Table of Contents

1. Introduction to the 2N3904 Transistor
2. Key Parameters and Specifications
3. Electrical Characteristics
4. Application Examples
5. Switch
6. Amplifier
7. Current Source
8. Package Types and Pinout
9. Handling and Soldering Precautions
10. Frequently Asked Questions (FAQ)
11. Conclusion

1. Introduction to the 2N3904 Transistor

The 2N3904 is a general-purpose NPN bipolar junction transistor manufactured by several semiconductor companies. It is designed for low to medium current amplification and switching applications in various electronic circuits. The transistor features a small signal current gain (hFE) range of 100 to 300, making it suitable for a wide range of projects.

2. Key Parameters and Specifications

To effectively use the 2N3904 transistor, it is crucial to understand its key parameters and specifications. The following table summarizes the essential characteristics:

Parameter	Symbol	Value
Collector-Emitter Voltage	VCEO	40 V
Collector-Base Voltage	VCBO	60 V
Emitter-Base Voltage	VEBO	6 V
Collector Current (Continuous)	IC	200 mA
Collector Current (Peak)	ICM	300 mA
Total Power Dissipation	PD	625 mW
Operating Temperature Range	TJ	-55 to 150 °C
DC Current Gain (Min)	hFE	100
DC Current Gain (Max)	hFE	300
Transition Frequency (Typ)	fT	300 MHz

These parameters define the safe operating limits and performance characteristics of the 2N3904 transistor. It is essential to design circuits that operate within these specified limits to ensure reliable operation and prevent damage to the component.

3. Electrical Characteristics

The 2N3904 Transistor Datasheet provides detailed information about its electrical characteristics under various operating conditions. Some of the key electrical parameters include:

3.1 Current-Voltage Characteristics

The current-voltage (I-V) characteristics of the 2N3904 transistor describe the relationship between the collector current (IC) and the collector-emitter voltage (VCE) at different base currents (IB). The following graph illustrates a typical I-V curve for the 2N3904:

As the base current increases, the collector current also increases, demonstrating the transistor’s ability to amplify current. The saturation region, active region, and cut-off region are clearly visible in the I-V curves.

3.2 Current Gain (hFE)

The current gain, or hFE, is a crucial parameter that indicates the transistor’s amplification capability. It is defined as the ratio of the collector current (IC) to the base current (IB) at a specified collector-emitter voltage (VCE). The 2N3904 has a typical current gain range of 100 to 300.

hFE = IC / IB

The current gain can vary with temperature, collector current, and collector-emitter voltage. The datasheet provides graphs that show the variation of hFE under different operating conditions.

3.3 Transition Frequency (fT)

The transition frequency (fT) is the frequency at which the current gain (hFE) of the transistor drops to unity (1). It is a measure of the transistor’s high-frequency performance. The 2N3904 has a typical transition frequency of 300 MHz, making it suitable for low to medium frequency applications.

4. Application Examples

The 2N3904 transistor finds use in a wide range of electronic circuits. Some common application examples include:

4.1 Switch

One of the most basic applications of the 2N3904 is as a switch. By applying a sufficient base current, the transistor can be turned on, allowing current to flow from the collector to the emitter. When the base current is removed, the transistor turns off, interrupting the current flow.

A simple transistor switch circuit using the 2N3904 is shown below:

In this circuit, a microcontroller or other digital device can control the base current through the resistor R1. When the base is driven high, the transistor turns on, allowing current to flow through the load connected to the collector.

4.2 Amplifier

The 2N3904 can be used as a small-signal amplifier to increase the voltage or current of a signal. The basic common-emitter amplifier configuration using the 2N3904 is shown below:

In this circuit, the input signal is applied to the base of the transistor through the coupling capacitor C1. The amplified output signal appears at the collector, with the gain determined by the ratio of the collector resistor (RC) to the emitter resistor (RE). The capacitor C2 provides AC coupling at the output.

4.3 Current Source

The 2N3904 can also be used to create a constant current source. By biasing the transistor in the active region and using a resistor in the emitter path, a stable current can be obtained. The following circuit shows a simple current source using the 2N3904:

The base voltage is set by the voltage divider formed by R1 and R2, which biases the transistor in the active region. The emitter resistor RE determines the current flowing through the transistor, which remains constant despite variations in the load connected to the collector.

5. Package Types and Pinout

The 2N3904 transistor is available in various package types, including TO-92, SOT-23, and SOT-223. The choice of package depends on the specific application requirements, such as power dissipation, mounting options, and space constraints.

The pinout for the most common TO-92 package is as follows:

• Pin 1: Emitter
• Pin 2: Base
• Pin 3: Collector

It is essential to refer to the specific datasheet for the exact pinout and dimensions of the chosen package type.

6. Handling and Soldering Precautions

When working with the 2N3904 transistor, proper handling and soldering techniques are crucial to ensure reliable operation and prevent damage to the component. Some key precautions include:

• Handle the transistor by its package, avoiding direct contact with the leads to prevent electrostatic discharge (ESD) damage.
• Use a grounded anti-static wrist strap or work on an ESD-safe mat when handling the transistor.
• Apply heat sparingly during soldering to prevent thermal damage. Use a temperature-controlled soldering iron and limit soldering time to a few seconds per lead.
• Avoid applying excessive mechanical stress to the leads during bending or cutting to prevent damage to the internal structure.

7. Frequently Asked Questions (FAQ)

7.1 What is the maximum collector current rating for the 2N3904 transistor?

The maximum continuous collector current rating for the 2N3904 is 200 mA, with a peak current rating of 300 mA.

7.2 Can the 2N3904 be used as a high-frequency amplifier?

The 2N3904 has a transition frequency (fT) of 300 MHz, making it suitable for low to medium frequency amplification. For high-frequency applications, transistors with higher fT values, such as RF transistors, are more appropriate.

7.3 Is the 2N3904 suitable for high-power applications?

No, the 2N3904 is designed for low to medium power applications, with a maximum power dissipation of 625 mW. For high-power applications, transistors with higher power ratings, such as power BJTs or MOSFETs, should be used.

7.4 Can the 2N3904 be replaced with other transistors?

In many cases, the 2N3904 can be replaced with similar NPN transistors, such as the BC547, BC548, or 2N2222, depending on the specific requirements of the application. However, it is essential to compare the key parameters and ratings of the alternative transistors to ensure compatibility.

7.5 What is the pinout of the 2N3904 in the TO-92 package?

In the TO-92 package, the 2N3904 pinout is as follows: Pin 1 – Emitter, Pin 2 – Base, Pin 3 – Collector.

8. Conclusion

The 2N3904 transistor is a versatile and widely used NPN bipolar junction transistor suitable for a range of low to medium power amplification and switching applications. By understanding the key parameters, electrical characteristics, and typical applications from the 2N3904 transistor datasheet, designers can effectively integrate this component into their electronic circuits.

When working with the 2N3904, it is crucial to operate within the specified limits, choose the appropriate package type, and follow proper handling and soldering precautions to ensure reliable performance and longevity.

By leveraging the information provided in the datasheet and considering the application requirements, engineers and hobbyists can harness the capabilities of the 2N3904 transistor to build efficient and robust electronic systems.