The BD140 is one of the most widely used medium-power PNP transistors in electronics. To fully understand its functionality, alternatives, specifications, working principles, and practical considerations, this article will discuss the BD140 transistor in complete detail.

Catalog

BD140 Transistor Overview

The BD140 General Purpose PNP Epitaxial Silicon Transistor is a medium-power device designed for reliable switching and amplification. With its PNP silicon epitaxial planar construction, it offers stable performance, a collector current rating up to 1.5 A, and voltage handling up to 80 V. Its TO-126 package supports efficient heat dissipation, making it suitable for audio circuits, driver stages, and general-purpose control applications. The transistor also provides a useful gain range, allowing designers flexibility in low- to medium-power electronics.

Commonly paired with its NPN complement, the BD139, the BD140 is frequently used in push-pull audio amplifiers, linear power supplies, and relay-driving circuits.

If you are interested in purchasing the BD140, feel free to contact us for pricing and availability.

BD140 Transistor CAD Models

BD140 Transistor Pinout Configuration

BD140 Transistor Alternatives & Equivalents

BD140 Transistor Specifications

BD140 Transistor Working in Circuit

BD140 Transistor as a Driver Stage

In the first circuit, the BD140 is used as part of a complementary push-pull transistor pair together with BD139. This arrangement allows the circuit to amplify signals efficiently by splitting the workload between the PNP (BD140) and NPN (BD139) transistors. When a small input signal is applied through the BC547, it biases the BD140, enabling it to conduct and drive the upper side of the load. As the signal changes polarity, the BD139 takes over, ensuring smooth and continuous output. This complementary action improves current handling, reduces distortion, and allows the circuit to drive heavier loads while maintaining stability.

BD140 Transistor as a High-Side Switch

In the second circuit, the BD140 functions as a high-side switch. When the switch (SW1) applies a small current to the base through resistor R1, the BD140 turns on and allows current to flow from the positive supply (V1) through the emitter to the collector and into the load. Once the base drive is removed, the transistor cuts off, stopping the current flow. This configuration is commonly used when a load needs to be powered from the positive rail, making the BD140 ideal for controlling motors, lamps, or other medium-power devices.

BD140 Transistor as a Push-Pull Driver for a Motor

In this circuit, the BD140 transistors function as the high-side drivers in a push-pull H-bridge used to control the motor. Each BD140 works together with a BD139 NPN transistor to form a complementary pair, allowing the circuit to deliver strong and efficient current to the motor. When the left BC547 transistor is activated by the input at B1, it drives the left BD140–BD139 pair, sourcing current from the supply through the BD140 and sinking it through the BD139. This powers the motor in one direction. When the right BC547 is triggered by A1, the opposite BD140–BD139 pair becomes active, reversing the polarity and spinning the motor in the other direction. Overall, the BD140 operates as a high-side power transistor that supplies the motor with the required current, enabling controlled forward and reverse operation.

BD140 Typical Characteristics Curve

Figure 1 – DC Current Gain (hFE vs. IC)

This graph shows how the transistor’s DC current gain (hFE) changes with collector current. At low currents, the gain starts around the mid-80s to about 90. It reaches a peak near a few hundred milliamps, then gradually decreases as current approaches the upper limit. This tells the user that the BD140 performs most efficiently - providing the strongest amplification - at moderate collector currents, with reduced gain at very low or very high operating currents.

Figure 2 – Collector-Emitter Saturation Voltage (VCE(sat) vs. IC)

This diagram illustrates how the saturation voltage increases as the collector current rises. Two curves are shown, corresponding to different base currents (IB = 10 mA and IB = 20 mA). A higher base current produces a lower saturation voltage, meaning the transistor can pass more current with less voltage dropped across it. The curve highlights that at small currents VCE(sat) is low, but it rises rapidly once the transistor approaches its higher current limits.

Figure 3 – Base-Emitter Voltage (VBE vs. IC)

This figure shows the base-emitter voltage required to drive different collector currents. As IC increases, VBE also rises, reflecting the typical exponential behavior of a bipolar junction transistor. Two curves are provided for different VCE conditions, but both indicate that VBE stays within the expected diode-like range (roughly 0.6–1.0 V). This helps designers understand the input voltage needed to achieve a desired output current.

Figure 4 – Safe Operating Area (SOA)

The Safe Operating Area chart maps out the combinations of collector current and collector-emitter voltage that the transistor can safely handle under various pulse durations. Higher currents are acceptable only at lower voltages, and short pulses allow greater stress than continuous operation. The SOA curve is essential for ensuring reliable operation, preventing damage from excessive power dissipation or secondary breakdown.

BD140 Transistor Applications

-General-purpose audio amplification in low-power audio stages

-Driver stages for power transistors in amplifiers

-Switching loads such as relays, LEDs, small motors, or solenoids

-Linear regulation circuits, including voltage regulators and current regulators

-DC motor control in low to medium current applications

-Signal amplification in analog circuits

-Inverter and converter driver circuits

-Battery-powered device control due to good gain and low saturation voltage

-Class-AB push-pull amplifier stages when paired with complementary BD139

-Protection circuits, such as overcurrent or thermal shutdown triggers

Comparison: BD140 vs BD136 vs TIP32C

BD140 Mechanical Dimensions

BD140 Advantages & Limitations

BD140 Advantages

-PNP transistor suitable for medium-power switching and amplification.

-Supports up to 1.5 A, making it useful for motors, LEDs, and driver stages.

-High 80 V voltage rating offers good protection against surges.

-Works well in audio amplifier push-pull stages with BD139.

-TO-126 package provides good heat dissipation for its size.

-Readily available and affordable.

-Stable operation in analog circuits due to decent gain (hFE 40–160).

-Wide operating temperature range (–55°C to +150°C).

BD140 Limitations

-Lower power dissipation (1.25 W) compared to larger power transistors (e.g., TIP32C).

-Gain can vary widely between units, requiring proper biasing.

-Not suitable for very high-current loads above 1.5 A without heatsinking.

-Switching speed is moderate; not ideal for high-frequency applications.

-TO-126 package is smaller and limits heat handling compared to TO-220 devices.

-Being a PNP transistor, it cannot directly replace NPN models like BD139.

Manufacturer

ON Semiconductor, known today simply as onsemi, is a leading global semiconductor manufacturer recognized for producing high-performance, energy-efficient electronic components. The company focuses heavily on power solutions, automotive electronics, industrial automation, and IoT technologies. With a strong emphasis on reliability and innovation, onsemi operates advanced manufacturing facilities worldwide and follows strict quality-control standards to ensure consistent device performance across its product lines.