MMBT3904 NPN Transistor for General-Purpose Circuits

ElecSuppliers Part Number GuideSep 06, 2026

MMBT3904 is a widely used NPN bipolar junction transistor designed for low-power switching and amplification applications. It is commonly selected for signal processing, transistor switching, driver stages and control circuits where a compact surface-mount device is preferred.

The device is based on the well-established 2N3904 transistor family and is offered in a small SOT-23 surface-mount package. Its combination of low power requirements, compact size and general-purpose characteristics makes MMBT3904 suitable for a broad range of electronic designs.

MMBT3904 Electrical Characteristics

MMBT3904 is an NPN transistor with a collector-emitter voltage rating of up to 40V. The device is intended for relatively low-power applications rather than high-current power switching.

Its collector current capability is typically specified up to 200mA, providing sufficient capacity for many control and signal-level switching tasks. This makes the transistor useful for driving LEDs, small relays, signal lines and other low-current loads when controlled by a microcontroller or logic circuit.

The transistor also provides useful current gain for amplification and switching stages. Actual gain depends on operating current, device conditions and the selected manufacturer version, so circuit designers should refer to the corresponding datasheet when precise gain limits are important.

Compact SOT-23 Package

One of the main advantages of MMBT3904 is its small surface-mount package. The SOT-23 form factor occupies significantly less PCB area than traditional through-hole transistor packages.

This makes it suitable for compact control boards, embedded electronics, communication equipment and other products where PCB space is limited.

The three-terminal configuration provides straightforward integration into conventional NPN transistor circuits, including common-emitter amplifier stages and low-side switching circuits.

MMBT3904 in Switching Applications

MMBT3904 can be used as a low-side switching device when a relatively small load needs to be controlled by a logic signal.

A microcontroller output, sensor signal or other control source can drive the transistor's base through an appropriate resistor. When sufficient base current is applied, the transistor conducts between collector and emitter and can control a connected load.

Typical applications include LED indicators, small signal relays, buzzers, interface circuits and discrete control functions.

For switching designs, the base resistor should be selected according to the required collector current and the characteristics of the driving circuit. This helps ensure that the transistor operates reliably without placing excessive current demand on the control device.

MMBT3904 for Signal Amplification

Beyond switching, MMBT3904 can function as a small-signal amplifier. Its NPN structure allows it to be configured in common-emitter, common-collector or other transistor amplifier circuits.

The common-emitter configuration is particularly useful when voltage gain is required. Depending on the surrounding components and bias conditions, MMBT3904 can be used in audio, sensor, interface and general analog signal circuits.

Because it is a small-signal transistor, PCB designers can also use it as an individual amplification stage within larger analog or mixed-signal circuits.

PCB Design Considerations

When incorporating MMBT3904 into a circuit, designers should consider collector current, collector-emitter voltage, power dissipation and junction temperature under the actual operating conditions.

The SOT-23 package is well suited to compact PCBs, but thermal performance depends on PCB copper area, ambient temperature and the amount of power dissipated by the device.

For switching applications, minimizing unnecessary base current while maintaining reliable transistor saturation can help improve overall circuit efficiency.

Common Uses for MMBT3904

MMBT3904 is suitable for a wide range of low-power electronic functions. It can be found in transistor switching circuits, signal amplifiers, LED drivers, interface circuits, sensor systems, small relay drivers and general-purpose control electronics.

Its familiar NPN architecture also makes it convenient for designs that need an inexpensive and readily available discrete transistor rather than a dedicated integrated driver.

For new PCB designs requiring a compact NPN transistor for general-purpose switching or signal amplification, MMBT3904 remains a practical component choice.