The 2N3904 is a popular general-purpose NPN bipolar junction transistor (BJT) designed for low-power switching and signal amplification. Supplied in a compact TO-92 through-hole package, it is widely used in electronic projects, control circuits, relay drivers, LED drivers, signal amplifiers, and educational applications.
With a collector-emitter voltage rating of up to 40V and collector current capability up to 200mA, the 2N3904 is suitable for controlling small loads and amplifying low-level signals.
Pin Configuration
For the commonly used TO-92 version, the pin configuration is typically:
Pin 1 – Emitter: Current exits through the emitter in conventional-current terms.
Pin 2 – Base: Controls transistor conduction.
Pin 3 – Collector: Current enters through the collector in conventional-current terms.
Important: Pin arrangement can vary by manufacturer. Always verify the datasheet or markings for the exact component supplied before installation.
Technical Specifications
-
Transistor Type: NPN Bipolar Junction Transistor (BJT)
-
Package: TO-92
-
Collector-Emitter Voltage (VCEO): 40V Max.
-
Collector-Base Voltage (VCBO): 60V Max.
-
Emitter-Base Voltage (VEBO): 6V Max.
-
Continuous Collector Current (IC): 200mA Max.
-
DC Current Gain (hFE): Device- and operating-condition dependent; up to approximately 300 for specified conditions
-
Mounting Type: Through-Hole
-
Typical Use: Switching and Signal Amplification
Using the 2N3904 as a Switch
The 2N3904 can be used as an electronic switch by operating between the cut-off and saturation regions.
When insufficient base current is applied, the transistor remains in cut-off and very little collector current flows. When sufficient base current is supplied, the transistor can enter saturation and conduct between collector and emitter.
A base resistor should normally be used to limit current from the controlling circuit.
For switching applications, the base resistor can be estimated using:
RB = (VCONTROL − VBE) / IB
where VBE is typically around 0.6–0.7V when conducting, rather than 5V as stated in the original description.
The required base current should be selected according to the collector load current and the desired saturation conditions.
Using the 2N3904 as an Amplifier
The 2N3904 can also operate in its active region as a small-signal amplifier.
Common transistor amplifier configurations include:
Common Emitter – Frequently used for voltage and signal amplification.
Common Collector – Also known as an emitter follower and commonly used for buffering.
Common Base – Used in selected high-frequency and specialized amplifier circuits.
The transistor's DC current gain is expressed as:
hFE = IC / IB
where IC is collector current and IB is base current.
Actual gain varies with collector current, temperature, device variation, and manufacturer specifications.
Applications
The 2N3904 is commonly used in relay drivers, LED drivers, transistor switching circuits, audio and signal amplifiers, sensor interfaces, oscillators, logic interfaces, pulse circuits, microcontroller projects, Arduino projects, educational electronics, and general-purpose PCB designs.
When driving inductive loads such as relays, a suitable flyback diode should normally be installed across the load to protect the transistor from inductive voltage spikes.
Important Notes
The 200mA collector current is an absolute maximum rating, not a recommended continuous operating target. Actual allowable current depends on power dissipation, collector-emitter voltage, ambient temperature, PCB design, and the specific manufacturer's datasheet.
The 6V specification refers to maximum emitter-base reverse voltage (VEBO). It is not the normal forward base-emitter operating voltage.
For reliable circuit design, always verify the electrical characteristics and pin configuration against the datasheet for the exact 2N3904 manufacturer supplied.
Package Includes
1 × 2N3904 NPN Transistor
