bipolar junction transistor. These
need a small amount of current to turn
on, usually between 1 and 20 milliamps.
They're great for amplifying signals and
doing basic switching jobs, and they can
operate at speeds up to a few hundred
khz. FE stands for field effect
transistor. These are a bit different.
They're controlled by voltage rather
than current. That means they draw
almost no input current at all, which
makes them much more efficient. As a
result, FETSS can switch way faster,
often in the tens or even hundreds of
megahertz. But like with most things,
there's a trade-off. BJTs are simpler
and cheaper, which makes them popular in
everyday circuits and lowcost designs.
FETSS, while more efficient and
longerlasting, are a bit more complex
and tend to cost more. So, they're
usually reserved for when battery life,
speed, or heat management really matter.
In most basic electronics projects,
you'll likely see BJTs. But when
performance matters, FETSS start to
shine. Let's start with BJTs since
they're the most common type of
transistor you'll run into when first
learning electronics. Every BJT has
three terminals. Two of them work kind
of like the ends of a regular switch.
One connects to the load, the other to
power or ground. These are called the
collector and the emitter. The third
terminal is the control input. It's what
decides when the transistor turns on or
off. That one's called the base. Now,
BJTs come in two types based on how
they're built internally, NPN and PNP.
In an NPN transistor, a thin layer of
ptype material is sandwiched between two
ntype layers. In a PNP, it's the other
way around. An Nype layer is between two
P types. But honestly, you don't need to
memorize the internal structure to use
them. What really matters is the
direction of current flow. In an NPN
transistor, current flows from collector
to emitter. In a PNP transistor, it
flows from emitter to collector. That
simple difference affects how you
connect and control them in a circuit.
And we'll explore that later. We always
mark the direction of current at the
emitter terminal. And that little arrow
is super helpful. Just by looking at it,
you can instantly tell whether the
transistor is NPN or PMP. If the arrow
points out, it's an NPN. If it points
in, it's a PNP. A quick trick to
remember, NPN arrow not pointing in. One
of the biggest differences between NPN
and PNP transistors is how you turn them
on. For an NPN transistor, the base
voltage needs to be about 0.7 volts
higher than the emitter. For a PNP
transistor, it's the opposite. The base
needs to be about 0.7 volts lower than
the emitter. So, if you have an NPN with
the emitter connected to ground 0 volt,
you need to apply at least 0.7 volts to
the base to switch it on. But for a PNP
transistor, if the emitter is at plus 5
V, the base must be at 4.3 volt or lower
to turn it on. It all comes down to the
voltage difference between the base and
emitter. Get that right. and the
transistor does its job. When you're
using a regular switch to control a
load, like a light bulb, you can put the
switch on either the positive side or
the negative side of the circuit. As
long as current flows through the load,
it works. Transistors follow the same
basic idea. For an NPN transistor, the
current flows from collector to emitter.
That usually means the collector
connects to power or the emitter goes to
ground. For a PNP transistor, it's the
reverse. Current flows from emitter to
collector. So you connect the emitter to
power or the collector to ground. At
first glance, it might seem like you
could just use either an NPN or a PNP
transistor to switch the positive or
negative side of a circuit just like a
regular switch. Technically, yes, you
could do that, but in practice, it's not
quite that simple. Here's the issue. To
control a transistor, you need to set
the base voltage relative to the
emitter. For an NPN, the base must be
about 0.7 volts above the emitter. For a
PNP, needs to be 0.7 volts below the
emitter. Now, when you're using an NPN
transistor for lowside switching with
the emitter connected to ground, it's
simple. Just apply 0.7 volts or more to
the base and it switches on. Easy. Same
with a PNP used for high-side switching.
If the emitter is at plus 5 volts, you
just pull the base down to 4.3 volts or
lower. Boom. Transistor on. But here's
where it gets tricky. If you try to use
an NPN for high-side switching, the
emitter isn't at a fixed voltage
anymore. It changes based on the load,
and that makes it hard to know exactly
what voltage the base needs to turn the
transistor on. The same problem happens
if you use a PNP for low side switching.
the emitter floats and your control
signal becomes unreliable.
So here's the rule of thumb. Use NPN
transistors for lowside switching with
the emitter connected to ground. Use PNP
transistors for high-side switching with
the emitter connected to VCC. Why?
Because this keeps the emitter voltage
steady, which makes it easy to apply the
correct base voltage and reliably turn
the transistor on. And just so you know,
NPNs are more commonly used in beginner
and microcontroller circuits. That's
because they play nicely with low
control voltages like the 3.3 volts or
5V signals from most digital outputs.
Now that we've got a solid understanding
of how to use NPN and PNP transistors as
switches, let's put that knowledge into
action. We're going to break down two
simple light sensing circuits. One will
turn on when it's dark using an NPN
transistor for low side switching. The
other will turn on when it's bright
using a PNP transistor for high-side
switching. Let's see how each one works.
To sense light, we'll use a light
dependent resistor or LDR. An LDR
changes its resistance based on how much
light hits it. In complete darkness, its
resistance can shoot up to around 1
megga, but as light increases, the
resistance drops, sometimes down to just
a few hundred ohms. Now when we connect
an LDR with a regular resistor in this
configuration, we create a voltage
divider. That means the voltage at the
center point depends on how much light
the LDR is getting. In the dark, the
LDR's resistance is high. So most of the
voltage appears at the midpoint, almost
equal to the supply voltage. As the
light increases, the LDR's resistance
drops and the midpoint voltage goes
down. Here's where the transistor comes
in. If we connect an NPN transistor to
this voltage divider, it turns on when
the base voltage rises above 0.7 volts.
So, it activates in the dark when the
voltage is high and turns off when it's
bright. That makes it a simple darkness
activated switch. Now, swap in a PNP
transistor. This one turns on when the
base voltage is about 0.7 volts below
the emitter. So, in a 6V system, that
means the base needs to drop below 5.3
volt. and that only happens when it's
bright. So, this version works as a
light activated switch. You can even add
a relay to either setup to control
higher power devices like a lamp or
motor. And for extra control, replace
the fixed resistor with a potentiometer.
That way, you can fine-tune exactly how
much light it takes to trigger the
switch. Now, let's take a closer look at
the voltage current characteristics of
an NPN transistor. This is where things
get really interesting. A lot of
students struggle with this part. So, if
that's you, don't worry. Stay with me
and I'll break it down step by step. To
explore this, I've set up a simple test
circuit with two variable voltage
sources, one connected to the base and
the other to the collector. This setup
lets us independently adjust each
voltage and observe exactly how the
transistor responds as we change them.
We'll begin by keeping VCC fixed at 5
volts and then slowly increase VBB, the
base voltage. As we do this, we'll track
four key values that reveal how the
transistor is behaving. In the first
graph, we'll plot the base emitter
voltage and the base current. In the
second graph, we'll look at the
collector emitter voltage and the
collector current. Each of these
measurements gives us a different window
into what's going on inside the
transistor as we adjust the input. Now,
let's start increasing VBB from zero and
watch how each parameter changes. Here's
what the graphs reveal. As we gradually
increase VBB, VBE rises slowly along
with VBB until it hits around 0.7 volts.
After that, it levels off and stays
nearly constant. That 0.7 volts is the
threshold voltage. The point where the
transistor starts to turn on. At first,
IB the base current remains at zero. But
as soon as VBE reaches that 0.7V mark,
current starts flowing into the base and
IB increases along with VBB. Meanwhile,
VCE stays close to the full supply
voltage 5 Vs while the transistor is
off. It acts like an open switch. But
once VBE hits 0.7 volt, VCE drops
sharply down near 0 volts, just like a
closed switch. And finally, IC the
collector current stays at zero until
the transistor turns on. Once VBE
reaches that threshold, the IC begins to
flow. At that point, you can calculate
the collector current using Ohm's law
based on the collector resistor and
So to sum up, when VBE is below 0.7
volts, the transistor is off. No current
flows between collector and emitter.
Once VBE crosses that point, the
transistor turns on and begins
conducting just like closing a switch.
The region where the transistor behaves
like an open switch is called the cutff
region. The region where it acts like a
fully closed switch is known as the
saturation region.
Now if we keep increasing VBB, IB will
continue to rise but only up to a point.
If we go too far, the current can exceed
what the transistor is rated for, and
that can lead to permanent damage. For
small transistors, the maximum base
current is usually under 20 milliamps.
Power transistors can handle more, but
either way, it's critical to check the
data sheet and stay within safe limits.
As long as we're within those limits,
VBE will stay around 0.7 volts and the
extra voltage from VBB will drop across
the base resistor, causing IB to
Now, let's move on to the second part of
our analysis. This time, we'll keep VBB
constant and slowly increase VCC to see
what happens. We already know that if
VBB is below 0.7 volt, the transistor
stays off. Nothing happens. So, let's
set VBB to 0.71 volt. That gives us a
VBE of about 0.7 volt, which turns the
transistor on and allows a small base
current IB to flow. Since we know VBE is
fixed at 0.7 volt and VBB is 0.71 volt,
that leaves 0.1 volt across the base
resistor. Using Ohm's law, we can
calculate the value of IB from that.
At first, everything behaves as
expected. Because VBB is fixed, both VBE
and IB stay constant. With VBE at 0.7
volts, the transistor switches on and
behaves like a closed switch. As we
begin increasing VCC, the collector
emitter voltage VCE drops close to zero
and the collector current IC rises just
like we'd expect. But as we keep raising
VCC, something interesting happens. At a
certain point, VCE starts to climb
again. It's as if the switch inside the
transistor isn't fully closed anymore.
It's starting to loosen. At the same
time, IC stops increasing. it hits a
ceiling and stays flat even though we're
still raising VCC.
Let's label this base current as IB1 and
collector current IC1.
Let's repeat the same experiment, but
VBE doesn't change much. It still holds
steady around 0.7 volt, but the base
current increases slightly. That's
because with VBE fixed at 0.7 volt, the
extra voltage 0.02 volt now appears
across the base resistor RB. And that
small increase is enough to boost IB.
We'll call this new base current IB2.
Looking at the graph, we see the same
curve shape, but with some important
changes. The collector current IC rises
higher before leveling off. We'll call
this new peak IC2.
Also, VCE remains low for longer, only
beginning to rise later as we continue
Try it once more with VBB set to 0.73 volts.
volts.
And we see the same trend again. A bit
more base current, a higher collector
Now, let's repeat the experiment using
real current values. And this time we'll
just focus on plotting IB and IC. If we
set VBB so that the base current IB is
10 micro amps, we observe that IC rises
and then levels off at 1,000 micro amps.
That's 1 milliamp. Next, we increase VBB
to make IB equal to 20 micro amps. And
now IC levels off at 2,000 micro amps or
2 milliamps. This pattern continues. For
each increase in IB, IC climbs to a
higher maximum value. Each one directly
related to its corresponding base current.
current.
Now if we observe the IC plot, we can
clearly see two distinct regions of
operation. First is the saturation
region where the transistor behaves like
a closed switch. In this region, as we
increase VCC, IC also increases. Just
like current through a typical resistor
or switch. But beyond a certain point,
IC stops rising. Even as we continue to
increase VCE, it flattens out.
This is the active region.
Here's where things get interesting. If
we plot IB against IC in the active
region, we get a straight line. That
means IC increases linearly with IB. And
the scale difference is dramatic. While
IB might only change by a few micro
amps, IC can change by milliamps. For
example, a 10 micro ampere change in IB
might produce a 1 milliamp change in IC.
That's a current gain of about 100
times. And that's the heart of
transistor amplification. A small signal
at the base controls a much larger
current at the collector. This gain is
called the common emitter current gain,
and it's usually represented by the
Greek letter beta.
Now if we go back and look at each IC
versus VCE graph, we can clearly
identify the different regions of
transistor operation.
When we combine all those curves onto a
single graph, the pattern becomes even
clearer. At the beginning of each curve,
there's a region where IC increases
along with VCE. This is the saturation
region where the transistor behaves like
a fully closed switch and current flows
freely from collector to emitter as VCC
rises. In this region, VCE stays close
to zero. But once IC stops increasing,
even though VCE continues to rise, we
move into the active region. Here IC
stays constant and it's completely
controlled by the base current IB. So to
summarize, in the saturation region, VCE
is nearly zero and IC increases with VCC
just like a switch that's fully on. In
the active region, IC stays steady even
as VCE increases. This is where the
transistor acts as an amplifier with
small changes in base current producing
much larger changes in collector current.
current.
Now let's try to combine the results of
both experiments into one complete
picture. We'll capture all these
behaviors in a single graph of IC versus
VCE. First, if IC is zero, even as VCE
increases, the transistor is completely
off. This is the cutff region where VBE
is below 0.7 volts and no current flows.
Next, if VCE is close to zero but IC is
flowing, the transistor is acting like a
closed switch. This is the saturation
region where both the base and collector
currents are active and the transistor
is fully on. Then we have the region
where both IC and VCE are non zero. This
is the active region. Here IC is no
longer just about VCE. It's directly
controlled by the base current IB.
Finally, if VCE goes much higher than
the rated value for the transistor, it
will most likely fail and possibly burn
out. This is called the breakdown
region. It's beyond normal operation and
should always be avoided. Most data
sheets will clearly list the maximum VCE
rating and staying under that limit is
crucial for safe and reliable circuit
design. So, putting it all together,
this graph gives us a complete view of
how a transistor transitions between
being off, on, amplifying, and finally
breaking down. All depending on the
voltages and currents at its terminals.
So far, we've looked at the ideal
behavior of a transistor, but in the
real world, things aren't always that
perfect. For example, in the saturation
region, VCE isn't exactly zero. Even
when the transistor is fully on, you'll
typically see about 0.2 to 0.3 volts
across the collector and emitter. And in
the cutff region where the transistor is
supposed to be completely off, you might
still notice a tiny leakage current
flowing through the collector, usually
around 1 micro amp or less. These small
imperfections don't matter much in basic
circuits, but they can become important
when you're designing precision or high
sensitivity electronics like analog
amplifiers or low-noise sensor systems.
Now that we've covered the basics, let's
explore how a transistor works as an
amplifier. To make amplification
possible, the transistor needs to
operate in the active region. That's the
sweet spot where it responds linearly to
small input changes. To set this up, we
keep the collector supply voltage VCC
fixed and we vary the base voltage VBB,
which controls the base current or IB.
As we saw earlier, changes in IB cause
corresponding changes in the collector
current IC. And in the active region,
this relationship is nicely
proportional. Here's the key part. IC
flows through a resistor connected to
the collector known as RC. According to
Ohm's law, the current through that
resistor equals the voltage across it
divided by its resistance. This equation
tells us how IC and VCE are related in
the active region. And if we rearrange
it, we get a standard linear equation
just like the format y = mx + c. In this
case, the slope m is -1 / rc, which
means the line slopes downward. Now
let's look at the two ends of this line.
If IC= 0, then VCE= VCC. This point is
called the cutff where the transistor is
off and no collector current flows. On
the other hand, if VCE equals zero, then
IC equals VCC / RC. That's the
saturation point where the transistor
acts like a fully closed switch and the
collector current is at its maximum.
When we connect these two points on a
graph, IC on the vertical axis and VCE
on the horizontal, we get a straight
line called the load line. This line
shows all the possible operating points
for the transistor in that particular
circuit. And it's more than just a line.
It's a powerful visualization tool that
lets us predict exactly how the
transistor will respond when we apply
different signals. So now let's take
this load line and use it to visualize
how amplification actually works.
To use a transistor as an amplifier, the
first thing we need to do is set a
steady operating point. A point where
VCE and IC are fixed when there's no
input signal. This is known as the bias
point. Let's say we choose a VCE of 2 V.
According to the load line, that
corresponds to a collector current IC of
3 milliamps. This value of IC comes from
setting the base current IB to 30 micro
as we control by adjusting VBB just like
Now let's vary the base current and
observe how IC responds. If we gradually
increase IB to 50 micro amps, the
transistor responds by shifting along
the load line to match this new base
current. As a result, IC rises to about
5 milliamps.
Likewise, if we reduce IB to 20 micro
amps, the transistor moves to a
different point along the load line and
So with just a small change in the base
current, we see a much larger change in
That's the core principle of
amplification. The transistor takes a
tiny input signal, just a few micro
amps, and turns it into a much larger
output current at the collector,
measured in milliamps. That's what makes
it such a powerful building block in electronics.
electronics.
There's one final concept we need to
cover when it comes to transistors, and
that's the bias point. As we've seen,
before a transistor can amplify any
changes in base current, we first need
to set it up in the active region. That
means choosing the right starting values
for both VCE and IB even before any
input signal is applied. Let's look at
three different bias points and see how
each one affects amplification. In the
first case, the bias point is set right
in the middle of both the VCE and IC
ranges. To place the transistor here, we
set IB to 30 micro amps, which gives us
IC equals 3 milliamps, just like we
Now, if we slightly increase or decrease
IB, those small micro amp changes are
amplified into clean symmetrical swings
in IC. This is exactly what we want from
a well-biased amplifier.
In the second case, the bias point is
set too low. VCE is close to zero, right
near the saturation region. Here, IB is
set to 50 micro as gives us IC= 5
milliamps, already near the circuit's
If we try to increase IB any further,
the transistor leaves the active region
and enters full saturation. At this
point, VCE drops sharply and IC
instantly jumps to its maximum value,
just like a fully closed switch. Once
it's saturated, IC stops responding to
changes in IB. So, even if we continue
varying IB like before, the transistor
no longer amplifies the full signal.
Only a portion of the waveform gets
amplified, the rest is flattened or
clipped. In other words, the output
becomes distorted and the signal is only
partially amplified with no headroom
left for IC to increase. This bias point
is not suitable for clean linear amplification.
Now let's look at the third case where
the bias point is set too high. VCE is
near VCC and IC is very low putting the
transistor near the cutff region. To
reach this state, we set IB to just 10
While this setup allows IB to increase
and push IC upward, there's very little
room for IB to decrease. If IB drops to
zero, the transistor hits off and IC
drops to zero with it. So, this bias
point restricts how far the signal can
swing downward. Again, not ideal for
clean amplification.
So when you're using a transistor as an
amplifier, it's crucial to choose a bias
point right in the center of the active
region. This gives your signal plenty of
room to swing both upward and downward
without hitting saturation or cutff and
without causing distortion. As a general
rule of thumb, we place the bias point
right at the midpoint of the load line.
That means the collector emitter voltage
VCE should be around half of VCC and the
collector current IC should be
approximately VCC divided by RC divided
by two. In other words, about halfway up
the load line. This setup gives you
maximum undistorted amplification, keeps
the transistor operating squarely in the
active region, and ensures your
amplifier performs exactly as intended.
And that's all for this lesson. I hope
you now have a solid understanding of
what a transistor is, how to use it as a
switch or an amplifier, and the
importance of the qoint in setting up a
stable amplifier. If you find my content
valuable and want to support what I do,
you can encourage me by joining the
Patreon community. And don't forget to
like, subscribe, and stay tuned to
Provmad for more exciting and
educational videos. I always welcome
your comments and questions and I do my
best to reply as quickly as I can. But
if there's ever a delay, I appreciate
your patience. Feel free to reach out to
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