Darlington Circuits & Applications

The transistor Darlington pair lends itself to a variety of applications and circuits where high current gain transistors are required.

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There are many different circuits that use the Darlington pair as the key active element.

The high gain of the Darlington pair lends it to many circuits and applications where relatively low level inputs need to be amplified to significantly amplified to drive much bigger loads.

Circuits used in association with light sensor, audio amplifiers and power supplies are some common examples of circuits using Darlington pair transistors.

In these circuits the Darlington pair may be a single device containing the integrated Darlington, or they can be circuits that are made from the discrete transistors.

Also the Darlington pair is used within many analogue integrated circuits where high levels of gain are needed.

Darlington pair circuit in linear power regulator

The Darlington pair circuit finds many applications as the series pass element in a linear regulator. Here the Darlington pair provides significant levels of current gain – a feature that is important in linear regulators.

Circuit using Darlington as PSU series pass transistor
Circuit using Darlington as PSU series pass transistor

In power supply applications, the series pass element is required to pass high levels of current. Transistors capable of high current levels tend to have lower levels of current gain, Beta Β. Sometimes this may even be as low as between 20 to 50.

Lower current transistors, like those capable of driving the output device tend to have much higher levels of current gain. By combining the two transistors in a Darlington configuration, the circuit is able to provide the required current handling capacity along with the required current gain.

Darlington transistor in audio output circuit

Another Darlington circuit application is within audio power amplifiers. In instances where the output stages need to drive loudspeakers, high levels of current gain are needed as speakers have a low impedance.

Even for headphones the audio output stages for lower power devices can benefit from the use of a configuration based on the Darlington circuit.

Photo-darlington circuit

The Darlington configuration is used in many photo applications. Phototransistor performance can be enhanced by using a photo-darlington, and as a result, the configuration is often used in this application.

Led drivers

The Darlington transistor circuit lends itself to use in small driver chips for LEDs and other small devices.

The high current gain of the Darlington circuit means that these small driver chips can be used to interface LEDs and other devices to logic chips.

By Ian Poole

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