Need to switch a relay or amplify a weak sensor signal on a compact PCB? The MMBT3904 transistor is probably already in your design toolkit. It is the surface-mount version of the classic 2N3904 NPN junction transistor (BJT). Engineers rely on it for switching and small-signal amplification in a compact SOT-23 package.
Key Takeaways
- NPN BJT in SOT-23 package: The MMBT3904 is the SMD version of the 2N3904. It fits high-density board designs easily.
- 40 V collector-emitter voltage: VCEO reaches 40 V. This suits most low-voltage digital and analog circuits.
- 200 mA max collector current: IC supports up to 200 mA continuous. It handles small relay coils and LED drivers well.
- High DC current gain (hFE 100–300): The gain range suits both switching and amplification. Select parts for tighter gain ranges when needed.
- 300 MHz transition frequency: fT at 300 MHz enables audio and RF pre-amplifier use. It works well up to the lower VHF band.
- Pin 1 = Base, Pin 2 = Emitter, Pin 3 = Collector: Verify pinout with the datasheet. Incorrect orientation causes immediate failure.
- Complementary pair with MMBT3906: Use the MMBT3906 (PNP) for push-pull and complementary circuit designs.
MMBT3904 Transistor Overview
The MMBT3904 is a general-purpose NPN transistor. Zetec, Diodes Inc., and Fairchild manufacture it widely. It operates across consumer, industrial, and IoT electronics sectors. Its SOT-23 footprint suits high-density PCB layouts.
What Is the MMBT3904 Transistor?
The MMBT3904 is an NPN silicon BJT. It belongs to the 2N3904 family of general-purpose transistors. The ‘MMBT’ prefix indicates a surface-mount SOT-23 package. In NPN transistors, current flows from collector to emitter. A small base current controls a larger collector current. This makes the MMBT3904 ideal for switching and amplification tasks.
MMBT3904 vs 2N3904: Key Differences
The 2N3904 uses a through-hole TO-92 package. The MMBT3904, however, uses a surface-mount SOT-23 package. Their electrical parameters are nearly identical. The MMBT3904 suits automated SMT assembly lines. Furthermore, its smaller footprint saves valuable PCB space for high-density PCB designs.
MMBT3904 Transistor Pinout Explained
The SOT-23 package has three pins. Correct orientation is critical for circuit function. Always verify the MMBT3904 pinout using the manufacturer datasheet.
- Pin 1 (Base): Controls transistor switching. Apply a small signal here.
- Pin 2 (Emitter): Current exits the transistor here. Connect it usually to ground.
- Pin 3 (Collector): Main current enters here. Connect it to the load or supply.
The flat side of the SOT-23 package faces toward pin 1. However, always cross-reference with the specific manufacturer datasheet. Some alternative-source parts may differ slightly.
MMBT3904 Transistor Technical Specifications
The table below summarizes the key MMBT3904 electrical parameters. These apply at 25 degrees Celsius ambient unless otherwise stated.
| Parameter | Symbol | Value / Range | Unit | Notes |
| Collector–Emitter Voltage | VCEO | 40 | V | Max breakdown voltage |
| Collector–Base Voltage | VCBO | 60 | V | Reverse bias limit |
| Emitter–Base Voltage | VEBO | 6 | V | Forward bias limit |
| Collector Current (DC) | IC | 200 | mA | Continuous max |
| Power Dissipation | PD | 350 | mW | At 25°C ambient |
| DC Current Gain (hFE) | hFE | 100–300 | — | At IC = 10 mA |
| Transition Frequency | fT | 300 | MHz | Typical at IC = 10 mA |
| Collector–Emitter Saturation | VCE(sat) | 0.2 | V | At IC = 10 mA |
| Operating Temperature | TJ | -55 to +150 | °C | Junction temperature |
| Package | — | SOT-23 | — | Surface-mount (3-pin) |
The hFE range of 100 to 300 matters for circuit design. Do not assume a fixed gain value in critical designs. Measure or characterize parts for precision circuits. Also note that PD derates above 25 degrees Celsius.
MMBT3904 Key Features and Advantages
Low Saturation Voltage for Efficient MMBT3904 Switching
VCE(sat) is typically 0.2 V at 10 mA. This low value minimizes power loss in switching mode. As a result, efficiency improves in battery-powered designs.
High Transition Frequency for MMBT3904 RF Use
The fT of 300 MHz enables RF pre-amplification. For example, it suits FM receiver front-end stages. In addition, it handles audio amplifier designs up to several MHz.
SOT-23 Package for Modern MMBT3904 PCB Designs
The compact SOT-23 package saves board space. It is compatible with standard SMT reflow processes. Therefore, it integrates easily into high-volume production lines.
Wide Temperature Operating Range
Junction temperature ranges from -55 to +150 degrees Celsius. This suits automotive and industrial environments. However, derate PD at temperatures above 25 degrees Celsius.
MMBT3904 Transistor Common Applications
Digital Logic Level Shifting with the MMBT3904
MCUs often output 3.3 V logic signals. The MMBT3904 shifts these to 5 V or 12 V. Connect the base to the MCU output via a resistor. Then connect the collector to the higher-voltage load. Finally, connect the emitter to ground.
LED and Relay Switching Circuits
The 200 mA IC rating drives most LEDs directly.For example, use the MMBT3904 to switch small relay coils up to 100 mA. Similar transistor switching techniques are commonly used in switched-mode power supplies (SMPS). Calculate the base resistor using hFE and IC for saturation. Also, add a flyback diode across inductive loads to protect the transistor.
Small-Signal Audio Amplification
The MMBT3904 amplifies microphone and audio pre-amp signals effectively. Its fT of 300 MHz far exceeds audio bandwidth requirements. Therefore, it performs well in common-emitter amplifier stages. Use AC coupling capacitors to block DC bias from the signal path.
Sensor Interface and Signal Conditioning
Many sensors output weak current or voltage signals. The MMBT3904 buffers these signals for ADC inputs. In addition, it converts current signals to voltage. This simplifies microcontroller analog input interfacing.
MMBT3904 Transistor Comparison Table
Use this table to compare the MMBT3904 with similar NPN and PNP SMD transistors.
| Feature | MMBT3904 | MMBT2222A | MMBT3906 (PNP) |
| Type | NPN | NPN | PNP |
| VCEO | 40 V | 40 V | 40 V |
| IC (max) | 200 mA | 600 mA | 200 mA |
| hFE (typical) | 100–300 | 100–300 | 100–300 |
| fT | 300 MHz | 300 MHz | 250 MHz |
| Package | SOT-23 | SOT-23 | SOT-23 |
| Best Use | Low-power switching | Higher-current loads | Complementary PNP pair |
Choose the MMBT2222A when load current exceeds 200 mA. Use the MMBT3906 for complementary PNP circuits. However, for most low-power switching tasks, the MMBT3904 is sufficient.
MMBT3904 Circuit Design Tips
MMBT3904 Base Resistor Calculation
First, determine the required collector current (IC). Next, divide IC by the minimum hFE to get the required base current (IB). Finally, calculate the resistor value: RB = (VIN − VBE) / IB. Use VBE = 0.7 V as a standard value.
For example, to drive a 50 mA load with a minimum hFE of 100, IB = 0.5 mA. So RB = (3.3 V − 0.7 V) / 0.5 mA = 5.2 kΩ. In practice, use a standard 4.7 kΩ resistor for this value.
Avoiding Common MMBT3904 Design Errors
- Always verify pinout before soldering. SOT-23 pins are easily confused.
- Add a base resistor. Never drive the base directly from a logic pin without one.
- Use a flyback diode for inductive loads. The transistor has no internal protection.
- Derate PD at high ambient temperatures. Check the derating curve in the datasheet for specific values at your operating temperature.
- Do not exceed VCEO of 40 V. Higher voltages damage the transistor immediately.
MMBT3904 Procurement and Supply Chain
The MMBT3904 is a high-volume commodity component. Availability is generally excellent worldwide. Multiple manufacturers produce pin-compatible alternatives. For example, Diodes Inc., ON Semiconductor, and Nexperia all offer it. However, always verify the datasheet for your specific source.
Standard packaging options include tape and reel (T&R) for SMT lines. MOQ for T&R is typically 3,000 pieces. Loose bulk packaging is also available for prototyping. Lead times are usually under two weeks from major distributors.
MMBT3904 Transistor FAQ
What Is the MMBT3904 Transistor Used For?
The MMBT3904 serves switching, amplification, and signal conditioning tasks. Common applications include LED drivers, relay switches, and audio pre-amplifiers. Engineers also use it for logic level shifting between 3.3 V and 5 V systems. Its compact SOT-23 package makes it suitable for space-constrained PCB designs. In addition, its wide hFE range of 100 to 300 supports both low-gain switching and higher-gain amplifier configurations. Choose this transistor when collector current stays below 200 mA.
What Is the Pinout of the MMBT3904?
Pin 1 is the Base, Pin 2 is the Emitter, and Pin 3 is the Collector. The flat side of the SOT-23 package faces pin 1 as a visual reference. However, always confirm the pinout with the manufacturer datasheet before soldering. Different sources may use slightly different markings. Incorrect pin orientation causes immediate transistor failure. Verify orientation under magnification during prototype assembly.
What Is the Max Current of the MMBT3904?
The maximum continuous collector current (IC) is 200 mA. Do not exceed this value to prevent transistor damage. For loads above 200 mA, use the MMBT2222A instead, which supports up to 600 mA. Also note that power dissipation (PD) limits to 350 mW at 25°C. As a result, both IC and PD constrain the maximum safe operating point. Always calculate both limits before finalizing your design.
Is MMBT3904 the Same as 2N3904?
Yes, they are electrically equivalent in most parameters. However, the MMBT3904 uses a surface-mount SOT-23 package, while the 2N3904 uses a through-hole TO-92 package. Choose the MMBT3904 for SMT assembly lines and compact PCB designs. Choose the 2N3904 for breadboard prototyping or through-hole builds. Both share the same hFE range, VCEO, and transition frequency. In addition, both suit the same switching and amplification applications.
What Replaces the MMBT3904?
The MMBT2222A is a common NPN replacement with a higher IC rating of 600 mA. The BC817 is another SOT-23 NPN alternative with similar characteristics. For PNP applications, use the MMBT3906 as the complementary pair. Always compare key specs — including VCEO, IC, hFE, and fT — before substituting. Furthermore, verify that the pinout matches before dropping in a replacement. Some alternatives may have a different pin assignment in the SOT-23 package.
MMBT3904 Transistor Conclusion
The MMBT3904 is a reliable, versatile NPN BJT that engineers have trusted for decades. Its SOT-23 package suits modern SMD circuit designs. Its wide hFE range and 300 MHz fT support switching, amplification, and signal conditioning tasks. For your next design, start with the base resistor calculation above and grab the datasheet from your preferred manufacturer.
Find What You Need on LCSC
Finding the right MMBT3904 transistor for your design is easy on LCSC. LCSC stocks the MMBT3904 from multiple manufacturers, including Diodes Inc., ON Semiconductor, and Nexperia. You can filter by manufacturer, packaging, and quantity. Browse the full MMBT3904 catalogue today.
References
ON Semiconductor / onsemi — MMBT3904 Datasheet: https://www.onsemi.com/pdf/datasheet/mmbt3904-d.pdf
Diodes Inc. — MMBT3904 Product Page: https://www.diodes.com/part/view/MMBT3904/
Nexperia — MMBT3904 Datasheet: https://assets.nexperia.com/documents/data-sheet/MMBT3904.pdf
