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You are building an electric circuit and one of the components that you will need is a transistor. Probably more than one transistor. Buying a transistor may sound like an easy task to do, but in reality, it is not. There are many types of transistors and each type has a unique set of properties. Before you buy one, you need to ensure that it is a perfect fit for your electric circuit. In this guide, we are going to help you choose the most appropriate transistor for your circuit. We will also highlight key tips for buying transistors, including choosing the best suppliers and manufacturers of transistors.
Basics of transistors: what are they?
Before you get into the process of choosing transistors, you first have to know what they are.
A transistor is an electronic component that is used for various purposes in electric circuits. Amplification and switching are among the top two functions of transistors in circuits. It is always considered among the most important electric inventions due to the important roles that it has played in the modern advancement of technology.
There are two main types of transistors in the market. These are the BJTs (Bipolar Junction Transistors) and FETs (Field Effect Transistors). Regardless if the types, both are designed to operate on the two operation modes, switching and amplification. It all depends on how they are configured into the circuits.
The semiconductor materials are the main or primary material used for making the transistors with silicon and germanium being the preferred materials. They are designed to have two regions, n-type and p-type zones. These two zones are created through the addition of impurities. This process is known as doping. Boron is the preferred material for creating the p-type region as it will create the positive charge or holes. On the other hand, Phosphorous is used for forming the n-type region of the transistors. This is because it has five valence electrons and four of the electrons will pair up with four silicon electrons. This type of pairing leaves one electron free to move around.
Also, a typical transistor comprises three layers which are the base, collector, and emitter. The doping in these three layers ends up creating different transistor configurations. These configurations are Negative-Positive-Negative (N-P-N) and the other one is the Positive-Negative-Positive (P-N-P). It is through these configurations that we end up with the two types of transistors, BJT and FET.
So, what’s the difference between these two types of transistors?
The bipolar junction transistors (BJT) transistors have two p-n junctions that are connected to each other. Here you will still have the two configuration options, PNP and NPN. It will all depend on the doping that you choose. The doping will depend on the power requirements of the circuit. Silicon is the primary substrate option for the BJT transistor. Like all transistors, BJT has the base, collector, and emitter terminals.
On the other hand, FET also has three terminals that are used for creating connections to the circuit. These terminals are the drain, gate, and source. However, unlike BJT, FET has one material that serves as the substrate. This can either be p-type or an n-type. The gate terminal of the FET transistor is linked to the primary substrate while the source to the p-type as the drain is connected to the n-type.
The working mechanism of the transistors
Before you proceed to choose the right transistor for your circuit, it will be prudent to know how it works. This way, it will be easy to foresee the transistor’s impact and how it will behave when you connect it to the circuit.
As we have already mentioned, there are two things that a transistor will do in a circuit. To work as an amplifier or to act as a switch. When used as an amplifier, a transistor will convert the low input current to a high output current. Let’s say you are connecting a powerful device to the circuit yet the incoming current is low, you will need a transistor to amplify the current.
When used as a switch, the transistor will determine the flow of current in a circuit. It controls how the current flows through the terminals so that at the end of the day the circuit can either be an ON state or OFF state. When there is no input signal applied across the base, the transistor will act as an open switch. When a positive input signal is applied at the terminals, the transistor will serve as a closed switch.
The operation of a transistor encompasses the flow of electrons through the p-n junctions. The n-type region has the majority of electrons while the p region has the majority number of holes.
So, let’s say we have the NPN transistor. When we apply negative voltage on the n-type region of this transistor, the electrons will flow from the negative region to the p-type region. This way, the emitter-base region transistor will be forward-biased.
Once the electrons from the n-type region get into the p-type region, they will combine with the holes and will form a collector current. The base, which is connected to the positive voltage can be used to collector current. This implies that it controls all the operation and behavior of the transistor.
In overall, the working of the transistor entails the movement of the electrons from the emitter to the collector. The flow can be controlled at the base whereby you will need to vary the input voltage.
What to look for when choosing transistors
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At this point, we can confidently assume that you know the basics and slight details if the transistors and how they work. Let’s proceed to look at the key factors that you should consider when choosing transistors for your circuit. Some of these factors are basically technical parameters of transistors that define the performance of the device.
Maximum collector current
You should confirm the maximum collector current of the transistor. For most normal transistors, this value is indicated in terms of the milliamps and it is amps in case you are using power transistors.
Current gain
This is defined as the ratio of the gain current to the collector current. Current gain is usually denoted by the symbol β or hfe. It is an important parameter that determines the amplification capability of the transistor. When buying transistors for amplification purpose, you should choose one that has a higher current gain. However, keep in mind that different applications have different requirements for the current gain.
Breakdown voltage
When choosing transistors, pay attention to their two major breakdown voltages. These are the collector-emitter breakdown voltage and the collector-base breakdown voltage. It mainly refers to the maximum voltage at which the transistor can operate without exceeding its insulation limit. The operation of the circuit should not exceed the breakdown as it will damage the transistor.
Collector-Emitter Voltage (Vceo)
This is the maximum voltage that the collector-emitter junction of the transistor is designed to handle at any give time. The Vceo of most applications is more than 30V.
Emitter-Base Voltage (Vebo)
This is the maximum voltage that can be allowed through the emitter-base junction of the transistor. In most applications, it is 6V.
Collector-Base Voltage (Vcbo)
This is the maximum voltage that you can apply at the collector-base junction of the transistor. In most applications, it is 50V.
Total power dissipation (Ptot)
This is the power that is released from the transistors and is measured in watts. The total power dissipation tends to vary from one transistor to another. High-power transistors tend to dissipate more power than others.
Operating temperature
You should check the ideal operating temperature range of the transistor. This will determine whether it will remain stable and suitable for your application. Using the transistor anywhere beyond the stated value will be a red flag.
Polarity and configuration
As we have stated, there are two main transistor configurations. These are the PNP and NPN configurations. They tend to directly affect the overall polarity of the transistors. Confirm the output requirements of the applications before you choose the polarity.
Transistor material
You should also pay attention to the material that has been used to make the transistor. Silicon is the most preferred material thanks to its numerous physical and electrical properties. Gallium arsenide and germanium are other alternative materials.
Conclusion
It is important to consider all the above-listed factors when choosing transistors for your circuit. This way, your circuit will be able to operate smoothly as expected.
Pay special attention to the operating current, source voltage, and even power dissipation of the transistor. They tend to directly affect all the other key parameters that determine the operation and compatibility of the transistor with the circuit.
The good thing is you can verify all these details from the product manual or datasheet.
And when it comes to buying transistors, ensure you get them from the right sources. Buying from reputable transistor suppliers in China will guarantee you quality and flawless transistors for sale.
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