The BD140 transistor is a widely used medium-power PNP device valued for its balance of voltage capability, current handling, and stable linear performance. Commonly paired with complementary NPN transistors, it is used in audio amplifiers, driver stages, and control circuits where reliability, symmetry, and predictable behavior are a must.
What Is the BD140 Transistor?
The BD140 is a medium-power PNP bipolar junction transistor (BJT) made with silicon technology and housed in a TO-126 package. It is designed to handle moderate current and voltage levels, with ratings up to 1.5 A and 80 V, and can dissipate about 12.5 W when proper heat sinking is used. As part of a complementary transistor family, it pairs with NPN devices such as the BD139 and BD135, making it suitable for circuits that require balanced or push-pull operation and stable linear performance, particularly in audio and driver stages.
BD140 Pinout Configuration
| Pin Number | Pin Name | Description |
|---|---|---|
| 1 | Emitter | Connects to the higher potential side of the circuit in PNP operation |
| 2 | Collector | Connects to the load and conducts current during operation |
| 3 | Base | Controls biasing and switching |
BD140 Features and Technical Specs
| Parameter | Specification |
|---|---|
| Transistor type | PNP bipolar junction transistor (BJT) |
| Maximum collector current (IC) | −1.5 A |
| Collector–emitter voltage (VCE) | −80 V |
| Collector–base voltage (VCB) | −80 V |
| Emitter–base voltage (VEBO) | −5 V |
| DC current gain (hFE) | Typically, 25 to 250 |
| Maximum power dissipation | 12.5 W |
| Transition frequency (fT) | Up to 190 MHz |
| Operating temperature range | −55 °C to +150 °C |
| Package type | TO-126 |
BD140 Equivalent and Replacement Transistors
Replacement
• BD238G – A medium-power PNP transistor with similar voltage and current ratings, commonly used in driver and audio stages where stable linear performance is needed.
• BD170 – Offers higher voltage tolerance than the BD140, making it suitable for circuits with higher supply rails while maintaining comparable current handling.
• BD180 – Designed for higher voltage applications and moderate current levels, often used in audio output and regulator circuits as a robust alternative.
• BD231 – Provides similar power dissipation capability and is frequently used in driver stages where thermal stability is important.
Alternatives
• MJE171 – A higher-power PNP transistor with increased current and power dissipation capability. It is suitable for heavier driver or control loads but typically requires bias and heat-sink adjustments due to its different thermal and gain characteristics.
• MJE702 – Designed for higher voltage and power handling than the BD140, making it suitable for demanding driver or control applications. Its internal design results in a much higher current gain, so base-drive and bias stability must be carefully reviewed before substitution.
• BD790 – A high-power PNP transistor commonly used in output stages. It offers greater current capability than the BD140 but operates with different gain behavior and thermal requirements, making it unsuitable as a direct drop-in replacement without circuit changes.
• BD792 – Closely related to the BD790 and optimized for complementary audio output stages. Proper bias adjustment is critical to ensure stable operation and to prevent crossover distortion or thermal stress.
Working Principle of the BD140
The BD140 follows standard PNP transistor operation, optimized for higher power handling and fast response. The emitter is typically connected to the higher potential supply, while the collector feeds the load.
When a small current flows out of the base, it allows a much larger current to flow from the emitter to the collector. When the base current is removed, conduction stops as the internal junctions return to their non-conductive state, turning the transistor off.
Common Applications of the BD140
• Audio amplifier driver and output stages – Used in push-pull and complementary designs where smooth linear response and matched behavior with NPN counterparts are important.
• Medium-current switching below 1.5 A – Suitable for controlling loads that require moderate current without the complexity of power MOSFETs.
• Battery charging circuits – Acts as a pass or control transistor to regulate charging current and protect the battery from overcurrent conditions.
• Regulated power supplies – Commonly used in linear regulators as a series pass element or control device for voltage and current regulation.
• Motor and relay drivers – Drive small DC motors or relay coils when paired with proper base resistors and protection components.
• Darlington pair configurations – Combined with another transistor to increase current gain, allowing low control currents to manage higher load currents.
How to Use the BD140 Transistor in a Circuit?
The BD140 is a current-controlled PNP transistor in which a small base current regulates a larger collector current. It turns on when the base voltage is sufficiently lower than the emitter voltage and turns off as the base approaches the emitter potential.
Base current should always be limited using a resistor to ensure controlled operation and predictable switching behavior. The base pin must never be left floating, as this can lead to unstable operation or unintended conduction. A pull-up resistor between the base and the emitter supply is commonly used to keep the transistor reliably off when not driven.
BD140 vs BD139 vs BD136 vs MJE702 Comparison
| Parameter | BD140 | BD139 (NPN) | BD136 | MJE702 |
|---|---|---|---|---|
| Collector-base voltage (VCB) | −80 V | 80 V | −45 V | −80 V |
| Collector-emitter voltage (VCE) | −80 V | 80 V | −45 V | −80 V |
| Emitter-base voltage (VEBO) | −5 V | −5 V | −5 V | −5 V |
| Collector current (IC) | −1.5 A | 1.5 A | −1.5 A | −4 A |
| Maximum power dissipation | 12.5 W | 12.5 W | 12.5 W | 40 W |
| Junction temperature | 150 °C | 150 °C | 150 °C | 150 °C |
| Transition frequency (fT) | 190 MHz | 190 MHz | 190 MHz | — |
| DC gain (hFE) | 25–250 | 25–250 | 10–250 | ~750 |
| Package | TO-126 | TO-126 | TO-126 | TO-126 |
The MJE702 exhibits a significantly higher DC current gain than the BD140 family due to differences in internal structure and intended operating range. This higher gain does not indicate direct equivalence. When substituting higher-gain devices, base-drive current, bias stability, and thermal behavior must be carefully evaluated to avoid overdrive or thermal stress.
Conclusion
The BD140 remains a dependable choice for medium-power PNP applications that require stable linear operation, predictable gain, and reliable thermal performance. With correct pin identification, proper biasing, and adequate heat sinking, it performs consistently in audio amplifiers, driver stages, and regulated power circuits. Its wide availability and compatibility with common complementary and replacement transistors make it a practical and enduring solution in modern electronic designs.
Frequently Asked Questions [FAQ]
What is the typical base-emitter voltage of a BD140 transistor?
The BD140 typically requires about 0.6–0.7 V between the base and emitter (with the base more negative than the emitter) to begin conduction. This value can increase slightly at higher currents or elevated temperatures.
Can the BD140 be used directly with microcontroller outputs?
Yes, but a base resistor is mandatory to limit base current. Since the BD140 is a PNP transistor, it is usually driven through a pull-up arrangement or via an intermediate NPN transistor when interfacing with low-voltage logic signals.
Does the BD140 require a heatsink in normal operation?
A heatsink is not always required, but it becomes necessary when power dissipation exceeds a few watts. Continuous operation near higher currents or voltages will quickly raise junction temperature without adequate heat sinking.
Is the BD140 suitable for high-frequency signal amplification?
The BD140 can handle moderate signal frequencies, but it is not ideal for RF applications. Its transition frequency is sufficient for audio and driver stages, but specialized RF transistors perform better at very high frequencies.
What happens if the BD140 base is left unconnected?
Leaving the base floating can cause unpredictable switching or noise pickup, leading to unintended conduction. A pull-up resistor to the emitter supply is recommended to keep the transistor reliably turned off when not driven.