What is Bipolar Junction Transistor?
Bipolar Junction Transistor (BJT) is an electronics device which is widely used in electronics circuits which act as a current controlled switch or amplifier. Its operation involves both majority and minority charge carriers, i.e., electrons and holes, which is why it is called “Bipolar.” This makes it different from unipolar devices like field-effect transistors (FETs).
It is used in microcontrollers to handle loads that require more current thanthe microcontroller’s GPIO pins. Overall, bipolar junction transistor amplifies or switches electrical signals in electronic circuits involving microcontrollers, connectors, sensors, relays, and power supply modules.
Bipolar Junction Transistor Symbol
Bipolar junction transistor symbol consists of three terminals, i.e., Emitter (E), Base(B), and Collector (C). In BJT, the collector is usually set at the top of the symbol, the base in the middle, and the emitter at the bottom, which makes schematic diagrams easy to interpret. Bipolar junction transistor is of two types - NPN and PNP transistor.
In an NPN transistor, an arrow on the emitter side points outward, which signifies conventional current flow out of the emitter. In PNP transistor, an arrow on the emitter side points inward, which indicates that current is flowing into the emitter.
Function of Bipolar Junction Transistor
- The main application of bipolar junction transistor is to act as an electronic switch or amplifier in various electronic circuits.
- By its switching mechanism, BJT turns the electric signals on or off, which allows it to control high current loads with a low power input signal.
- As an amplifier, BJT is used to increase the strength of weak input signals, which provides signal amplification for audio, radi,o and sensor circuits.
- BJT’s impedance matching function ensures maximum power transfer between circuit stages in analog and digital systems.
- By configuring the BJT in different modes like common-emitter, common-base, and common-collector, it can be used for specific amplification or switching functions.
Bipolar Junction Transistor Symbol
Depending on the arrangement of the n-type and p-type semiconductor layers, BJT can be constructed in two types - NPN and PNP. In NPN transistor, a thin p-type base region is placed between two n-type regions, forming an emitter-base and base-collector junction, and in PNP transistor, a thin n-type base region is sandwiched between two p-type regions.
To inject a large number of charge carriers (electrons in NPN, holes in PNP) into the base, the emitter region is heavily doped. To allow most of the carriers from the emitter to pass through to the collector, the base region is very thin and lightly doped, and to collect charge carriers efficiently and dissipate heat generated during operation, the collector region is moderately doped.
Operation of Bipolar Junction Transistor
The operation of bipolar junction Transistor (BJT) is based on the movement of charge carriers through its three regions, i.e., emitter, base, and collector. BJT operates in three regions: cut-off, active, and saturation region.
Active region allows the BJT to amplify signals in which the emitter-base junction is forward-biased while the collector-base junction is reverse-biased. In the cut-off region, no current flows from collector to emitter because both junctions are reverse-biased and the transistor acts like an open switch. In the saturation region, both junctions are forward-biased, causing maximum current flow, and the transistor behaves like a closed switch.
There is a small base current which controls larger current flowing from collector to emitter, which is the basis for signal amplification.
Types of Bipolar Junction Transistor
Based on the arrangement of semiconductor layers, bipolar junction transistor is classified into NPN and PNP transistors.
1. NPN Bipolar Junction Transistor
NPN transistor consists of p-type base region sandwiched between two n-type regions the emitter and collector. In this BJT, electrons are the majority charge carriers that flow from the emitter to the collector when a small base current is applied.
In circuit diagrams, the symbol for an NPN transistor shows an arrow pointing outward from the emitter terminal, indicating the direction of conventional current flow. The NPN type is widely used in switching and amplification applications because it offers better electron mobility, resulting in higher current gain.
2. PNP Bipolar Junction Transistor
PNP transistor is constructed with a thin n-type base region between two p-type regions, the emitter and collector. The symbol for a PNP transistor has an arrow pointing inward towards the emitter, showing the conventional current entering the transistor.
Holes are the majority charge carriers in PNP BJT, and current flows from the emitter to the collector when the base is made more negative than the emitter. PNP transistors are often used where a negative supply is needed or where sourcing current to a load is required.
Configuration of Bipolar Junction Transistor
Configurations of BJT depend on how emitter, base, and collector are connected within a circuit.
1. Common Emitter
In the Common Emitter configuration, the emitter terminal is common to both input and output circuits. It provides high current and voltage gain, making it the most widely used configuration for amplifiers and switches. The Common Emitter configuration also inverts the input signal, producing a 180-degree phase shift between input and output.
2. Common Base
In the Common Base configuration, the base terminal is common to both input and output. This configuration offers low input impedance, high output impedance, and is mainly used for high-frequency applications because of its excellent stability.
3. Common Collector
The Common Collector configuration, also known as an emitter follower, has the collector terminal common to input and output. It provides high input impedance and low output impedance, making it ideal for impedance matching and buffering signals.
Applications of Bipolar Junction Transistor
- Bipolar junction transistors are used as signal amplifiers in audio amplifiers, radio frequency amplifiers, and sensor signal conditioning circuits to boost weak analog signals.
- They act as electronic switches in microcontroller-based systems, controlling high-current devices like motors, solenoids, and actuators with low-power logic signals.
- In relay driver circuits, BJTs provide the necessary current to energize relay coils, ensuring safe and efficient control of AC or DC loads.
- BJTs are integral in power supply modules, where they function in voltage regulator circuits, current limiters, and overcurrent protection systems.
- They are used in oscillator circuits for generating stable waveforms required in communication and signal processing applications.
- BJTs also play a role in signal modulation and demodulation circuits, essential for data transmission in communication systems.
FAQ- Bipolar Junction Transistor
1. What is the Difference Between BJT and FET?
BJT is a current-controlled device that uses both electron and hole charge carriers. At the same time, a FET is a voltage-controlled device that relies on either electrons (n-channel) or holes (p-channel) for operation.
2. Why is BJT Called Bipolar?
BJT is called bipolar because it uses both electrons and holes as charge carriers for its operation.
3. How Does a BJT Amplify Current?
BJT amplifies current by using a small base current to control a much larger collector current flowing from collector to emitter.
4. How to test a BJT Transistor Using a Multimeter?
To test a BJT transistor using a multimeter, check the forward and reverse bias of the base-emitter and base-collector junctions, similar to diodes — both should conduct in only one direction.
5. What is the Role of Base, Collector, and Emitter in a BJT?
In a BJT, the base controls the transistor’s operation, the emitter injects charge carriers, and the collector collects them to enable current flow and amplification.
