The 2N3904 is a widely used bipolar junction transistor (BJT) of the NPN type, fabricated from silicon. Introduced in the 1960s, it remains a staple in electronics due to its reliability, low cost, and versatility. Designed for low-power amplification and switching, it operates effectively in a variety of circuits, from hobbyist projects to industrial applications.

Key Features and Specifications
Maximum Ratings:

• Collector-Emitter Voltage (V_CEO): 40 V

• Collector-Base Voltage (V_CBO): 60 V

• Emitter-Base Voltage (V_EBO): 6 V

• Continuous Collector Current (I_C): 200 mA

• Total Power Dissipation (P_tot): 625 mW (at 25°C)

• Operating Temperature Range: -55°C to +150°C

Electrical Characteristics:

• DC Current Gain (h_FE): 100–300 (varies with collector current and temperature)

• Transition Frequency (f_T): 300 MHz (typical)

• Saturation Voltages:

• V_CE(sat): 0.2 V (at Ic=10 mA, Ib=1 mA)

• V_BE(sat): 0.65–0.85 V (active region)

Package and Pin Configuration
The 2N3904 is housed in a TO-92 plastic package, with pins arranged as follows (front view, flat side facing you):

1. Emitter (E): Left pin.

2. Base (B): Middle pin.

3. Collector (C): Right pin.

Applications

1. Amplification: Used in small-signal amplification (e.g., audio preamps, RF stages) due to moderate gain and bandwidth.

2. Switching: Capable of switching loads up to 200 mA (e.g., LEDs, relays).

3. Darlington Pairs: Combined with PNP transistors (e.g., 2N3906) for high-gain configurations.

4. Oscillators and Signal Generators: Utilized in LC or RC oscillator circuits.

5. Voltage Regulators: Acts as a pass element in low-power regulator designs.

Comparisons with Similar Transistors

• 2N2222: Higher current capacity (800 mA) but lower f_T (250 MHz); packaged in TO-18.

• BC547: Comparable specs, but pin configuration may differ.

• 2N4401: Higher I_C (600 mA) and power dissipation.

Thermal Considerations

• Power Dissipation: Exceeding 625 mW without a heat sink risks thermal damage.

• Thermal Runaway: Mitigated by using emitter resistors and proper biasing.

Example Circuits

1. Switching Circuit:

• Conditions: Vcc=5V, LED Vf=2V, Ic=10 mA.

• Rc = (5V - 2V - 0.2V)/10 mA = 280 Ω.

• Assuming hFE=100, Ib=0.1 mA.

• Rb = (5V - 0.7V)/0.1 mA ≈ 43 kΩ (use 10 kΩ for saturation).

• Driving an LED:

2. Common-Emitter Amplifier:

• Midpoint biasing for linear operation.

• Voltage Gain ≈ Rc/Re (with bypassed emitter resistor).

Testing and Troubleshooting

• Multimeter Testing:

• Base-Emitter/Base-Collector junctions behave like diodes (conduct in forward bias).

• Emitter-Collector should be open in reverse bias.

• Common Issues:

• Overheating due to excessive current.

• Incorrect pin configuration or missing base resistor.

Alternatives

• BC547, 2N4401, PN2222 (TO-92 version of 2N2222).

Conclusion
The 2N3904’s enduring popularity stems from its adaptability in low-power applications, cost-effectiveness, and robust performance. Whether amplifying signals or switching loads, it remains a cornerstone of electronic design, supported by a wealth of documentation and complementary components like the 2N3906 PNP transistor. Engineers and hobbyists alike continue to rely on this transistor for its proven reliability and ease of use.