2N3904 Transistor (40V, 0.2A)

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

• Transistor Type: NPN

• Maximum Collector Current ($I_C$): 200 mA (0.2 A)

• Collector-Emitter Voltage ($V_{CE}$): 40 V

• Power Dissipation ($P_C$): 625 mW

• Package Type: TO-92

• 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:

1. Emitter

2. Base

3. 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

• 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.

• Amplification: Used extensively in audio pre-amplifiers and radio frequency (RF) signal amplification.

• 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:

Usage Best Practices

• • 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.

  • Complementary Pair: The 2N3906 is the direct PNP complement to the 2N3904. Together, they are frequently used in complementary symmetry circuits.

  • 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 $I_B \approx 0.1 \times I_C$.