Transistors Explained - How transistors work

This is a transistor

It is one of the most important devices ever

to be invented.

So, we're going to learn how they work in detail in this video.

What is a transistor?

Transistors come in many shapes and sizes.

There are two main types, the bipolar and the field effect.

We're going to mostly focus on the bipolar version in this video.

Transistors are small electronic components with two main functions.

It can act as a switch to control circuits

and they can also amplify signals.

Small low power transistors are enclosed

in a racing case to help protect the internal parts.

But higher power transistors will have a partly metal case, which is used to help

remove the heat which is generated as this will damage the components over time.

We usually find these metal body transistors

attached to a heat sink, which helps remove the unwanted heat.

For example, inside this DC Bench power supply

We have some mosfet transistors which are attached to very large heat sinks.

Without the heat sink

the components quickly reach 45 degrees Celsius or 113 degrees Fahrenheit.

With a current of just 1.2A.

They will become much hotter as the current increases.

But for electronic circuits with small currents, we can just use these resin body transistors

which do not require a heat sink.

On the body of the transistor.

We find some text.

This will tell us the part number which we

can use to find the manufacturers datasheet.

Each transistor is rated to handle

a certain voltage and current, so it is important to check these sheets.

Now with the transistor we have three pins labelled E, B and C.

This stands for the emitter, the base and the collector.

Typically with these resin body type TRANSISTORS

with a flat edge,

the left pane is the emitter,

the middle is the base, and the right side is the collector.

However, not all transistors use this configuration.

So do check the manufacturers datasheet.

We know that if we connect a light bulb to a battery, it will illuminate.

We can install a switch into the circuit

and control the light by interrupting the power supply.

But this requires a human to manually control the switch.

So how can we automate this?

For that, we use a transistor.

This transistor is blocking the flow of current.

So the light is off.

But if we provide a small voltage to the base pane in the middle,

it causes the transistor to start allowing current to flow in the main circuit.

So the light turns on.

We can then place a switch on the controlling pin to operate it remotely

or we can place a sensor on this to automate the control.

Typically, we need to apply at least 0.6V

to 0.7 volts to the base pin for the transistor to turn on.

For example, this simple transistor circuit

has a red LED and a nine volt power supply across the main circuit.

The base pin is connected to the DC Bench power supply

The circuit diagram looks like this.

When the supply voltage to the base pin is

0.5V the transistor is off.

So the LED is also off

at 0.6V the transistor is on, but not fully.

The LED is dim because the transistor is not yet letting the full current flow

through the main circuit.

Then at 0.7V the lead is brighter because the tran

sistor is letting almost the full current through.

At 0.8V, the LED is at full brightness.

The transistor is fully open.

So what's happening is we're using a small

voltage and current to control a larger voltage and current.

We saw that a small change to the voltage on the base pin

causes a large change on the main circuit.

Therefore, if we input a signal to the base pin,

the transistor acts as an amplifier.

We could connect a microphone which varies

the voltage signal on the base pin, and this will amplify a speaker in the main circuit

to form a very basic amplifier.

Typically, there is a very small current on the base pin,

perhaps just 1mA or even less.

The collector has a much higher current, for example, 100mA.

The ratio between these two is known as the current gain and uses the symbol beta

We can find the ratio in the manufacturers datasheet.

In this example, the collector current is 100mA

and the base current is 1mA.