BD681 Pinout, Equivalent, Applications, Datasheet

The BD681 is a high-gain NPN Darlington transistor in a TO-126 package designed for medium-power amplification and switching. This guide covers BD681 pinout, datasheet, and practical applications.

Introduction to BD681 NPN Transistor

The BD681 is a silicon NPN Darlington power transistor known for its high current gain, excellent performance, and reliability in amplification and switching applications. Packaged in a TO-126 case, it provides an efficient balance between power handling and compactness, making it ideal for audio amplifier stages, voltage regulators, and driver circuits.

Its Darlington pair configuration combines two NPN transistors internally to achieve very high current amplification (hFE). This allows the BD681 transistor to deliver large collector currents with minimal base drive, making it a popular choice in control, automation, and signal amplification systems.

Due to its electrical characteristics and complementary pairing with BD682 (PNP), it is widely used in push-pull amplifier circuits for high-efficiency audio output and signal processing.

BD681 NPN Transistor

Pinout of BD681

Understanding the BD681 Pinout Configuration

The BD681 pinout follows the standard layout for TO-126 NPN Darlington transistors, ensuring compatibility with a wide range of circuit designs. Proper pin identification is essential to prevent incorrect wiring and device failure.

Pin Configuration of BD681 Pinout

Key Features of BD681 Transistor

• High gain NPN Darlington transistor for power amplification
• Collector current up to 4A for medium-load applications
• High DC gain with minimal base drive requirement
• Compact TO-126 package with efficient heat dissipation
• Ideal for audio and voltage control circuits
• Complementary to BD682 (PNP Darlington transistor)
• Excellent linearity and performance stability
• Low saturation voltage and fast switching response

BD681 Transistor Datasheet and Specifications

• Collector-Emitter Voltage (Vceo): 100 V
• Collector-Base Voltage (Vcbo): 100 V
• Emitter-Base Voltage (Vebo): 5 V
• Collector Current (Ic): 4 A
• Power Dissipation (Ptot): 40 W
• DC Current Gain (hFE): Minimum 750
• Transition Frequency (fT): 3 MHz typical
• Junction Temperature (Tj max): 150 °C
• Polarity: NPN
• Package Type: TO-126

According to the BD681 transistor datasheet, this component is suitable for medium-power audio and control circuits. It delivers high efficiency with low base current input and can be easily used in amplifier designs or as a relay driver in industrial applications.

Working Principle of BD681 NPN Transistor

The BD681 transistor operates as a Darlington pair, internally combining two NPN transistors in a single package. This design provides extremely high current amplification, allowing a small base current to control a large collector current.

When a positive voltage is applied to the base relative to the emitter, the transistor conducts and allows current to flow from collector to emitter, amplifying the input signal. The Darlington configuration ensures minimal signal loss and improved input impedance, key advantages for precision amplifiers, drivers, and switching control systems.

More Circuit Layouts

Typical Applications of BD681

• Audio power amplifiers and preamplifiers
• Relay and motor drivers
• Voltage regulation and control circuits
• Power switching and industrial automation systems
• Push-pull output stages with BD682 (PNP)
• LED and lamp drivers
• DC motor speed controllers and servo circuits

PNP Complementary Transistor

The PNP complementary transistor of BD681 is BD682. Both transistors are engineered as matched pairs for use in push-pull amplifier configurations, where one transistor handles the positive signal cycle (NPN) and the other handles the negative cycle (PNP).

This complementary arrangement enhances signal symmetry, reduces distortion, and increases overall amplifier efficiency, making BD681 and BD682 an excellent choice for high-quality audio and control circuits.

Equivalent Transistors and Alternatives

If BD681 is unavailable, several equivalent or alternative NPN Darlington transistors can be used depending on circuit requirements.

Equivalent / Replacement Transistors:

• BD679 (lower gain NPN Darlington)
• TIP122 (NPN Darlington transistor, TO-220 package)
• MJE800 (NPN power Darlington)
• 2N6045 (NPN Darlington transistor)

Before substitution, verify the voltage, current, and package compatibility to ensure reliable operation in your specific circuit.

Comparison BD681 vs BD682 Complementary Transistors

The BD681 (NPN) and BD682 (PNP) form a complementary pair of Darlington power transistors, designed for symmetrical operation in audio, amplifier, and driver circuits.

BD681 conducts when its base is driven positive relative to the emitter, while BD682 conducts with a negative base drive. Together, they provide full-cycle amplification, improving sound fidelity and efficiency in push-pull amplifier stages.

Both transistors share identical voltage and current ratings (80V, 4A) and offer excellent gain and linearity, making them highly reliable for medium-power electronic systems.

Frequently Asked Questions (FAQ)

What is the BD681 transistor used for?

The BD681 is used for power amplification, motor control, and switching applications requiring high current gain.

What type of transistor is BD681?

It is an NPN Darlington transistor designed for medium-power switching and audio applications.

What is the BD681 pin configuration?

From left to right (flat side facing you): Emitter, Collector, Base.

What is the complementary transistor for BD681?

The complementary transistor is BD682 (PNP).

Can I use TIP122 instead of BD681?

Yes, TIP122 can be used as a substitute if circuit design and power requirements match.

Conclusion

The BD681 NPN Darlington transistor is a robust, high-gain device perfect for medium-power amplifier and control applications. With a collector current rating of 4A and voltage tolerance of 80V, it offers dependable performance in audio, motor driver, and automation circuits.

When used with BD682, the pair provides efficient and symmetrical amplification, ensuring low distortion and high output quality. Its TO-126 package makes it ideal for compact yet powerful circuit designs.

Datasheet of BD681 NPN Transistor

Click the following Button to download the datasheet of BD681 Transistor :

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