Darlington Transistor (Darlington Pair)

Darlington Transistor (Darlington Pair)
Darlington Transistor (Darlington Pair)

The Darlington Pair, or Darlington Transistor, amplifies current with two bipolar transistors. In 1953, Sidney Darlington introduced the idea, hence its name. This transistor design is popular because it provides high current gain and better performance than a single transistor. We shall define the Darlington Transistor, its function, and its benefits in this section.

Darlington Transistor Definition

A Darlington Transistor is a pair of bipolar transistors, usually NPN, with the emitter connected to the base. The first transistor increases the current, which drives the second transistor to amplify it. The Darlington Transistor has a much larger current gain than a single transistor because it multiplies the gains of the two transistors. It is a single transistor with strong common-emitter current gain.

Darlington Transistor purpose

The Darlington Transistor solves bipolar transistors’ limited current gain when larger current amplification is needed. For high-current amplification applications, the Darlington Transistor has a larger current gain by cascading two transistors. It can drive low-power control signals to high-power loads for efficient signal processing. The Darlington Transistor is used in power amplifiers, motor control circuits, and switching.

Darlington Transistor benefits

Darlington Transistor configurations have many benefits. First, its high current gain amplifies low-level input signals efficiently. This makes it suited for high-power applications. Second, the Darlington Transistor requires low input current, relieving driving circuitry. It also features high input and low output impedances, making it suitable with many signal sources and loads. It also has rapid switching and thermal stability, ensuring reliable performance in harsh operational situations. The Darlington Transistor outperforms single transistors in performance and adaptability.

Transistors are familiar, but what’s a Darlington transistor? Many of your everyday electronics use this powerful but misunderstood technology. Darlington transistors, also known as Darlington pairs, are two transistors coupled to dramatically amplify the input signal. This aids circuit design in numerous ways. This page will explain the Darlington transistor’s operation, applications, and comparisons to other transistor layouts. This fundamental building piece of modern electronics will be clear by the end.

Definition of Darlington Transistor.

Darlington transistors, also known as Darlington pairs, are two transistors that behave as one with a larger current gain. It boosts the input current to increase the output current.Sidney Darlington invented the Darlington pair configuration. It has two identical transistors wired in a certain way. The first transistor amplifies the input signal and drives the second transistor, which amplifies it further for a high output current gain.This design benefits the Darlington transistor. It controls a high output current with a small input current because to its high input impedance. Its 10,000+ current gain lets a modest input signal drive a big output current. A tiny input voltage change causes a big output voltage change in the Darlington pair due to its high voltage gain.

Common Darlington transistor uses include:

Power amplifiers: The Darlington transistor’s high current gain makes it perfect for boosting low-level signals.Motor control circuits: A Darlington pair’s high input impedance allows low-power signals to control it while delivering high output power for motors.Switching applications: The Darlington transistor can regulate a large output power with a tiny input signal.In conclusion, the Darlington transistor design provides considerable current and voltage gains compared to a single transistor. It is useful for controlling high output power with a low-level input signal. The Darlington pair is fundamental to modern electronics because to its basic but intelligent construction.

How Do Darlington Transistors Work?

How does the Darlington transistor work? All depends on current amplification.

The Darlington Pair

The Darlington transistor, named after its inventor, Sidney Darlington, has two bipolar transistors coupled. As usual, the first transistor amplifies the input. However, it sends the amplified signal to the second transistor’s input instead of the output. First transistor increases signal, then second transistor enhances signal.This dual amplification gives a large current gain compared to a transistor. A normal bipolar transistor can only reach a few hundred gains, whereas the Darlington pair can get 10,000 or more. This high gain powers the Darlington transistor and lets it drive high-current loads that a single transistor cannot.

How Current Amplifies

The Darlington transistor amplifies current in two stages:

The first transistor amplifies the input signal normally. A tiny input current controls the first transistor, allowing a larger collector current. The first transistor amplifies its output to feed the second transistor.The first transistor’s signal is amplified by the second transistor. If each transistor has a gain of 100, 10mA flowing into the first transistor will allow 1A to flow into the second. 100A will flow from the second transistor to the output! This cascading amplification produces huge gains. This allows a modest input signal to regulate a huge output current. Darlington transistors are great for controlling motors, relays, and other power loads due to their high-gain, high-power capabilities. A concealed Darlington transistor may be working behind a light switch or appliance when you turn it on!

Key Darlington Transistor Benefits

Darlington transistors provide advantages over single transistors. Here are several Darlington pair advantages:

Greater current gain

A single transistor has substantially lower current gain than a Darlington design. Two transistors connected together multiply the current gain of the first transistor by the second. Due to its high current gain, Darlington pairs can amplify signals better than transistors.

Higher input impedance

The Darlington arrangement has higher input impedance than a transistor. First transistor in pair isolates input signal from output via input impedance. Many amplifier applications use this high input impedance to avoid loading the input signal source.

A higher saturation voltage

Higher saturation voltage is another Darlington advantage. The saturation voltage is the minimum voltage between collector and emitter to keep the transistor “on”. Darlington pairs can accommodate higher power signals without distortion because to their higher saturation voltage.


Improved thermal stability is another Darlington benefit. Darlington pair transistors thermally stabilize each other due to their proximity. As one transistor heats up, the other does too. The balancing effect prevents thermal runaway that could harm a transistor.

Simpler biasing

Finally, Darlington can be biased more easily than a high-power transistor. Biasing the Darlington pair input lets current flow through both transistors. No biasing circuit is needed to bias the second transistor, which is “self-biasing.”. The setup is simpler than biasing two high-power transistors.In conclusion, the Darlington transistor design outperforms a single transistor in high-power, high-current applications. The Darlington pair is helpful in many circuits due to its high current gain, input impedance, saturation voltage, thermal stability, and simple biasing.

Common Darlington Transistor Uses

Darlington transistors are often used in switching applications like high-power device management. The Darlington can handle higher output currents than a transistor due to its high current gain. This makes it excellent for driving heavy loads like:In hobby electronics and robotics, the Darlington transistor controls DC motor speed and direction. It can operate large motors with low-power logic signals due to its high current capacity.

Relays—A Darlington pair can operate electromagnetic relays, which can switch larger voltages and currents than the Darlington. The Darlington boosts a low-power control signal to activate the relay.Solenoids—like valve control devices, require higher currents than typical logic circuits. A Darlington transistor can boost solenoid current while being regulated by a low-power signal.• High-power LEDs—Large LED displays, spotlights, and grow lights require more current than ordinary logic signals can deliver. A low-power microcontroller or logic gate can control high-power LED strings with a Darlington transistor.

Darlington transistors may drive small heaters like 3D printer hot ends and appliances. Remember that the Darlington lets a low-power control signal drive a high-power load.Each application uses the Darlington transistor as a power amplifier to drive a high-current load with a low-current control signal. This allows it to interface low-power logic or control circuits with high-power components in various devices.

In comparison to other transistor configurations

The Darlington pair has a far larger current gain than a transistor. A Darlington transistor can have 5,000 to 10,000 current increases, compared to 50 to 200 for a single transistor. Darlingtons are ideal for switching circuits for motor and relay driving due to their high current and low saturation voltage.The Darlington arrangement has drawbacks compared to a transistor. The saturation voltage is 1 to 3 volts, compared to 0.2 to 0.5 volts for a single transistor, due to the more junctions. The Darlington’s switching speed and frequency responsiveness are reduced due to two transistors’ capacitance.

The Darlington differs from the Sziklai pair and triple emitter follower. The Sziklai pair has higher input impedance but lower output impedance, while the triple emitter follower has high input, output, and current gain but lower voltage gain.Darlington transistors are simpler, smaller, and cheaper than integrated circuit operational amplifiers (op-amps), however the latter have higher input impedance, gain, and bandwidth. Opamps can be adjusted to invert or not, providing positive or negative gain values. Op-amp performance can be modified by external components, whereas Darlington performance is fixed by design.

In conclusion, the Darlington transistor provides higher current gain, faster switching, and lower saturation voltage than a single transistor, but it has weaker frequency responsiveness, higher saturation voltage, and less flexibility than an op-amp. However, the Darlington pair is still a good choice for high-power, low-frequency switching applications that require affordability and simplicity.


So there, the Darlington transistor basics explained. You now understand its operation and why it’s important in so many power applications. The Darlington brings up new power electronics possibilities with its strong current gain and signal amplification. After seeing one of those familiar pairs of transistors joined together, you’ll understand what’s going on. You might even design Darlington pair high-power circuits. After learning the basics, the possibilities are unlimited.

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