2N4240

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The 2N4240 is a silicon NPN transistor, categorized as a single bipolor junction transistor (BJT). Based on its physical appearance and electrical characteristics, it is intended for use in low-frequency switching and low-level linear amplification applications.

The term “NPN” refers to the type of semiconductor structure formed when the two types of semiconductor material, p-type and n-type, are sandwiched together. The negative-type material is placed on the bottom and topped with a positive-type material in the middle layer, with electrons as the majority charge carriers in the p-type layer and holes as the majority charge carriers in the n-type layer. This type of structure creates a transistor that can be used for both currents and voltages. The 2N4240 has a 2 amperes maximum collector current (Ic) and a 1.3 volts collector-base breakdown voltage (Vcb).

The 2N4240 is a general purpose silicon transistor, which means it can be used for a variety of purposes, from low-frequency signal amplification to high-frequency switching. Due to its low current and voltage ratings, it is best suited for low-level signal amplifications, such as in audio circuits or for general amplification purposes in digital logic circuits. It is also commonly used to perform relay switching in automotive electronics due to its low power dissipation at frequencies up to 1 MHz.

The two most important parameters to understand when studying the 2N4240 are the current gain (hfe) and the gain-bandwidth product (fT). The hfe is the ratio of collector current (Ic) to base current (Ib) and is usually measured using a current probe or a DC voltmeter. The gain-bandwidth product (fT) is the frequency at which the current gain (hfe) of the transistor drops to unity, or essentially becomes nonexistent. A higher fT indicates that the transistor can handle more frequency before its gain drops off.

The two important parameters of hfe and fT define the working principle and application field of the 2N4240. In most cases, the transistor is operated in its active region, meaning that its collector current is between its saturation and cutoff voltages. For optimum performance, the transistor should not be operated over the maximum allowable collector-base voltage, as this can cause extreme variations in the collector current, resulting in unwanted distortion and increased power dissipation. Generally, when operating in its active region, hfe can range from 128 to 327, depending on the particular transistor and its frequency of operation. An fT value of 50–100 MHz is typical for the 2N4240 and indicates its optimum performance frequency range.

In summary, the 2N4240 is a low-level bipolar transistor suitable for low-frequency linear and switching applications. Its physical characteristics and electrical parameters enable it to achieve a good balance between gain and bandwidth for the majority of low-frequency applications. Furthermore, its low power-dissipation makes it useful for relay switching applications in automotive electronics. Knowing its parameters and characteristics, engineers can easily identify its best application field, making the 2N4240 a popular choice amongst designers.

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