Importance and Applications of BJTs

Bipolar Junction Transistors (BJTs) form the fundamental building blocks of a myriad of electronic devices. They are at the heart of most electronic circuits, from simple amplifiers in your radios to complex logic gates in your computers. Their primary applications include amplification, regulation, switching, and signal processing.

The invention of the BJT in the late 1940s revolutionized the electronics industry. The BJT was the first type of transistor to be mass-produced. Bell Labs’ scientists John Bardeen and Walter Brattain, under the supervision of William Shockley, were the pioneers who unveiled this groundbreaking technology, an achievement that later earned them the Nobel Prize in Physics.

Structure of BJTs

A BJT is a type of transistor that uses both electron and hole charge carriers. They are made of three layers of semiconductor material, either silicon or germanium, which can be doped to create two different types of BJTs: NPN and PNP.

The NPN transistor consists of two layers of N-type semiconductor material, with a layer of P-type sandwiched between them. Conversely, a PNP transistor consists of two layers of P-type material with an N-type layer in the middle.

These semiconductor layers form three terminal regions in the transistor: the emitter, base, and collector. The emitter emits (injects) charge carriers into the base, which controls the number of carriers that reach the collector.

Symbol and Modes of Operation

In circuit diagrams, the symbols for NPN and PNP transistors are depicted differently. The arrow in the symbol always points from the P-type material to the N-type material.

A BJT operates in three modes: active, cutoff, and saturation. The mode of operation is determined by the biasing (voltage levels) applied to the junctions between the emitter, base, and collector.

• In the active mode, the base-emitter junction is forward-biased, and the base-collector junction is reverse-biased. This mode is used when the BJT is applied for amplification purposes.
• In the cutoff mode, both junctions are reverse-biased. The transistor is “off” in this mode – it does not conduct current from the collector to the emitter.
• In the saturation mode, both junctions are forward-biased. The transistor is “on” in this mode – it conducts a maximum current from collector to emitter. This mode is used when the BJT is employed as a switch.

Understanding the Basics of Bipolar Junction Transistors (BJTs)

The BJT: A Three-Layer, Two-Junction Device

As we have learned, a Bipolar Junction Transistor (BJT) is a type of transistor that uses both electron and hole charge carriers for its operation. It consists of three alternating layers of N- and P- type semiconductor materials, which form two pn junctions.

The two types of BJTs are NPN and PNP, named based on how these layers are arranged. In an NPN transistor, an N-type semiconductor layer is sandwiched between two P-type layers. Conversely, in a PNP transistor, a P-type layer is sandwiched between two N-type layers. These arrangements lead to differing operational characteristics between NPN and PNP transistors.

The three layers in the transistor correspond to the three terminals of the BJT: the emitter, base, and collector. The emitter emits carriers into the base, while the collector collects them. The base controls the flow of carriers.

BJT Symbols and Modes of Operation

BJTs are represented by specific symbols in circuit diagrams. The symbol consists of a triangular arrangement, with a line representing each of the three terminals. An arrow on the emitter terminal indicates whether the transistor is NPN (arrow pointing out) or PNP (arrow pointing in).

BJTs can operate in three different modes depending on the voltages applied to their two pn junctions (the base-emitter junction and the base-collector junction). These modes are:

1. Active Mode: When the base-emitter junction is forward-biased and the base-collector junction is reverse-biased, the BJT is in the active mode. In this mode, the transistor can amplify signals. This is the typical operating mode for BJTs used in amplifiers.
2. Cutoff Mode: When both junctions are reverse-biased, the BJT is in the cutoff mode. In this state, the transistor is “off,” and no current flows from the collector to the emitter.
3. Saturation Mode: When both junctions are forward-biased, the BJT is in the saturation mode. Here, the transistor is “on,” and a maximum current flows from the collector to the emitter. This mode is common when the BJT is used as a switch in digital circuits.

BJTs as Current Amplifiers

The heart of the BJT’s usefulness in many circuits is its ability to act as a current amplifier. This aspect is what allows it to fulfill various roles, such as signal amplification and switching.

Current Relations in a BJT

The currents in the three regions of a bipolar junction transistor (BJT) — the emitter (Ie), the base (Ib), and the collector (Ic) — are related by the equation:

Ie = Ib + Ic

In other words, the emitter current is the sum of the base and collector currents. The emitter emits carriers into the base, which controls the number of carriers that reach the collector.

In an NPN transistor, the emitter emits electrons into the base, while in a PNP transistor, it emits holes. The base current Ib is typically a lot smaller than the collector current Ic. This is due to the thinness and low doping of the base region, which means that a majority of the injected carriers reach the collector.

Current Gain (β or hfe)

The current gain, or beta (β), of a BJT is the ratio of the collector current (Ic) to the base current (Ib). It is also denoted as hfe in some contexts.

β = Ic / Ib

This current gain is a vital characteristic of the BJT, as it quantifies the amplification capability of the transistor. The β value typically ranges from 20 to 1000, depending on the particular transistor and the biasing conditions.

It is important to note that β is not a constant value; it can vary depending on the collector current, temperature, and from one device to another. However, for design purposes, it is often treated as a constant.

BJT Biasing

Bipolar Junction Transistors (BJTs) can serve a myriad of functions in electronic circuits, from amplifying signals to switching electrical currents. To perform these functions effectively, BJTs must be properly biased.

Purpose of BJT Biasing

Biasing refers to the process of setting up the DC (Direct Current) operating conditions for a transistor. These conditions, referred to as the operating point or Q-point, determine the mode in which the BJT operates – active, cutoff, or saturation.

Proper biasing is crucial because it ensures the BJT operates in the desired region for a given application. For example, for a BJT to amplify an AC (Alternating Current) signal without distortion, it needs to operate in the active region, where the input-output characteristics are almost linear.

Types of BJT Biasing

Several methods are used to bias a BJT, and they differ mainly in how the base-emitter junction is forward-biased and how the base current is supplied. Here are some common types of BJT biasing:

1. Fixed Bias (or Base Bias): In this simplest form of biasing, a single DC supply is connected to the base of the BJT through a biasing resistor. The base-emitter junction is forward-biased, while the base-collector junction is reverse-biased, putting the BJT in the active mode. However, this method is rarely used in practice because it does not provide good stability against variations in β and temperature.
2. Emitter Bias: This method uses two power supplies to independently control the base and collector currents. The base current is controlled by a base resistor, and the emitter current is controlled by an emitter resistor. This method improves stability but requires two power supplies, which can be a disadvantage in some applications.
3. Voltage-Divider Bias: Also known as self-bias, this method is the most widely used because it provides good stability with only one power supply. It uses two resistors to form a voltage divider network that sets the base current, and an emitter resistor to stabilize the emitter current.

Each biasing method has its advantages and disadvantages, and the choice depends on the specific requirements of the circuit.

BJT Applications

Bipolar Junction Transistors (BJTs) are versatile devices used in various electronic applications. Their ability to control current flow and amplify signals makes them invaluable components in many circuits. In this reading, we’ll explore two common applications of BJTs: as a switch and as an amplifier.

BJT as a Switch

In digital electronics, BJTs are commonly used as electronic switches. The idea is to operate the transistor in either cutoff or saturation mode, corresponding to the “off” and “on” states of the switch, respectively.

1. Cutoff Mode (Off State): In this state, both the base-emitter and base-collector junctions are reverse-biased. No current flows from the collector to the emitter (Ic ≈ 0). The BJT is considered to be “off.”
2. Saturation Mode (On State): In this state, both the base-emitter and base-collector junctions are forward-biased. Maximum current flows from the collector to the emitter, limited only by any external circuitry connected to the transistor. The BJT is considered to be “on.”

The ability to quickly switch between these two states makes BJTs suitable for use in digital logic circuits and pulse (square wave) generators, among other applications.

BJT as an Amplifier

One of the most common uses of BJTs is to amplify signals. This is achieved by operating the BJT in the active mode. In this mode, a small change in the base current results in a large change in the collector current, due to the transistor’s current gain (β).

There are several different amplifier configurations, each with its unique properties:

1. Common Emitter (CE) Amplifier: This is the most frequently used amplifier circuit, known for its high voltage and power gain. However, it inverts the phase of the signal.
2. Common Base (CB) Amplifier: This configuration is less common and is characterized by a high voltage gain but a current gain of less than one. It does not invert the phase of the signal.
3. Common Collector (CC) Amplifier: Also known as an emitter follower, this configuration has a voltage gain of approximately one but provides high current gain and high input impedance. It does not invert the phase of the signal.

These applications highlight the importance of BJTs in electronics. Understanding how they function as switches and amplifiers forms the basis for studying more complex applications and circuits.