Transistors Explained - How transistors work
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This is a transistor
This is a transistor
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It is one of the most important devices ever
It is one of the most important devices ever
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to be invented.
to be invented.
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So, we're going to learn how they work in detail in this video.
So, we're going to learn how they work in detail in this video.
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What is a transistor?
What is a transistor?
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Transistors come in many shapes and sizes.
Transistors come in many shapes and sizes.
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There are two main types, the bipolar and the field effect.
There are two main types, the bipolar and the field effect.
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We're going to mostly focus on the bipolar version in this video.
We're going to mostly focus on the bipolar version in this video.
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Transistors are small electronic components with two main functions.
Transistors are small electronic components with two main functions.
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It can act as a switch to control circuits
It can act as a switch to control circuits
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and they can also amplify signals.
and they can also amplify signals.
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Small low power transistors are enclosed
Small low power transistors are enclosed
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in a racing case to help protect the internal parts.
in a racing case to help protect the internal parts.
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But higher power transistors will have a partly metal case, which is used to help
But higher power transistors will have a partly metal case, which is used to help
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remove the heat which is generated as this will damage the components over time.
remove the heat which is generated as this will damage the components over time.
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We usually find these metal body transistors
We usually find these metal body transistors
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attached to a heat sink, which helps remove the unwanted heat.
attached to a heat sink, which helps remove the unwanted heat.
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For example, inside this DC Bench power supply
For example, inside this DC Bench power supply
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We have some mosfet transistors which are attached to very large heat sinks.
We have some mosfet transistors which are attached to very large heat sinks.
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Without the heat sink
Without the heat sink
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the components quickly reach 45 degrees Celsius or 113 degrees Fahrenheit.
the components quickly reach 45 degrees Celsius or 113 degrees Fahrenheit.
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With a current of just 1.2A.
With a current of just 1.2A.
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They will become much hotter as the current increases.
They will become much hotter as the current increases.
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But for electronic circuits with small currents, we can just use these resin body transistors
But for electronic circuits with small currents, we can just use these resin body transistors
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which do not require a heat sink.
which do not require a heat sink.
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On the body of the transistor.
On the body of the transistor.
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We find some text.
We find some text.
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This will tell us the part number which we
This will tell us the part number which we
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can use to find the manufacturers datasheet.
can use to find the manufacturers datasheet.
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Each transistor is rated to handle
Each transistor is rated to handle
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a certain voltage and current, so it is important to check these sheets.
a certain voltage and current, so it is important to check these sheets.
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Now with the transistor we have three pins labelled E, B and C.
Now with the transistor we have three pins labelled E, B and C.
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This stands for the emitter, the base and the collector.
This stands for the emitter, the base and the collector.
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Typically with these resin body type TRANSISTORS
Typically with these resin body type TRANSISTORS
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with a flat edge,
with a flat edge,
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the left pane is the emitter,
the left pane is the emitter,
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the middle is the base, and the right side is the collector.
the middle is the base, and the right side is the collector.
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However, not all transistors use this configuration.
However, not all transistors use this configuration.
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So do check the manufacturers datasheet.
So do check the manufacturers datasheet.
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We know that if we connect a light bulb to a battery, it will illuminate.
We know that if we connect a light bulb to a battery, it will illuminate.
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We can install a switch into the circuit
We can install a switch into the circuit
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and control the light by interrupting the power supply.
and control the light by interrupting the power supply.
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But this requires a human to manually control the switch.
But this requires a human to manually control the switch.
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So how can we automate this?
So how can we automate this?
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For that, we use a transistor.
For that, we use a transistor.
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This transistor is blocking the flow of current.
This transistor is blocking the flow of current.
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So the light is off.
So the light is off.
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But if we provide a small voltage to the base pane in the middle,
But if we provide a small voltage to the base pane in the middle,
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it causes the transistor to start allowing current to flow in the main circuit.
it causes the transistor to start allowing current to flow in the main circuit.
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So the light turns on.
So the light turns on.
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We can then place a switch on the controlling pin to operate it remotely
We can then place a switch on the controlling pin to operate it remotely
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or we can place a sensor on this to automate the control.
or we can place a sensor on this to automate the control.
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Typically, we need to apply at least 0.6V
Typically, we need to apply at least 0.6V
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to 0.7 volts to the base pin for the transistor to turn on.
to 0.7 volts to the base pin for the transistor to turn on.
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For example, this simple transistor circuit
For example, this simple transistor circuit
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has a red LED and a nine volt power supply across the main circuit.
has a red LED and a nine volt power supply across the main circuit.
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The base pin is connected to the DC Bench power supply
The base pin is connected to the DC Bench power supply
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The circuit diagram looks like this.
The circuit diagram looks like this.
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When the supply voltage to the base pin is
When the supply voltage to the base pin is
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0.5V the transistor is off.
0.5V the transistor is off.
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So the LED is also off
So the LED is also off
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at 0.6V the transistor is on, but not fully.
at 0.6V the transistor is on, but not fully.
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The LED is dim because the transistor is not yet letting the full current flow
The LED is dim because the transistor is not yet letting the full current flow
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through the main circuit.
through the main circuit.
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Then at 0.7V the lead is brighter because the tran
Then at 0.7V the lead is brighter because the tran
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sistor is letting almost the full current through.
sistor is letting almost the full current through.
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At 0.8V, the LED is at full brightness.
At 0.8V, the LED is at full brightness.
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The transistor is fully open.
The transistor is fully open.
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So what's happening is we're using a small
So what's happening is we're using a small
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voltage and current to control a larger voltage and current.
voltage and current to control a larger voltage and current.
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We saw that a small change to the voltage on the base pin
We saw that a small change to the voltage on the base pin
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causes a large change on the main circuit.
causes a large change on the main circuit.
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Therefore, if we input a signal to the base pin,
Therefore, if we input a signal to the base pin,
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the transistor acts as an amplifier.
the transistor acts as an amplifier.
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We could connect a microphone which varies
We could connect a microphone which varies
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the voltage signal on the base pin, and this will amplify a speaker in the main circuit
the voltage signal on the base pin, and this will amplify a speaker in the main circuit
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to form a very basic amplifier.
to form a very basic amplifier.
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Typically, there is a very small current on the base pin,
Typically, there is a very small current on the base pin,
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perhaps just 1mA or even less.
perhaps just 1mA or even less.
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The collector has a much higher current, for example, 100mA.
The collector has a much higher current, for example, 100mA.
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The ratio between these two is known as the current gain and uses the symbol beta
The ratio between these two is known as the current gain and uses the symbol beta
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We can find the ratio in the manufacturers datasheet.
We can find the ratio in the manufacturers datasheet.
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In this example, the collector current is 100mA
In this example, the collector current is 100mA
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and the base current is 1mA.
and the base current is 1mA.
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So the ratio is 100mA divided by 1mA, which gives us 100.
So the ratio is 100mA divided by 1mA, which gives us 100.
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We can also rearranges formula to find the currents also.
We can also rearranges formula to find the currents also.
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NPN and PNP transistors
NPN and PNP transistors
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We have two main types of bipolar transistors,
We have two main types of bipolar transistors,
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the NPN and the PNP type, the two transistors look nearly identical.
the NPN and the PNP type, the two transistors look nearly identical.
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So we need to check the part number to tell which is which.
So we need to check the part number to tell which is which.
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With an NPN transistor.
With an NPN transistor.
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We have the main circuit and the control circuit.
We have the main circuit and the control circuit.
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Both are connected to the positive of the battery.
Both are connected to the positive of the battery.
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The main circuit is off until we press the switch on the control circuit.
The main circuit is off until we press the switch on the control circuit.
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We can see the current is flowing through both wires to the transistor.
We can see the current is flowing through both wires to the transistor.
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We can remove the main circuit and the control circuit lED
We can remove the main circuit and the control circuit lED
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will still turn on when the switch is pressed
will still turn on when the switch is pressed
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as the current is returning to the battery through the transistor.
as the current is returning to the battery through the transistor.
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In this simplified example,
In this simplified example,
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when this switch is pressed, there are 5mA flowing into the base pin.
when this switch is pressed, there are 5mA flowing into the base pin.
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There are 20mA flowing into the collector pin
There are 20mA flowing into the collector pin
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and 25mA flowing out of the emitter.
and 25mA flowing out of the emitter.
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The current therefore combines in this transistor
The current therefore combines in this transistor
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With a PNP transistor.
With a PNP transistor.
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We again have the main circuit and the control circuit,
We again have the main circuit and the control circuit,
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but now the emitter is connected to the positive of the battery.
but now the emitter is connected to the positive of the battery.
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The main circuit is off until we press the switch on the control circuit.
The main circuit is off until we press the switch on the control circuit.
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We can see with this type that some of the current flows out of the base pin and returns to the battery.
We can see with this type that some of the current flows out of the base pin and returns to the battery.
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The rest of the current flows through
The rest of the current flows through
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the transistor and through the main led and then back to the battery.
the transistor and through the main led and then back to the battery.
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If we remove the main circuit, the control circuit, LED will still turn on.
If we remove the main circuit, the control circuit, LED will still turn on.
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In this example, when the switch is pressed,
In this example, when the switch is pressed,
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there are 25mA flowing into the emitter,
there are 25mA flowing into the emitter,
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20mA flowing out of the collector and 5mA flowing out of the base.
20mA flowing out of the collector and 5mA flowing out of the base.
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The current, therefore, divides in this transistor
The current, therefore, divides in this transistor
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I'll place these side by side so you can see how they compare.
I'll place these side by side so you can see how they compare.
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Transistors are shown on electrical drawings
Transistors are shown on electrical drawings
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with symbols like these, the arrow is placed on the emitter.
with symbols like these, the arrow is placed on the emitter.
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The arrow points in the direction
The arrow points in the direction
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of conventional current so that we know how to connect them into our circuits.
of conventional current so that we know how to connect them into our circuits.
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How does a transistor work
How does a transistor work
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To understand how a transistor works,
To understand how a transistor works,
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I want you to first imagine water flowing through a pipe.
I want you to first imagine water flowing through a pipe.
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It flows freely through the pipe until we block it with a disc.
It flows freely through the pipe until we block it with a disc.
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Now, if we connect a smaller pipe into the main one and place a swing gate
Now, if we connect a smaller pipe into the main one and place a swing gate
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within this small pipe, we can move the disc using a pulley.
within this small pipe, we can move the disc using a pulley.
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The further the swing gate opens,
The further the swing gate opens,
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the more water is allowed to flow in the main pipe.
the more water is allowed to flow in the main pipe.
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The swing gate is a little heavy,
The swing gate is a little heavy,
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so a small amount of water won't be enough to open it.
so a small amount of water won't be enough to open it.
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A certain amount of water is required to force the gate to open.
A certain amount of water is required to force the gate to open.
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The more water we have flowing in this small pipe,
The more water we have flowing in this small pipe,
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the further the valve opens and allows
the further the valve opens and allows
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more and more water to flow in the main pipe.
more and more water to flow in the main pipe.
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This is essentially how an NPN transistor works.
This is essentially how an NPN transistor works.
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You might already know that when we design electronic circuits,
You might already know that when we design electronic circuits,
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we use conventional current.
we use conventional current.
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So in this NPN transistor circuit,
So in this NPN transistor circuit,
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we assume that the current is flowing from the batteries positive
we assume that the current is flowing from the batteries positive
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into both the collector and the base pin and then out of the emitter pin.
into both the collector and the base pin and then out of the emitter pin.
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We always use this direction to design our circuits.
We always use this direction to design our circuits.
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However, that's not what's actually occurring.
However, that's not what's actually occurring.
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In reality, the electrons are flowing
In reality, the electrons are flowing
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from the negative to the positive of the battery.
from the negative to the positive of the battery.
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This was proved by Joseph Thompson, who carried out some experiments
This was proved by Joseph Thompson, who carried out some experiments
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to discover the electron and also prove they flowed in the opposite direction.
to discover the electron and also prove they flowed in the opposite direction.
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So in reality,
So in reality,
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electrons flow from the negative into the emitter and then out
electrons flow from the negative into the emitter and then out
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of the collector and the base pin. We call this electron flow.
of the collector and the base pin. We call this electron flow.
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I'll place the side by side so you can see the difference in the two theories.
I'll place the side by side so you can see the difference in the two theories.
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Remember, we always design circuits using the conventional current method.
Remember, we always design circuits using the conventional current method.
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But scientists and engineers know that electron flow is how it actually works
But scientists and engineers know that electron flow is how it actually works
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by the way, we have also covered how
by the way, we have also covered how
9:25
a battery works in detail in our previous video.
a battery works in detail in our previous video.
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Do you check that out
Do you check that out
9:29
links can be found in the video description down below.
links can be found in the video description down below.
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OK, so we know that electricity is the flow of electrons through a wire.
OK, so we know that electricity is the flow of electrons through a wire.
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The copper wire is the conductor and the rubber is the insulator.
The copper wire is the conductor and the rubber is the insulator.
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Electrons can flow easily through
Electrons can flow easily through
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the copper, but they can't flow through the rubber insulator.
the copper, but they can't flow through the rubber insulator.
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If we look at this basic model of an atom
If we look at this basic model of an atom
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of a metal conductor, we have the nucleus at the centre and this
of a metal conductor, we have the nucleus at the centre and this
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is surrounded by a number of orbital shells which hold the electrons.
is surrounded by a number of orbital shells which hold the electrons.
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Each shell holds a maximum number
Each shell holds a maximum number
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of electrons, and an electron needs to have a certain
of electrons, and an electron needs to have a certain
10:05
amount of energy to be accepted into each shell.
amount of energy to be accepted into each shell.
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The electrons located furthest away from the nucleus hold the most energy.
The electrons located furthest away from the nucleus hold the most energy.
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The outermost shell is known as the valence shell.
The outermost shell is known as the valence shell.
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A conductor has between one and three electrons in its valence shell.
A conductor has between one and three electrons in its valence shell.
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The electrons are held in place by the nucleus,
The electrons are held in place by the nucleus,
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but there is another shell known as the conduction band.
but there is another shell known as the conduction band.
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If an electron can reach this, then it can break free from the atom
If an electron can reach this, then it can break free from the atom
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and move to other atoms. With a metal atom such as copper.
and move to other atoms. With a metal atom such as copper.
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The valence shell and the conduction band overlap,
The valence shell and the conduction band overlap,
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so it's very easy for the electrons to move
so it's very easy for the electrons to move
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with an insulator the outermost shell is packed.
with an insulator the outermost shell is packed.
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There's very little to no room for an electron to join.
There's very little to no room for an electron to join.
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The nucleus has a tight grip
The nucleus has a tight grip
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on the electrons and the conduction band is far away.
on the electrons and the conduction band is far away.
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So the electrons can't reach this to escape.
So the electrons can't reach this to escape.
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Therefore, electricity cannot flow through this material.
Therefore, electricity cannot flow through this material.
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However, there's another material known as a semiconductor.
However, there's another material known as a semiconductor.
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Silicon is an example of a semiconductor.
Silicon is an example of a semiconductor.
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With this material,
With this material,
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there's one too many electrons in the valence shell for it to be a conductor.
there's one too many electrons in the valence shell for it to be a conductor.
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So it acts as an insulator.
So it acts as an insulator.
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But as the conduction band is quite close,
But as the conduction band is quite close,
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if we provide some external energy, some electrons will gain enough energy
if we provide some external energy, some electrons will gain enough energy
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to make the jump into the conduction band and become free.
to make the jump into the conduction band and become free.
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Therefore, this material can act as both an insulator and a conductor.
Therefore, this material can act as both an insulator and a conductor.
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Pure silicon has almost no free electrons.
Pure silicon has almost no free electrons.
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So what engineers do is dope the silicon
So what engineers do is dope the silicon
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with a small amount of another material which changes its electrical properties.
with a small amount of another material which changes its electrical properties.
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We call this P type and N type doping.
We call this P type and N type doping.
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We combine these materials to form the PN junction.
We combine these materials to form the PN junction.
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We can sandwich these together to form an NPN or PNP transistor.
We can sandwich these together to form an NPN or PNP transistor.
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Inside the transistor we have
Inside the transistor we have
12:07
the collector pin and the emitter pin
the collector pin and the emitter pin
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between these in an NPN transistor,
between these in an NPN transistor,
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we have two layers of N type material and one layer of P type.
we have two layers of N type material and one layer of P type.
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The base wire is connected to the P type layer
The base wire is connected to the P type layer
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in a PNP transistor this is just configured the opposite way.
in a PNP transistor this is just configured the opposite way.
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The entire thing is enclosed in a resin to protect the internal materials.
The entire thing is enclosed in a resin to protect the internal materials.
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Let's imagine the silicon hasn't been doped yet,
Let's imagine the silicon hasn't been doped yet,
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so it's just pure silicon inside.
so it's just pure silicon inside.
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Each silicon atom is surrounded by four other silicon atoms.
Each silicon atom is surrounded by four other silicon atoms.
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Each atom wants eight electrons in its valence shell
Each atom wants eight electrons in its valence shell
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but the silicon atoms only have four electrons in their valence shell,
but the silicon atoms only have four electrons in their valence shell,
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so they sneakily share an electron
so they sneakily share an electron
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with their neighbouring atom to get the 8 desire.
with their neighbouring atom to get the 8 desire.
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This is known as covalent bonding.
This is known as covalent bonding.
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When we add the N type material such as phosphorus,
When we add the N type material such as phosphorus,
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it will take the position of some of the silicon atoms.
it will take the position of some of the silicon atoms.
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The phosphorus atoms have five electrons in their valence shell.
The phosphorus atoms have five electrons in their valence shell.
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So as the silicon atoms are sharing electrons to get their desired eight,
So as the silicon atoms are sharing electrons to get their desired eight,
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they don't need this extra one, which means there's now extra electrons
they don't need this extra one, which means there's now extra electrons
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in the material and these are free to move around
in the material and these are free to move around
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with P type doping we add in a material such as aluminium.
with P type doping we add in a material such as aluminium.
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This atom has only three electrons in this valence shell.
This atom has only three electrons in this valence shell.
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It therefore can't provide its four neighbours with an electron to share.
It therefore can't provide its four neighbours with an electron to share.
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So one of them will have to go without.
So one of them will have to go without.
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This means a hole has been created where an electron can sit and occupy.
This means a hole has been created where an electron can sit and occupy.
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We now have two doped pieces of silicon,
We now have two doped pieces of silicon,
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one with too many electrons and one we not enough electrons.
one with too many electrons and one we not enough electrons.
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The two materials join to form a PN junction.
The two materials join to form a PN junction.
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At this junction we get what's known as a depletion region
At this junction we get what's known as a depletion region
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in this region some of the excess electrons
in this region some of the excess electrons
14:05
from the N side will move over to occupy the holes in the P side.
from the N side will move over to occupy the holes in the P side.
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This migration will form a barrier
This migration will form a barrier
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with a build up of electrons and holes on opposite sides.
with a build up of electrons and holes on opposite sides.
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The electrons are negatively charged and the holes are therefore considered positively charged,
The electrons are negatively charged and the holes are therefore considered positively charged,
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so this Build-Up causes a slightly negatively charged region
so this Build-Up causes a slightly negatively charged region
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and a slightly positively charged region.
and a slightly positively charged region.
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This creates an electric field
This creates an electric field
14:32
and prevents more electrons from moving across.
and prevents more electrons from moving across.
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The potential difference across this region is typically around 0.7V
The potential difference across this region is typically around 0.7V
14:41
when we connect a voltage source across the two ends
when we connect a voltage source across the two ends
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with the positive connected to the P type material.
with the positive connected to the P type material.
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This will create a forward bias and the electrons will begin to flow.
This will create a forward bias and the electrons will begin to flow.
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The voltage source has to be greater than the 0.7V barrier.
The voltage source has to be greater than the 0.7V barrier.
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Otherwise, electrons cannot make the jump
Otherwise, electrons cannot make the jump
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when we reverse the power supply so that the positive is connected to the N type material.
when we reverse the power supply so that the positive is connected to the N type material.
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The electrons held in the barrier will be pulled back towards the positive terminal
The electrons held in the barrier will be pulled back towards the positive terminal
15:12
and the holes will be pulled back towards the negative terminal.
and the holes will be pulled back towards the negative terminal.
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This has caused a reverse bias
This has caused a reverse bias
15:19
in a NPN transistor.
in a NPN transistor.
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We have two layers of N type material, so we have two junctions and therefore two barriers,
We have two layers of N type material, so we have two junctions and therefore two barriers,
15:28
so no current can flow through it ordinarily.
so no current can flow through it ordinarily.
15:31
The emitter N type material is heavily doped,
The emitter N type material is heavily doped,
15:35
so there are a lot of excess electrons here.
so there are a lot of excess electrons here.
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The base P type is lightly doped, so there are a few holes here.
The base P type is lightly doped, so there are a few holes here.
15:44
The collector N type is moderately doped,
The collector N type is moderately doped,
15:47
so there are a few excess electrons here.
so there are a few excess electrons here.
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If we connect a battery across the base and the emitter with the positive
If we connect a battery across the base and the emitter with the positive
15:54
connected to the P type layer,
connected to the P type layer,
15:57
this will create a forward bias.
this will create a forward bias.
15:59
The forward bias causes the barrier to collapse
The forward bias causes the barrier to collapse
16:02
as long as the voltage is at least 0.7V.
as long as the voltage is at least 0.7V.
16:06
So the barrier diminishes
So the barrier diminishes
16:08
and the electrons rush across to fill the space within the P type material.
and the electrons rush across to fill the space within the P type material.
16:13
Some of these electrons will occupy a hole
Some of these electrons will occupy a hole
16:16
and they will be pulled towards the positive terminal of the battery.
and they will be pulled towards the positive terminal of the battery.
16:20
The P type layer is thin
The P type layer is thin
16:22
and was lightly doped on purpose so that there i
and was lightly doped on purpose so that there i
16:25
s a low chance of electrons falling into a hole.
s a low chance of electrons falling into a hole.
16:29
The rest will remain free to move around the material.
The rest will remain free to move around the material.
16:33
Therefore, only a small current will flow
Therefore, only a small current will flow
16:36
out of the base pin, leaving an excess of electrons in the pitot material
out of the base pin, leaving an excess of electrons in the pitot material
16:41
if we then connect another battery between the emitter and the collector
if we then connect another battery between the emitter and the collector
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with the positive connected to the collector,
with the positive connected to the collector,
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the negatively charged electrons within the collector
the negatively charged electrons within the collector
16:52
will be drawn to the positive terminal, which causes a reverse bias.
will be drawn to the positive terminal, which causes a reverse bias.
16:57
If you remember, with the reverse bias, the electrons and holes of the barrier are
If you remember, with the reverse bias, the electrons and holes of the barrier are
17:02
pulled back across, so the electrons on the P type side
pulled back across, so the electrons on the P type side
17:07
of the barrier are pulled across to the N type side
of the barrier are pulled across to the N type side
17:11
and the holes on the N type side are pulled back to the P type side.
and the holes on the N type side are pulled back to the P type side.
17:15
They are already an excess number of electrons in the P type material.
They are already an excess number of electrons in the P type material.
17:20
So they will move to occupy these holes and some of them will be pulled across
So they will move to occupy these holes and some of them will be pulled across
17:26
because the voltage of this battery is greater.
because the voltage of this battery is greater.
17:29
So the attraction is much higher.
So the attraction is much higher.
17:31
As these electrons are pulled across, they flow into the battery.
As these electrons are pulled across, they flow into the battery.
17:36
So a current develops across the reverse bias junction.
So a current develops across the reverse bias junction.
17:40
A higher voltage on the base pin fully opens the transistor,
A higher voltage on the base pin fully opens the transistor,
17:44
which means more current and more electrons moving into the P type layer.
which means more current and more electrons moving into the P type layer.
17:49
Therefore, more electrons are pulled across the reverse bias.
Therefore, more electrons are pulled across the reverse bias.
17:53
We also see more electrons flowing
We also see more electrons flowing
17:56
in the emitter side of the transistor compared to the collector side.
in the emitter side of the transistor compared to the collector side.
18:01
OK, that's it for this video,
OK, that's it for this video,
18:02
but to continue learning about electronics engineering, click on one of the videos
but to continue learning about electronics engineering, click on one of the videos
18:07
on screen now and I'll catch you there for the next lesson.
on screen now and I'll catch you there for the next lesson.
18:10
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18:12
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