Everything You Need To Know About Bipolar Junction Transistor

Bipolar junction transistors are three-terminal semiconductor devices. They are mainly used as amplifiers, switches, and voltage-controlled current sources.

A bipolar junction transistor is made of two types of semiconductor material — n-type and p-type. Depending on the type of semiconductor, these transistors are classified as either bipolar NPN or bipolar PNP. This article covers everything you need to know about BJT.

Table of Contents

How Does a Bipolar Junction Transistor Work?

A bipolar junction transistor is a three-terminal semiconductor device. It is a combination of two separate diodes: an n-type and a p-type semiconductor. The two junctions create an NPN or a PNP structure.

The two diodes are connected in parallel and are called the “emitter” and the “collector”. Each of these diodes has a very high resistance when no current flows through it. A small current through the emitter-base junctions controls a large current flow through the collector-emitter junctions.

Bipolar Junction Transistor Operation

When it comes to the actual operation, NPN and PNP transistors operate in a similar manner. The following is a general overview of how they work:

-When the input voltage (Vbe) is lower than the base voltage (Vce), the base-emitter junction will be reverse biased. This means that no current will flow through the transistor.

-When Vbe is higher than Vce, the base-emitter junction will be forward biased. This means that current will flow through the transistor.

The amount of current flowing through the transistor depends on how much Vce and Vbe differ from each other. This difference is called “Voltage Gain”. The higher this difference, the more current will flow through it, making it a very useful feature for amplifying signals among many other uses.

Operating regions of a BJT

Regardless of the type,a bipolar junction transistor has three main operating regions: These are active, saturation region, and cut-off region. What happens in these regions?

-Active region: This is the region where a BJT is able to amplify the input signal. This region can be divided into two sub-regions, linear and non-linear. The linear region is where the transistor is able to control current flow with a small voltage change. The non-linear region is where the transistor requires a large voltage change in order to control current flow.

-Saturation Region: This is the region where a BJT has maximum current flow with minimum base-emitter voltage (Vbe).

-Cutoff Region: This is the region where a BJT has minimum current flow with maximum base-emitter voltage (Vbe). In this region, we can comfortably say that the transistor is fully off since there is no current flowing through.

Structure of a BJT

The basic structure of a bipolar junction transistor looks like this — Both the n-type and p-type semiconductors are sandwiched between two layers of conducting materials. They are separated by the two p-n junctions. The layers of conducting material are referred to as the collector, base, and emitter.

When the layers of semiconductor material and the layers of conducting material are all stacked together, they form the structure of a BJT.

Types of Bipolar Junction Transistors

There are two types of BJTs – the NPN and the pnp transistor. The two types of transistors are very similar in design and functionality, but they are made from different semiconductor materials. Let’s have a brief overview of the two:

NPN bipolar junction transistor

In this type of BJT, the p-type semiconductor is sandwiched between two n-type semiconductors. The name “NPN” is derived from this structure. The two n-type conductors in this transistor function as an emitter and collector respectively. The p-type in the middle will function as a base.

In the NPN BJT, current enters the transistor via the collector. The current then passes through the base region and exits from the emitter. The base and emitter form a junction, which is where the name ‘Bipolar Junction Transistor’ comes from.

PNP Bipolar Junction Transistor

This is the opposite of the NPN BJT. In this transistor, the n-type semiconductor is sandwiched between the two p-type semiconductors. The two p-type act as collectors and emitters while the n-type serves as a base.

BJT Configurations

At this point, we know that a bipolar unction transistor has three terminals. So, how is it connected to an electric circuit?

There are three configurations for connected BJT to a circuit. These are;

-Common Emitter- Here, the collector is connected to the positive terminal of a voltage source, and the emitter is connected to ground.

-Common Base- Here, the base is connected to the positive terminal of the voltage source. The emitter and collector are both connected to ground.

-Common Collector- In this configuration, the collector is attached to electric ground, while both base and emitter are attached to a positive voltage source.

The NPN and PNP transistors can be configured in any one of these three ways. However, keep in mind that BJTs in general will have different characteristics when configured differently.

For instance, when it comes to a common emitter configuration, there will be a high power gain coupled with medium voltage/current gains. This configuration also tends to have a high output impedance and exhibits a 10 degrees phase angle.

For the common base configuration, the power gain and the current gain will be low while the voltage gain will be high. It will not have any phase angle about will exhibit high output impedance.

What about the common collector configuration? The BJT will have a medium power gain and a very low voltage gain. It will also have a low output impedance and a zero phase angle.

Advantages of Bipolar Junction Transistors

Bipolar junction transistors have various features and characteristics that make them not only ideal but also suitable for their respective areas of application. Some of these advantages include:

-Can accept low input current and transform it into high output: As we have mentioned, BJTs serve as amplifiers. Do you know why? It is because of their ability to accept low input current, then transform them into high output.

-Low input bias current: Bipolar transistors are capable of accepting very small biases and can operate in the very small currents.

-Low power dissipation: The bipolar transistor has a low power dissipation compared to the other transistor types. For example, if a bipolar transistor is put at a given temperature, it will dissipate less power than the other types of transistors.

-Wide power supply voltages: The bipolar transistor is suitable for operating with wide power supply voltages because it can tolerate these higher voltages without any problem.

-High phase angle: The bipolar transistor has an extremely high phase angle compared to other types of transistors. This feature makes it suitable for use in high speed circuits such as digital circuits and FFTs.

-When used as switches, BJTs are affected by physical bounce like their mechanical counterparts. A bounce is a phenomenon wherein the energy stored in the transistor is released suddenly when a switch is opened. This sudden release of energy can have effects on the other transistors in a circuit, especially the ones used for amplification.

Disadvantages of Bipolar Junction Transistors

Bipolar junction transistors are not without their flaws. These transistors have a high current flow through them. They also have low voltage ratings. Another disadvantage of using bipolar junction transistors is that they are not suitable for high-frequency applications. They are also not very stable when their environment changes. The current flow in a BJT changes when the temperature of the surrounding environment changes. This is not a good thing for circuits that need constant output values.

Uses of BJT

Bipolar junction transistors have three main uses. These are amplifiers, oscillators, switch, and as demodulators.

As an amplifier, a BJT can amplify very small signals. This is because they are able to control a large amount of current. They can also amplify a signal over a wide range of frequencies.

As a switch, a BJT can be used as a switch. This is because they quickly turn on and off when the input signal goes low and high.

A bipolar junction transistor oscillator can produce high frequencies. These transistors have a wide frequency range that makes them suitable for applications that need high speed operation.

The bipolar junction transistors are used as demodulators in radio receivers, which convert radio waves into audio signals and vice versa.

What are the application areas of BJT?

Now that you know their uses, let’s look at some of the application areas. These include;

-Radio: We all know that a radio consists of an antenna, radio tuner, amplifier and loudspeaker. The receiver is called the demodulator and it uses the BJT to demodulate the signals.

-Microphones: These are used to convert sound waves into electrical signals. They use a BJT to amplify these signals so that they can be heard clearly on a speaker.

-Lighting: The bulbs in your house have tungsten filaments that are heated by electricity to produce light. A BJT is used in the circuit to control how much current flows through the filament. This is why you can adjust the brightness of your lights using a dimmer switch.

-Transistors are also used in electronic calculators, computers, and cell phones.

-Instrumentation: In this application, a BJT is used as an amplifier for electrical measurements because it can handle high voltages that are needed for this application area.

-Electrical machines: A transistor is used in a motor to control the speed of the motor and also in an electric clock.

-Television: In this application, a transistor is used as an amplifier for the picture signal so that it can be displayed on the screen.

They are also used in interrupters, amplifiers, and oscillators (oscillators use them to generate audio signals).

-Telecommunication systems such as mobile phones and radio receivers where the transistor is used for the amplification of the signals.

Specifications of a Bipolar Junction Transistor

Not all the BJts are the same. They come with different specifications. When buying a bipolar junction transistor, it is vital to pay attention to the following specs:

-Maximum collector current (Ic): This is the current that can be passed through the collector terminal of the transistor. It is measured in amps.

-Maximum collector-emitter voltage (Vce): This is the voltage that can be applied to a transistor’s collector terminal. It is usually in volts.

-Maximum base current (Ibc): This is the current that can be passed through the base terminal of a transistor. The maximum base current depends on the type of IC.

-Maximum emitter-to-emitter voltage (Vee): This is the voltage that can be applied to two transistors’ emitter terminals and depends on their types too.

-Collector to emitter breakdown voltage (BVce): This is the voltage that can be applied to two transistors’ collector terminals and depends on their types too.

-Maximum collector base-emitter current (Icb): This is the current that can be passed through the base terminal of a transistor. It is measured in amps.

-Maximum base-to-emitter voltage (Vbe): This is the voltage that can be applied to two transistors’ base terminals and depends on their types too.

Like other electronics, ensure that the specifications of the BJT that you are about to buy match the specifications of the circuit that you are about to build.

Where to buy a bipolar junction transistor?

You should buy from reputable and reliable online shopping websites that have good feedback and reputation. If you are planning to buy BJT in China, it is advisable to use a sourcing agent to help you with the purchase.

For example, ICRFQ is a well-known sourcing agent that has a wide range of electronic components and also offers prices, shipping costs, payment methods, return policies and other details of the products that you buy.

You should also check the specifications and datasheets of the BJT for your project. If they match, then you are free to proceed with the purchase.

Conclusion

Bipolar junction transistors are quite useful electric component in modern applications. They are commonly used in electronic circuits to amplify electrical signals, control power and even switch on or off other circuits.

If you have any question about bipolar junction transistor, please feel free to contact us for advice.