Description
The 2N3904 is one of the most widely used general-purpose NPN bipolar junction transistors (BJT) in electronics. It is prized for its high speed, low current requirements, and versatility in switching and amplification applications.
Key Specifications
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Transistor Type: NPN
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Maximum Collector Current ($I_C$): 200 mA (0.2 A)
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Collector-Emitter Voltage ($V_{CE}$): 40 V
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Power Dissipation ($P_C$): 625 mW
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Package Type: TO-92
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Transition Frequency ($f_T$): 300 MHz (Excellent for high-frequency small-signal applications)
Pinout Configuration
When looking at the flat side (the face with the markings) of the TO-92 package with the leads pointing down, the pin arrangement from left to right is:
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Emitter
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Base
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Collector
Note: The 2N3904 pinout is different from the BC-series transistors (like the BC547 or BC337). Always verify your pinout, as the 2N3904 follows the EBC order, while the BC-series follows the CBE order.
Key Applications
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High-Speed Switching: Because of its high transition frequency ($f_T$), it is often used in digital logic circuits and pulse-width modulation (PWM) applications.
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Amplification: Used extensively in audio pre-amplifiers and radio frequency (RF) signal amplification.
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General Purpose: It is a standard component for driving small loads like low-current LEDs, buzzers, or acting as a buffer in sensor circuits.
Comparison: 2N3904 vs. BC547
Both are NPN transistors, but they have different origins and pin configurations:
| Feature | 2N3904 | BC547 |
| Max $I_C$ | 200 mA | 100 mA |
| Max $V_{CE}$ | 40 V | 45 V |
| Pinout (L to R) | E – B – C | C – B – E |
| Origin | North American standard | European standard |
Usage Best Practices
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Pinout Awareness: Because the Emitter and Collector pins are swapped compared to the BC547/548, you cannot simply swap these parts on a breadboard without re-routing your connections.
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Complementary Pair: The 2N3906 is the direct PNP complement to the 2N3904. Together, they are frequently used in complementary symmetry circuits.
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Saturation: To use the 2N3904 as a switch, ensure you provide enough base current ($I_B$) to reach saturation. A common design approach is to use a base resistor such that $I_B \approx 0.1 \times I_C$.
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