Last Updated on October 22, 2023 by Kevin Chen
Image source Nuts and Volts Magazine
Switching and amplification are the two main applications of the transistors. It can be used to regulate the behavior of the electric current and as well as amplify the electric signals. Transistors can be used as switches for switching both low-voltage and high-voltage devices. This switch application is also quite common in both analog and digital electronics.
In this article, we will discuss how a transistor works as a switch. We will examine the different operating modes that allow transistors to work as switches. Are you planning to buy transistors in switching circuits, we will give you the best tips for choosing reputable transistor suppliers in China.
Overview of switching circuit
Before we look at the roles of transistors in switching circuits, let’s have a brief outlook on these circuits. What is a switching circuit? What are the functions of these circuits and where are they used?
Switching circuits are common in modern electronics. They are also known as logical circuits and sometimes digital circuits. They are called switching circuits because of their ability to switch or alter the behavior of the output based on the input signals and the current state of the circuit.
For example, if the current state of the circuit is OFF, the switching circuit can influence it to go OFF and the conditions can be reversed. When in the mechanical application, if the current is connected to an object, the switching action will facilitate a disconnection.
Switching circuits are the main drivers of modern electronic devices that are heavily characterized by digital systems. The circuits make it possible for the digital electronics to perform binary and logical operations which feature the use of 0 and 1.
Furthermore, switching circuits are closely related to the operations of the logic gates. These operations include AND, OR, NOT,XOR, NAND, flip-flops, and multiplexers, among others. These logic gate operations are used for running devices in different devices. Common applications include processors and memory systems.
Switching circuits can operate as single independent units or can be a combination of several circuits brought together to function in a single device. Regardless of the number of circuits involved, a common feature in all of them is the output will be altered depending on the inputs and the operations performed.
Transistors as switches
For a transistor to be used in a switching circuit, first, it must serve as a switch. To make this possible, both the NPN and PNP regions of the bipolar junction transistors (BJT) are biased to operate within the active regions. This way, the transistor will be able to operate in ON/OFF which in logic circuits can be interpreted as 0 and 1.
A typical BJT transistor comprises of two PN junctions that are connected next to each other and a common layer that is right in the middle of the junctions. BJT is called bipolar because it consists of holes and electrons, Holes are the positive charge carriers while electrons are negative charge carriers.
When used as a switch, the biasing can either be NPN or PNP. These two biasing options allow the transistor to operate on both sides of the current-voltage characteristics.
BJT transistor operation regions
Transistor switches have two operation regions: The cut-off region and the saturation regions. At the cut-off region, the transistor will turn off the switching circuit while at the saturation region, the transistor will turn on the switching circuit.
At the cut-off region, the transistor has zero input base current which directly translates to zero output base current. The collector voltage (Vce) is at the maximum level meaning that you will have a large depletion layer. There will be no flow of current through the transistor hence it will be at OFF state.
The saturation region is another operation region of this transistor switch. At this region, the transistor is biased allowing the base current to flow through to the collector region. The collector-emitter voltage will be at its lowest hence reducing the size of the depletion layer. This means that current will flow freely through the transistor and it will remain in an ON mode.
Both the cut-off region and the saturation regions of the transistors are used in switching digital circuits.
Examples of transistors used in the switching circuits
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We can illustrate this using a simple circuit that comprises of a power source LED bulb as a load and of course the transistor. When the main switch of the circuit is open, no current will flow through the base terminal of the transistor. This means that it will be at the cutoff state and the circuit will remain OFF.
On the other hand, when the switch is closed, electric current will flow through the base current and the transistor will be at the saturation state hence allowing the switch circuit to be ON.
This is an example of a transistor serving as a power switch. It dictates the flow of power to the load, which in this case is an LED lamp.
Another common application of transistors in switching circuits is in memory devices. This is where data to be stored is represented in the binary forms 0s and 1s. The transistor will switch to indicate the availability and absence of charges which will be a representation of data.
A switching regulator is another common application of transistors in switching circuits. These regulators are mainly used in DC power converters. The transistors facilitate the fast-switching speeds of the regulators.
At this point, you have the full information about the role of transistors in switching circuits. So, in case you are building an application that will require a switching circuit, you should consider adding transistors. They will enhance the effectiveness and efficiency of the circuit. Keep in mind that the specifications of the transistor should match with the circuit. Check out the power requirements, and other technical specifications of the transistor.
You should also buy genuine transistors from reputable electronic components suppliers. Other than performance, quality transistors are likely to meet the required safety standards for the switching circuits.
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