Part Number: ULN2804A

Manufacturer: STMicroelectronics

Description: Darlington Transistors

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Eight common-emitter Darlington transistors with built-in inductive load suppression diodes are found in the ULN2804A. Each Darlington has a 600 mA peak load current rating (500 mA continuous rating) and can resist at least 50 V in the OFF state. It is possible to parallelize outputs to enhance current capacity.

It has the practical input-opposite-output pinout, which makes the board layout simpler, and is available in an 18-lead plastic DIP with copper leads. But first, a brief introduction to Darlington transistors before we move on.

What is a Darlington Transistor?

Sidney Darlington invented the transistor and gave his name to the device we now know as the Darlington transistor. Connecting two bipolar junction transistors, PNP or NPN results in constructing a Darlington transistor. It is possible to produce a sensitive transistor with high current gain by connecting the emitter of one PNP transistor to the base of another PNP transistor.

This transistor type is useful in various applications where switching or amplifying signals is essential. It is possible to build the transistor pair used in the Darlington transistor using two BJTs coupled to one another in separate circuits. As a transistor may act as a switch and an amplifier, a BJT can serve as an on/off switch.

Transistor with a Darlington diode

A Darlington transistor, also known as a Darlington pair, is a piece of electronic equipment that connects two BJTs (Bipolar Junction Transistors) to enable a very high amount of current gain. This configuration results in the creation of a device that is referred to as a Darlington transistor. This is accomplished through a compounding amplification, in which the first transistor amplifies the current and is further amplified by the second transistor.

This particular transistor is known as a “Darlington Pair” because its compound structure is created from two BJTs. Because it only possesses a single emitter, collector, and base, this transistor operates similarly to a single transistor unit. In 1953, Sidney Darlington came up with the idea for what would later be known as the Darlington transistor.

If the current gain of a transistor is 1 and 2, then the overall current gain of a Darlington pair is 12. This transistor has an exceptionally high current gain compared to a standard transistor. Consequently, the transistor is also called a “Super Beta Transistor.”

Darlington Transistor Circuit

The Darlington Transistor comprises two PNP or NPN transistors coupled to each other in a back-to-back configuration. Both transistors share a single package with a shared collector terminal, and it is a single package overall.

A connection has been made between the second transistor’s base terminal and the first transistor’s emitter terminal. Therefore, the base supply is only provided to the first transistor, while the output current is only drawn from the second transistor. Both of these transistors are connected in series. As a result, it has a single base, a single emitter, and a single collector, as demonstrated in the image below.

Darlington Transistor Circuit

PNP and NPN Darlington Transistor

When both of the Darlington pair’s transistors are PNP varieties, the resulting device is referred to as a PNP Darlington Transistor. An NPN Darlington Transistor is formed when the Darlington pair is composed of two NPN transistors. The following illustration provides a connection diagram that may apply to NPN and PNP Darlington Transistors.

PNP and NPN Darlington Transistor

Both varieties of transistors share the Collector terminal. Neither kind has a dedicated Collector terminal. When a PNP transistor is used, the base current is sent via the second transistor and into its emitter terminal. Additionally, in an NPN transistor, the current from the emitter is routed to the base terminal of the second transistor.

Compared to the amount of space necessary for two individual transistors, the amount of space needed for a Darlington transistor is significantly smaller since the collector terminal of both transistors is shared in this configuration.

Advantages of Darlington Transistor

Compared to a standard transistor, the Darlington transistor, also known as a Darlington pair, possesses several advantageous qualities. The following is a list that provides a summary of these points:

  • The high current gain offered by a Darlington transistor is considered its primary benefit. Therefore, the transistor can be triggered by a negligible amount of base current.
  • It has a high input impedance, which results in an equivalent reduction in output impedance.
  • There is only one container included. It is far simpler to install components on a printed circuit board (PCB) than to connect two distinct transistors.

Disadvantages of Darlington Transistor

The following is a list that provides a concise summary of the drawbacks associated with using a Darlington transistor, often known as a Darlington pair:

  • It switches things over slowly.
  • The base-emitter voltage is nearly two times higher than a typical transistor.
  • It dissipates a lot of power in this application because of the high saturation voltage.
  • There’s a cap on the bandwidth.
  • The Darlington transistor causes the negative feedback circuit to exhibit a phase shift at a predetermined frequency.

What is ULN2804A Transistor

The ULN2804A from STMicroelectronics is a DIP-packaged, eight-Darlington through-hole array. With common emitters and built-in suppression diodes, the array’s eight Darlington transistors are able to handle inductive loads with ease.  Each Darlington can withstand a minimum of 50 volts when it is in the OFF state and has a maximum load current rating of 600 milliamperes (500 milliamperes continuously).

Additionally, a 10.5 kilohm input resistor for 6 to 15 volts CMOS is included to make the connection to standard logic families easier.

ULN2804A Transistor Features

There are eight common-emitter Darlington transistors in it.

  • 50 V output voltage
  • Diodes for integral suppression
  • Versions for every popular family of logic
  • Parallel output is possible.
  • To simplify the board layout, inputs are pinned to opposite outputs.

ULN2804A Transistor Manufacturer

As a leading developer and provider of semiconductor solutions for various microelectronics applications, STMicroelectronics is a global independent semiconductor corporation. The Company is at the cutting edge of System-on-Chip (SoC) technology. Its solutions are essential in supporting today’s convergence trends because of an unmatched combination of silicon and system knowledge, Intellectual Property (IP) portfolio,  manufacturing strength, and strategic partners.


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