Last revised: January 28, 2025

Darlington transistors are circuits that combine two bipolar transistors in a single device. They provide high current gain and require less space than configurations that use two discrete transistors.

In Darlington pairs, transistor collectors are tied together and the emitter of the first transistor is directly coupled to the base of the second transistor.

The total gain, which is often 1000 or more, is the product of the gain of the individual transistors.

Compared to single transistor configurations, Darlington transistor pairs have more phase shift at high frequencies and can become unstable with negative feedback more easily.

Darlington transistors also have a higher base-emitter voltage, which is the sum of both base emitter voltages. Sidney Darlington, an engineer at Bell Laboratories in the 1950s, is credited with first combing two transistors on a single chip.

Performance Specifications

Selecting Darlington transistors requires an analysis of performance specifications. The common emitter current gain, the ratio of collector current to base current (), characterizes the amplifying ability of bipolar transistors.

Collector-to-emitter breakdown voltage () is the maximum voltage than can be applied continuously in the reverse direction of the collector junction when the emitter is open. Other important considerations include collector-to-base breakdown voltage () and maximum collector current ()_.

Current gain bandwidth product () is the frequency at which the common emitter current gain is in unity. Power dissipation (), which is usually expressed in watts or milliwatts, is the total power consumption of the device.

Operating temperature () is the junction’s full-required range of ambient operating temperatures. Some Darlington transistors support a specific temperature range and feature mechanical and electrical specifications that are suitable for commercial or industrial applications. Other devices meet screening levels for military specifications (MIL-SPEC).

Features

Darlington transistors vary in terms of polarity, packaging, and packing methods.

NPN is a physical bipolar junction transistor (BJT) arrangement in which the emitter and the collector are made of N-type material and the base is made of P-type material.

By contrast, PNP is a BJT arrangement in which the emitter and the collector are made of P-type material and the base is made of N-type material.

In terms of packaging, Darlington transistors are available in small outline (SO), transistor outline (TO), small outline transistor (SOT), discrete packaging (DPAK), and flatpack.

Darlington transistors often use either surface mount technology (SMT) or through hole technology (SMT) and vary in terms of the number of leads. Packaging methods for Darlington transistors include tape reels, rails, bulk packs, tubes, and trays.

Standards

BS EN 120003 — Specification for harmonized system of quality assessment for electronic components - blank detail specification - phototransistors, photodarlington transistors, phototransistor —arrays.

MIL-M-38510/141 — Microcircuits, linear, Darlington transistor array, seven and eight gate, monolithic silicon.

MIL-PRF-19500/738 — Semiconductor device, transistor, NPN, silicon, power Darlington, types 2n7575, 2n7576, and 2n7577, jan, jantx, jantxv, and jans.

SMD 5962-86058 — Microcircuit, linear, high-voltage, high current, Darlington transistor arrays, monolithic silicon. 

Darlington Transistors FAQs

How do Darlington transistors differ from regular transistors in terms of performance specifications?

Darlington transistors differ from regular transistors in several key performance specifications:

Current Gain

Darlington transistors have a very high current gain because they consist of two transistors connected in such a way that the current gain of the pair is the product of the gains of the individual transistors. This results in a current gain that can be as high as 10,000, allowing a small base current to control a much larger collector current.

Voltage Requirements

To turn on a Darlington transistor, a voltage of 1.4 V is required across the base-emitter junctions, as they are connected in series. This is higher than the 0.7 V typically required for a regular transistor.

Collector Current

Darlington transistors can control higher collector currents compared to regular transistors. This is due to their configuration, which allows them to handle larger loads with a small input current.

Packaging and Technology

Darlington transistors are available in various packaging options such as small outline (SO), transistor outline (TO), and small outline transistor (SOT). They often use surface mount technology (SMT) or through-hole technology.

What are the applications of Darlington transistors?

Darlington transistors are widely used in various applications due to their high current gain and ability to control large loads with minimal input current. Here are some common applications:

Switching Applications

Darlington transistors are often used in switching applications where a small input current is used to control a larger output current. This makes them suitable for driving heavy loads such as motors, solenoids, and lamps.

Amplification

They are used in amplification circuits where high current gain is required. The configuration allows for significant amplification of weak signals, making them useful in audio amplifiers and other signal processing applications.

Motor Control

In motor control applications, Darlington transistors can be used to control the speed and direction of DC motors. The high current gain allows for efficient control of the motor with a small input signal.

Relay Driving

Darlington transistors can be used to drive relays, which are used to switch high power circuits with a low power signal. This is particularly useful in automation and control systems.

LED and Display Drivers

They are used in driving LED displays and other types of displays where multiple LEDs need to be controlled simultaneously. The high current gain allows for efficient control of multiple LEDs with a single input signal.

These applications leverage the unique characteristics of Darlington transistors, such as their high current gain and ability to handle larger loads, making them versatile components in various electronic circuits.

What is the working principle of a Darlington transistor?

The working principle of a Darlington transistor involves the use of two transistors connected in such a way that they function as a single unit with a very high current gain. Here's a detailed explanation:

Configuration

A Darlington transistor consists of two bipolar transistors connected in a configuration where the emitter of the first transistor is connected to the base of the second transistor. This setup allows the current amplified by the first transistor to be further amplified by the second transistor.

Current Gain

The overall current gain of a Darlington pair is the product of the current gains of the individual transistors. This results in a very high current gain, often around 10,000, which means that a very small base current is needed to control a much larger collector current.

Voltage Requirements

To turn on a Darlington transistor, a voltage of approximately 1.4 V is required across the base-emitter junctions, as they are connected in series. This is higher than the typical 0.7 V required for a single transistor.

Operation

When a small input current is applied to the base of the first transistor, it allows a larger current to flow from the collector to the emitter of the first transistor. This current then becomes the base current for the second transistor, allowing an even larger current to flow from the collector to the emitter of the second transistor. This cascading effect results in a significant amplification of the input current.

Applications

Due to their high current gain, Darlington transistors are used in applications such as switching, amplification, motor control, relay driving, and LED/display driving, where controlling large loads with minimal input current is required.

How do Darlington transistors compare to other types of transistors in terms of applications?

Darlington transistors have unique characteristics that differentiate them from other types of transistors in terms of applications.

Darlington transistors are known for their exceptionally high current gain, which is the product of the gains of the two transistors in the pair. This allows them to amplify a small input current into a much larger output current, making them suitable for applications requiring high current gain, such as driving heavy loads like motors and lamps.

Due to their high current gain, Darlington transistors are often used in switching applications where a small input current controls a larger output current. This makes them ideal for applications like motor control, relay driving, and LED/display driving.

Darlington transistors require a higher voltage to turn on (approximately 1.4 V) compared to regular transistors, which typically require around 0.7 V. This is due to the series connection of the base-emitter junctions in the Darlington pair.

They are used in amplification circuits where significant amplification of weak signals is needed, such as in audio amplifiers and other signal processing applications.

Darlington transistors are available in various packaging options and often use surface mount technology (SMT) or through-hole technology. This versatility in packaging allows them to be used in a wide range of electronic circuits.

What are the different packaging options available for Darlington transistors?

Darlington transistors are available in a variety of packaging options, which cater to different application needs and mounting technologies.

Small Outline (SO)

This is a compact packaging option suitable for applications where space is limited.

Transistor Outline (TO)

A common packaging style that provides robust protection and is often used in through-hole mounting applications.

Small Outline Transistor (SOT)

Another compact packaging option, typically used for surface mount technology (SMT).

Discrete Packaging (DPAK)

This packaging is used for discrete components and is suitable for surface mounting.

Flatpack

A packaging style that offers a low-profile design, often used in military and aerospace applications.

Mounting Technologies

Darlington transistors can be mounted using either surface mount technology (SMT) or through-hole technology.

Packaging Methods

They are available in various packaging methods such as tape reels, rails, bulk packs, tubes, and trays.

These packaging options provide flexibility in terms of design and application, allowing engineers to choose the most suitable package for their specific needs.

How do Darlington transistors work in motor control applications?

Darlington transistors are particularly useful in motor control applications due to their high current gain and ability to control large loads with minimal input current.

Configuration and Current Gain

A Darlington transistor consists of two transistors connected in a way that the current amplified by the first transistor is further amplified by the second. This configuration results in a very high current gain, often around 10,000, allowing a small base current to control a much larger collector current.

Motor Control

In motor control applications, the high current gain of Darlington transistors enables efficient control of the motor with a small input signal. This is particularly useful for controlling the speed and direction of DC motors.

Switching Large Loads

Darlington transistors can switch large loads, such as motors, by using a small input current to control a larger output current. This makes them ideal for applications where a microcontroller or other low-power device needs to control a high-power motor.

Voltage Requirements

To turn on a Darlington transistor, a voltage of approximately 1.4 V is required across the base-emitter junctions, which are connected in series. This is higher than the typical 0.7 V required for a single transistor, but it allows for the control of larger currents.

Darlington Transistors Media Gallery

References

GlobalSpec—Robotics

Electronics360—How to switch large loads with a microcontrolled transistor

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