Bipolar Junction Transistor
(BJT)
Description
BJT stands for Bipolar Junction Transistor. It is a very important part of circuits that use electricity. There would be no electronics without transistors. These tiny parts, called transistors, make your phone, computer, or even a simple calculator work. The Bipolar Junction Transistor, or BJT for short, is one of these common types of transistors.
What is a BJT?
The Bipolar Junction Transistor (BJT) is a type of transistor that has two poles. This is a small semiconductor that can either turn signals on or off or make them stronger. It controls the flow of current in an electronic circuit. It is termed bipolar because it uses two types of charge carriers: electrons and holes.
Semiconductors are the material of which BJT is made, predominantly silicon. It contains three parts or terminals, which are:
(i) Emitter
(ii) Base
(iii) Collector
Different types of BJT
There are generally two BJTs:
1. NPN Transistor: Two layers of N-type material sandwich a layer of P-type material.
2. PNP Transistor: Two layers of P-type material sandwich an N-type layer.
Both these types function in the same basic idea, only opposite directions of current flow.
How does BJT work?
BJT is an electrical valve. The base-to-collector current flow is very small relative to the collector-to-emitter current flow, which can be quite large.
A tiny base-to-emitter current in the NPN transistor is capable of letting a far greater collector-to-emitter current pass.
This characteristic has rendered BJTs very convenient in two primary tasks:
Amplification – enhancing the magnitude of weak input signal
A device which allows the current in a circuit to pass or not to pass.
Real-World Example
Consider a microphone. When you talk, your voice creates small audio waves, and when you are whispering, these are very small only. You can forward them to a loudspeaker or use BJT amplifier for clear recording. This is how BJT is functioning in real-life devices.
BJT parts and their roles
Let’s consider the three parts in more detail:
1. Emitter: Lets charge carriers (electrons or holes) go. Heavily doped (more charge carriers)
2. Base The middle layer Very thin and not very doped Controls how many charge carriers go to the collector
3. Collector: This part collects the charge carriers from the emitter. Doped to a moderate level
Made to work with high voltages
What really is the importance of BJT? BJT are:
• They have been around for decades and are therefore pretty reliable.
Fast—they can be switched on and off very fast.
Inexpensive—used in many things and doesn’t cost a lot to make.
Versatility: It may be employed across a broad array of electronic circuits.
Applications
They find extensive application in BJTs due to the above factors:
Amplification of sound
Radio transmission
Oscillation
Power regulation
Analog and/or digital logic circuits
Advantages of BJTs
They exhibit high gain thus amplifying signals quite distinctly.
Most suitable for analog circuits
Pleasant and friendly to use in analog circuits
BJTs can handle higher power levels than many MOSFETs, but this makes them more preferable to power applications and less ideal for digital systems where they must switch quickly. These devices are still widely used in various applications; for instance, conditions such as analog signal processing are most suitable for BJTs as compared to some of the noted disadvantages.
BJT against MOSFET
You may have heard about MOSFETs in today’s electronics – Metal Oxide Semiconductor Field Effect Transistors. They are pretty ubiquitous in digital systems. By comparison with BJTs:
BJTs are better suited to analog circuits.
MOSFETs are better suited to digital circuits.
They both each have their purpose, and many circuits will have both of them.
BJTs Work in Circuits
Normally, a symbol in circuit diagrams represents a BJT. You can see whether the symbol is for an NPN or PNP transistor just by arrow direction.
Some of the applications are:
BJTs amplify audio signals to make them louder in a sound amplifier.
BJTs help light sensors turn lights on or off depending on how bright it is.
They were used in early computer processors before integrated circuits came into the picture.
BJT is a small component that plays a huge role in electronics. It can accurately control, amplify, and switch electrical signals. Among the very first things you learn in electronics is BJTs.
This will help you to know more about the technology around you if you are a student, an amateur, or merely curious about understanding the functionality of gadgets. New devices are entering the market, but still, BJTs find application in most circuits. This tells us that sometimes just doing simple and effective things is all that’s needed.