2N3904 NPN Bipolar Junction Transistor Documentation

1. Introduction

The 2N3904 is a widely used general-purpose NPN bipolar junction transistor (BJT) designed for low-power amplification and switching applications. Known for its affordability, reliability, and versatility, the 2N3904 is a staple in many electronic projects, from hobbyist circuits to professional designs. It is particularly well-suited for applications requiring small signal amplification or low-current switching.

Common Applications:

• Signal amplification in audio and RF circuits
• Low-power switching in digital and analog circuits
• Driving small loads such as LEDs or relays
• Oscillator and timer circuits
• General-purpose use in prototyping and educational projects

The 2N3904 is often used in conjunction with microcontrollers like the Arduino UNO, making it a popular choice for beginners and experienced engineers alike.

2. Technical Specifications

The following table outlines the key technical specifications of the 2N3904 transistor:

Pin Configuration

The 2N3904 is housed in a TO-92 package with three pins. The pinout is as follows:

Below is a diagram of the TO-92 package for reference:

   _______
  |       |
  |  2N   |
  | 3904  |
  |_______|
   | | |
   E B C
  (Front View)

3. Usage Instructions

Using the 2N3904 in a Circuit

The 2N3904 can be used in two primary modes: switching and amplification.

1. Switching Mode

In switching mode, the transistor acts as an electronic switch. When a small current is applied to the base, it allows a larger current to flow from the collector to the emitter.

Example Circuit: Controlling an LED with an Arduino UNO

• Connect the emitter to ground.
• Connect the collector to one terminal of the LED (with a current-limiting resistor in series).
• Connect the other terminal of the LED to the positive voltage supply.
• Use a resistor (typically 1kΩ) between the Arduino's digital pin and the base of the transistor.

Arduino Code Example:

// Define the pin connected to the transistor's base
const int transistorBasePin = 9;

void setup() {
  pinMode(transistorBasePin, OUTPUT); // Set the pin as an output
}

void loop() {
  digitalWrite(transistorBasePin, HIGH); // Turn on the transistor (LED ON)
  delay(1000);                           // Wait for 1 second
  digitalWrite(transistorBasePin, LOW);  // Turn off the transistor (LED OFF)
  delay(1000);                           // Wait for 1 second
}

2. Amplification Mode

In amplification mode, the transistor amplifies a small input signal at the base into a larger output signal at the collector. This is commonly used in audio or RF circuits.

Example Circuit: Audio Amplifier

• Connect the emitter to ground through a resistor.
• Connect the base to the input signal through a coupling capacitor and a biasing resistor.
• Connect the collector to the positive voltage supply through a load resistor.

Important Considerations

• Base Resistor: Always use a resistor between the base and the control signal to limit the base current and prevent damage to the transistor.
• Power Dissipation: Ensure the transistor does not exceed its maximum power dissipation (625mW). Use a heatsink if necessary.
• Voltage Ratings: Do not exceed the maximum voltage ratings (e.g., 40V for V_CEO).
• Current Ratings: Ensure the collector current does not exceed 200mA.

4. Troubleshooting and FAQs

Common Issues and Solutions

Frequently Asked Questions (FAQs)

Q1: Can I use the 2N3904 to drive a motor?
A1: The 2N3904 can drive small motors with currents below 200mA. For larger motors, use a power transistor or MOSFET.

Q2: What is the purpose of the base resistor?
A2: The base resistor limits the current flowing into the base to prevent damage to the transistor and the control circuit.

Q3: Can the 2N3904 be used in high-frequency circuits?
A3: Yes, the 2N3904 has a transition frequency (f_T) of 300MHz, making it suitable for high-frequency applications.

Q4: How do I test if my 2N3904 is working?
A4: Use a multimeter in diode mode to check the base-emitter and base-collector junctions. Both should show a forward voltage drop (~0.6–0.7V) when forward-biased.

This documentation provides a comprehensive guide to understanding, using, and troubleshooting the 2N3904 NPN BJT transistor. Whether you're a beginner or an experienced engineer, the 2N3904 is a versatile and reliable component for your electronic projects.