What is the MMBT3904 transistor?

The MMBT3904 is an NPN transistor in a small SOT-23 surface-mount package. It is a member of the BC547 transistor family and is functionally equivalent to the 2N3904 transistor. The MMBT3904 is designed for low-power applications, with a maximum collector current of 200mA and a maximum collector-emitter voltage of 40V.

Key features of the MMBT3904 transistor

1. High current gain (hFE): 100 to 300
2. Low collector-emitter saturation voltage (VCE(sat)): 0.2V max at 10mA
3. Fast switching speeds: ton = 35ns, toff = 50ns
4. Low leakage current: ICBO = 50nA max at 30V
5. Small SOT-23 package: 2.9mm x 1.3mm x 1.0mm

MMBT3904 pinout and package information

The MMBT3904 transistor comes in a SOT-23 (Small Outline Transistor) package with three pins:

1. Base (B)
2. Emitter (E)
3. Collector (C)

The pinout for the MMBT3904 is as follows:

Pin	Function
1	Base
2	Emitter
3	Collector

MMBT3904 electrical characteristics

The following table summarizes the key electrical characteristics of the MMBT3904 transistor:

Parameter	Symbol	Value
Collector-Emitter Voltage	VCEO	40V
Collector-Base Voltage	VCBO	60V
Emitter-Base Voltage	VEBO	6V
Collector Current	IC	200mA
Power Dissipation	PD	250mW
DC Current Gain	hFE	100 to 300
Collector-Emitter Saturation Voltage	VCE(sat)	0.2V at 10mA
Base-Emitter On Voltage	VBE(on)	0.7V to 0.9V
Transition Frequency	fT	300MHz
Turn-On Time	ton	35ns
Turn-Off Time	toff	50ns

Applications of the MMBT3904 transistor

The MMBT3904 transistor is suitable for a wide range of applications, including:

1. Switching circuits
2. Logic gates
3. Amplifiers
4. Drivers for LEDs and small relays
5. Interface circuits
6. Signal conditioning
7. Pulse generators
8. Voltage regulators
9. Temperature sensors
10. Current sources

Example: MMBT3904 as a switch

One of the most common applications of the MMBT3904 is as a switch. In this example, we’ll demonstrate how to use the MMBT3904 to control an LED.

Schematic

        +5V
         |
        [R1]
         |
         |
        /
[LED]--<
        \
         |
         |
        [R2]
         |
        B|
MMBT3904 |E
        C|
         |
        GND

Components

• R1: 1kΩ resistor (current-limiting resistor for the LED)
• R2: 10kΩ resistor (base resistor for the MMBT3904)
• LED: Light-emitting diode
• MMBT3904: NPN transistor

Explanation

When a voltage greater than 0.7V is applied to the base of the MMBT3904 through R2, the transistor turns on, allowing current to flow from the collector to the emitter. This current flow causes the LED to illuminate. When the base voltage is removed or falls below 0.7V, the transistor turns off, and the LED stops emitting light.

The base resistor R2 is used to limit the current flowing into the base of the transistor, ensuring that the transistor operates within its safe operating area. The LED current-limiting resistor R1 is chosen based on the desired LED current and forward voltage drop.

MMBT3904 vs. other small-signal NPN transistors

There are several small-signal NPN transistors that can be used in place of the MMBT3904, depending on the specific application requirements. Some common alternatives include:

1. 2N3904: Through-hole version of the MMBT3904
2. BC847: Similar specifications, but with a slightly higher current gain
3. BC817: Higher voltage and current ratings compared to the MMBT3904
4. MMBTA42: Higher voltage rating (300V) and lower leakage current

When selecting a substitute for the MMBT3904, consider factors such as package type, voltage and current ratings, current gain, switching speed, and leakage current to ensure proper performance in your application.

Frequently Asked Questions (FAQ)

1. What is the difference between MMBT3904 and 2N3904 transistors?

The main difference between MMBT3904 and 2N3904 transistors is their package type. The MMBT3904 comes in a surface-mount SOT-23 package, while the 2N3904 is a through-hole device. Electrically, they are functionally equivalent and can be used interchangeably in most applications.

2. Can I use the MMBT3904 for high-power applications?

No, the MMBT3904 is a small-signal transistor designed for low-power applications. Its maximum collector current is 200mA, and its power dissipation is limited to 250mW. For high-power applications, consider using power transistors like the TIP31 or TIP41.

3. How do I determine the base resistor value for the MMBT3904?

To determine the base resistor value, you need to consider the desired collector current and the transistor’s current gain (hFE). Use the following formula:

R_base = (V_supply – V_BE) / (I_collector / hFE)

Where V_supply is the supply voltage, V_BE is the base-emitter voltage (typically 0.7V), I_collector is the desired collector current, and hFE is the current gain of the transistor.

4. What is the maximum switching frequency of the MMBT3904?

The MMBT3904 has a transition frequency (fT) of 300MHz, which is the frequency at which the current gain falls to unity. However, the practical switching frequency is lower and depends on factors such as the circuit design, load, and drive current. In general, the MMBT3904 is suitable for switching applications up to a few MHz.

5. Can I use the MMBT3904 as a linear amplifier?

Yes, the MMBT3904 can be used as a linear amplifier, particularly in low-power, low-frequency applications. However, its performance may be limited compared to transistors specifically designed for linear amplification, such as the BC546 or 2N5089. When using the MMBT3904 as a linear amplifier, ensure that it operates within its safe operating area and consider factors such as bias point stability and linearity.

Conclusion

The MMBT3904 is a versatile and reliable NPN switching transistor that finds use in a wide range of low-power applications. Its small SOT-23 package, high current gain, and excellent switching characteristics make it a popular choice for designers and hobbyists. By understanding its electrical characteristics, pinout, and typical applications, you can effectively incorporate the MMBT3904 into your projects, whether you’re building switching circuits, amplifiers, or simple LED drivers.