2N3904 Transistor: Parameter and Applications

The 2N3904 is an NPN transistor for general-purpose switching or low-power amplifier applications. It is mainly designed for medium voltage, low power applications and operates at medium and high speed.

Parameter of 2N3904 Transistor

The 2N3904 is a widely used NPN bipolar junction transistor (BJT) known for its versatility and availability. It's commonly used in low-power amplifying and switching applications.

2N3904 Transistor’s Key Specifications

Parameter	Value
Transistor Type	NPN
Maximum Collector-Emitter Voltage (VCEO)	40 V
Maximum Collector-Base Voltage (VCBO)	60 V
Maximum Emitter-Base Voltage (VEBO)	6 V
Collector Current (IC)	200 mA
Power Dissipation (PD)	625 mW
DC Current Gain (hFE)	100 to 300 (at IC = 10 mA)
Transition Frequency (fT)	250 MHz
Package	TO-92

Applications

Signal amplification

Low power switching

General-purpose electronics

Logic circuits

Pin Configuration (TO-92 Package):

   (Flat side facing you)

   ---------------------

   | E | B | C |

   |---|---|---|

   | 1 | 2 | 3 |

E: Emitter

B: Base

C: Collector

Specifications

High voltage gain: The 2N3904 has a high voltage gain, suitable for use in amplifier circuits.

Low Noise: Transistors have low noise figures and are very useful in low-level signal amplification applications.

High Current Gain: The 2N3904 has a high current gain, which is suitable for driving loads such as relays and lamps.

Fast switching speed: The transistors have fast switching speeds, making them ideal for switching applications.

Low cost: 2N3904 is cheap.

In the 2N3904 transistor, the majority charge carriers are electrons, so they are negatively charged. The operating state of this transistor can switch from reverse bias to forward bias with a small voltage at the base terminal (typically around 0.7V), turning the transistor ON.

2N3904 NPN Transistor Operating Principle Diagram

Normal Operating Conditions:

Base voltage (Vb) > Emitter voltage (Ve)

Collector voltage (Vc) > Base voltage (Vb)

If the base pin is connected to GND, then both the emitter and collector terminals are in reverse bias or remain open (transistor is OFF). Once a signal is applied to the base pin, it becomes forward biased, allowing conduction.

Amplification and Current Limits

The high gain value of the 2N3904, typically up to 300, determines its amplification capability.

The maximum collector current (Ic) is 200 mA, so any load requiring more than 200 mA should not be connected through this transistor.

When current is applied to the base terminal, the transistor becomes biased.

The base current (Ib) should be limited to 5 mA to prevent damage.

Saturation and Cutoff States

When the 2N3904 NPN transistor is fully biased, it allows a maximum current of 200 mA to flow between the collector and emitter terminals—this is called the saturation region.

The transistor can typically handle 40V between collector and emitter, and 60V between collector and base.

When the base current is removed, the transistor turns OFF, which is called the cutoff region, with the VBE (base-emitter voltage) being around 600 mV.

What transistor can be used to replace 2N3904

1. Equivalent transistor

BC636, BC547, BC549, BC639, 2N2222 TO-18, 2N2222 TO-92, 2N2369, 2N3906, 2N3055, 2SC5200, etc.

2. 2N3904 patch

2N3904 patch is MMBT3904

Applications of 2N3904 Transistor

Using 2N3904 as a Switch

The 2N3904 transistor can be effectively used as a switch in electronic circuits, allowing a small control signal to control a larger current or voltage. When used in switching mode, the transistor operates in either the cutoff region (OFF) or the saturation region (ON), providing binary operation for the circuit.

Step-by-Step Guide to Using 2N3904 as a Switch

1) Understand the Pin Configuration

Know the pinout of the 2N3904 transistor:

The Emitter is typically connected to ground.

The Base receives the control signal.

The Collector connects to the load or the part of the circuit being switched.

2) Determine Load Requirements

Identify the load to be controlled by the transistor switch, including its voltage and current specifications. These parameters influence resistor selection and power handling.

3) Select the Base Resistor

Calculate the base current (IB) needed to fully saturate the transistor:

IB = IL / hFE

Where:

IL = Load current

hFE = Transistor gain (typically 100–300)

Then calculate the base resistor (RB) to limit base current:

RB = (Vcontrol - VBE) / IB

Where:

Vcontrol = Control signal voltage

VBE ≈ 0.7V for a silicon BJT

Choose a resistor that limits current to about 1/10th of the calculated value for added safety.

4) Connect the Transistor

Wire the components according to the switching circuit:

Emitter to GND

Collector to load and then to VCC

Base to control signal via RB

Make sure all connections have correct polarity.

5) Test and Operate

Apply the control voltage to the base:

When base current flows and the transistor is saturated, it turns ON and conducts between collector and emitter—activating the load.

When the control signal is removed, the transistor enters cutoff and turns OFF—deactivating the load.

6) Add Protection

Use components like flyback diodes or clamping circuits to suppress voltage spikes from inductive loads, protecting the transistor and surrounding components.

Step-by-Step Guide to Using 2N3904 as an Amplifier

The 2N3904 transistor is commonly used as a general-purpose amplifier in electronic circuits. Its versatility and availability make it suitable for a wide range of applications where signal amplification is needed. When used as an amplifier, the 2N3904 operates in its active region, where a small input signal is amplified to produce a larger output signal.

1) Pin Configuration

Familiarize yourself with the pinout of the 2N3904 transistor (same as discussed before):

Emitter

Base

Collector

2) Biasing the Transistor

Apply appropriate DC bias voltage to the base-emitter junction to set the transistor's operating point (Q-point) in the active region.

Use a voltage divider (two resistors) to create the required bias voltage at the base.

Proper biasing ensures the transistor stays in the linear region for effective amplification.

Refer to the datasheet for recommended biasing conditions (e.g., VBE ≈ 0.7V, IC = a few mA).

3) Coupling Capacitors

Use coupling capacitors to block DC and allow only the AC components (signals) to pass:

One capacitor between the input signal and the base.

One capacitor between the collector and the output.

These capacitors prevent the DC bias from affecting the input and output signal lines.

4) Load Resistor

Connect a load resistor between the collector and the positive supply voltage (VCC).

This resistor helps define the voltage gain of the amplifier.

It also drops voltage based on the amplified current, generating the output signal.

5) Impedance Matching

Match the amplifier’s:

Input impedance with the source impedance, and

Output impedance with the load impedance

This ensures efficient signal transfer, maximizes power delivery, and minimizes signal reflection.

6) Testing and Tuning

Apply an input signal to the base, and observe the amplified output across the load resistor at the collector:

Fine-tune the bias resistors to optimize the transistor’s operating point.

Use an oscilloscope or multimeter to measure and analyze the input and output waveforms.

7) Stability and Compensation

Add capacitors or resistors as compensation components if needed:

These components help stabilize the amplifier and prevent oscillation.

Choose based on the desired frequency response and bandwidth of the amplifier.

8) Amplifier Configurations

The 2N3904 can be configured in several amplifier topologies:

Common Emitter (CE) – High voltage gain, moderate input and output impedance.

Common Collector (Emitter Follower) – Voltage buffer, high input impedance, low output impedance.

Common Base – Low input impedance, high voltage gain, wide bandwidth.