Amplifying Signals with Bipolar Junction Transistors (BJTs)

The world we live in today depends heavily, if not solely, on signals. In our homes and offices, the electronic devices we use send or receive signals in one way or another. Our smartphones are one of the best examples of devices that use signals for the purpose of communication. However, in remote locations, we often face the issue of weak signals. 

Ever wonder if there was any electronic component that could amplify signals so the issue of weak signals could at least be minimized if not completely eliminated? Bipolar junction transistors, or BJTs, do exactly that. But the question is, how does a BJT do that? And what considerations should be kept in mind while designing an amplification circuit? This article will tell you exactly that. Let's start!

Bipolar Junction Transistors (BJTs)

In simple words, a bipolar junction transistor is a semiconductor device that has three terminals and two p-n junctions. These p-n junctions are able to amplify signals. The three terminals of a bipolar junction transistor are the following.

• Base

• Collector 

• Emitter

It is important to know that a BJT is a current-controlled device that does not require an external source of DC power to amplify signals. 

Construction of BJTs

A bipolar junction transistor has three layers, each connected by an electrical terminal. As discussed above, these terminals are base (B), collector (C), and emitter (E). Moreover, it has two PN junctions: a base-emitter junction and a base-collector junction. 

In an NPN transistor, a P-type semiconductor is doped between two N-type semiconductors. In a PNP transistor, an N-type semiconductor is doped between two P-type semiconductors. 

In both these transistors, the heavily doped terminal is the emitter (E), whereas collector (C) is moderately doped, and base (B) is lightly doped. The outer layer of a bipolar junction transistor is wider than the layer at the center. 

Configurations of BJTs

A bipolar junction transistor comes with three different configurations generally. These are discussed below.

1.Common Base (CB) Configuration 

This configuration uses the base as a common terminal for input and output signals, hence the name "common base." In a CB configuration, the input is applied between the emitter and base, while the output signal is taken between the collector and base terminals. The base terminal is grounded. 

One thing to remember about a CB transistor is that the input emitter current must be higher than the collector and base currents. Otherwise, the transistor will not work. The amplifier circuit that uses the CB configuration is known as a non-inverting amplifier circuit. 

A common base circuit is mostly used in single-stage amplifier circuits. Examples include radio frequency amplifiers and microphone preamps. 

2.Common Collector (CC) Configuration 

Just like a transistor with a CB configuration uses the base as the common terminal, the common collector-configured transistors use the collector as the common terminal for input and output signals. This configuration is also called the emitter follower configuration because the emitter voltage in the CC configuration follows the base voltage. 

In this configuration, the input signal is applied between the base and collector, while the output signal is taken between the emitter and collector. The emitter current in this configuration is also higher than the base current and the collector current.

A common collector transistor is used as an impedance-matching device, as a switching circuit, and in digital circuits with logic gates. 

3.Common Emitter (CE) Configuration 

The emitter is used as a common terminal in transistors with the CE configuration. Contrary to the CB configuration, the common emitter configuration is an inverting amplifier circuit. The input signal in this configuration is applied between the base and emitter, and the output is taken from the collector-emitter region.

Transistor-based amplifiers used the CE configuration, making it the most used configuration among the three configurations discussed above. 

BJT Amplifier Circuit 

The transistor is the main electronic component of a signal amplifier. As the name suggests, a BJT amplifier is an electronic circuit that uses bipolar junction transistors in it. A bipolar junction transistor can operate in three zones. These are called Saturation, Active, and Cut-off zones. 

To work as an amplifier, a bipolar junction transistor must operate in the active or linear zone. Depending on the configuration of the BJT transistor used in them, there are three types of BJT amplifiers.

1. Common Base Amplifiers – they have low input resistance, high output resistance, high voltage gain, and a current gain almost equal to one.

2. Common Emitter Amplifiers – these amplifiers exhibit moderate input resistance, output resistance, voltage gain, and current gain.

3. Common Collector Amplifiers – they have high input resistance, low output resistance, high current gain, and a voltage gain that is approximately equal to one. 

Final Words

Bipolar Junction Transistors (BJTs) are the most important electronic component in an amplification circuit. Depending on the doping sequence, BJTs can be either Negative-Positive-Negative-Positive (NPN) or Positive-Negative-Positive (PNP). 

Additionally, different configurations of BJTs are available; the common emitter configuration is used more often than the common base or common collector configurations. Based on the kind of bipolar junction transistor used, the amplification circuits also have different types. 

Signal amplification is often needed, and those who know how to build electronic circuits prefer building their own signal amplifiers instead of buying them. This provides the flexibility to modify the amplification circuit according to one's requirements. However, such people are often unsure about where to buy transistors for their amplifiers. 

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