BD139 NPN Transistor: Pinout, Specification and Test Circuit

Post Date: 2025-06-26 , ON Semiconductor

BD139 Introduction

The BD139 is an epitaxial planar NPN transistor housed in a SOT-32 plastic package, designed primarily for use in audio amplifier circuits and driver stages, especially in complementary or quasi-complementary configurations. It pairs with its PNP counterpart BD140, offering reliable performance in medium power linear and switching applications.

BD139 Working Principle

The BD139 operates on the same basic principle as a standard bipolar junction transistor. It has three terminals: Base (B), Collector (C), and Emitter (E). When the base voltage is approximately 0.7V higher than the emitter voltage, the transistor turns on, allowing current to flow from the collector to the emitter. This enables current amplification, where a small input current at the base controls a much larger current through the collector. BD139 is commonly used in audio amplifiers and power drivers.

BD139 Pinout

B (Base) – The control terminal that determines whether the transistor is on or off.

C (Collector) – The main current input terminal, typically connected to the load.

E (Emitter) – The current output terminal, usually connected to ground or the negative rail of the circuit.

BD139 Symbol

BD139 Footprint

BD139 3D Model

BD139 Specification

Parameter	Value
Transistor Type	NPN
Package	TO-126
Collector-Emitter Voltage (VCE)	80 V
Collector Cutoff Current	100nA (ICBO)
Vce Saturation (Max) @ Ib, Ic	500mV @ 50mA, 500mA
DC Current Gain (hFE) (Min) @ Ic, Vce	40 @ 150mA, 2V
Emitter-Base Voltage (VEB)	5 V
Total Power Dissipation (Ptot)	1.25 W
Operating Temperature	-55°C ~ 150°C
DC Current Gain (hFE)	40 – 250 (at IC = 0.5 A)
Transition Frequency (fT)	190 MHz

BD139 Features

Products are pre-selected in DC current gain

These Devices are Pb−Free, Halogen Free/BFR Free and are RoHS Compliant

Designed for reliable operation in audio and switching applications.

Suitable for high power dissipation with excellent thermal stability.

Available in a standard TO-126 package for easy mounting and handling.

Tested for high surge current capability ensuring robust performance.

BD139 Applications

Audio amplifier circuits

Low to medium power switching devices

Driver stages for relays, motors, and other loads

Voltage regulators and power management circuits

Signal amplification in consumer electronics

General-purpose amplification in industrial and automotive equipment

BD139 Test Circuit

This circuit diagram illustrates a simple audio power amplifier test circuit based on the BD139 transistor. The input audio signal first passes through a 47μF coupling capacitor and enters the base of a small-signal transistor (2N3904), which functions as the preamplifier. The amplified signal from the 2N3904 is then AC-coupled via a 10μF capacitor to the base of the BD139, which serves as the main power amplifier driving a speaker connected to its collector.

The emitter of the BD139 is grounded, while the collector is connected in series with a 100Ω resistor and a 100μF capacitor before reaching the speaker and then the 12V power supply. This common-emitter configuration allows the BD139 to provide significant current amplification, making it suitable for driving low-impedance loads such as speakers. The circuit is straightforward in design and ideal for testing the amplification performance and current-handling capability of the BD139 transistor.

BD139 Complementary

The complementary transistor to the BD139 is typically the BD140. Specifically, BD135, BD137, and BD139 belong to the NPN transistor series, while their complementary counterparts BD136, BD138, and BD140 are PNP transistors. This complementary relationship means that BD139 and BD140 are often used together as a matched pair.

BD139 Package

BD139 adopts TO-126 package, which is a common upright plastic package with good heat dissipation performance, suitable for medium power transistor applications. The package is equipped with a metal heat sink, which can be fixed to the heat sink by screws to enhance the heat dissipation effect.

How to Use BD139?

When using the BD139, the first step is to determine whether it will function as an amplifier or a switch based on the circuit requirements.

If used as a switch, a current-limiting resistor should be placed in series with the base, connected to the control signal. The emitter is grounded, and the collector is connected to one end of the load, with the other end of the load connected to the positive supply. When a high-level signal is applied to the base, the transistor turns on, allowing current to flow through the load.

If used as an amplifier, a small signal is applied to the base, the collector is connected to the positive voltage supply, and appropriate biasing resistors are used to set the operating point. BD139 can then amplify the input signal. Be sure to consider power dissipation and provide a heatsink if necessary.