A deep dive into how transistors work
The video delves into the fundamental workings of one of the most significant inventions in electronics: the transistor. Introduced as a critical device in modern technology, the video aims to explain the intricate details of how transistors function, focusing primarily on the bipolar junction transistor (BJT).
Transistors, the video explains, come in various shapes and sizes, but they can be broadly classified into two main types: bipolar and field-effect transistors (FETs). This particular discussion centers around the bipolar type, which includes NPN and PNP transistors. These small electronic components serve two essential functions: they can act as switches to control circuits and amplify signals.
Transistors are typically encapsulated in resin cases to protect their internal parts. High-power transistors, however, often have a partly metal casing to dissipate heat, as excess heat can damage the components over time. These metal-bodied transistors are usually attached to heat sinks for efficient heat removal. For instance, in a DC bench power supply, MOSFET transistors are attached to large heat sinks to prevent overheating. The video mentions that without a heat sink, the components can quickly reach temperatures as high as 45°C (113°F) with just 1.2A of current.
Transistors are identified by part numbers inscribed on their bodies, which correspond to detailed specifications in manufacturers' datasheets. Each transistor has three pins: the emitter (E), the base (B), and the collector (C). The configuration of these pins varies, so checking the datasheet is essential.
The video demonstrates how a transistor can automate the control of a circuit. When a light bulb is connected to a battery, it illuminates, and a switch can manually control this illumination. To automate the control, a transistor can be used. By applying a small voltage to the base pin, the transistor allows current to flow through the main circuit, thereby turning on the light. This principle can be applied to create automated systems using sensors or remote switches.
In a simple circuit, a transistor is used to control the brightness of an LED using a 9V power supply. When the base pin receives a small voltage (around 0.6V to 0.7V), the transistor starts conducting, allowing current to flow through the main circuit and light up the LED. As the voltage on the base pin increases, the LED becomes brighter, demonstrating the transistor’s amplifying capability.
The video further explains the concept of current gain, represented by the symbol β (beta), which is the ratio of the collector current to the base current. For example, if the collector current is 100mA and the base current is 1mA, the current gain is 100.
The difference between NPN and PNP transistors is also discussed. In an NPN transistor, both the main circuit and the control circuit are connected to the positive terminal of the battery, with the current flowing through the transistor when the control switch is activated. Conversely, in a PNP transistor, the emitter is connected to the positive terminal, and the current flows differently.
Transistors are represented in circuit diagrams with specific symbols, where the arrow on the emitter indicates the direction of conventional current flow.
The video uses an analogy of water flowing through a pipe to explain how a transistor works. Imagine water flowing freely through a pipe until blocked by a disc. Introducing a smaller pipe with a swing gate that can move the disc represents how a small voltage can control a larger current in a transistor.
The discussion moves on to the structure and operation of semiconductors, which are the building blocks of transistors. Pure silicon, a semiconductor, can act as both a conductor and an insulator. Doping silicon with other materials, such as phosphorus (for N-type) or aluminum (for P-type), alters its electrical properties, creating excess electrons or holes, respectively.
In an NPN transistor, the emitter is heavily doped with excess electrons, the base is lightly doped, and the collector is moderately doped. When a voltage is applied, electrons flow from the emitter to the collector, allowing current to pass through the transistor.
The video concludes by emphasizing the importance of understanding conventional current flow and electron flow in circuit design. It encourages viewers to explore further resources on electronics engineering and to follow the channel on various social media platforms for more educational content.