Know all about Darlington Transistor Pair

In electrical and electronic circuits, Darlington transistor or Darlington transistor pair is an essential component. It consists of two bipolar transistors, that are connected in such a way that the current amplifies by the first transistor then by the second transistor. The configuration of the Darlington transistor gives a much higher current gain than a single transistor taken individually. The configuration of these transistors was invented by an engineer “Sidney Darlington ” from Bell laboratories in the year 1953.For more details regarding Darlington transistor follow the below link. Darlington transistor working along with its applications.

Darlington Transistor
Darlington Transistor

Darlington Transistor Pair

A Darlington transistor pair comprises of a couple of bipolar transistors that are coupled in order to deliver a very high-current gain from a low-base current. In this transistor, the emitter of the input transistor is connected to the base terminal of the output transistor, base and collectors of these transistors are wired together. Therefore, the current that is amplified by the first transistor then by the second transistor.

Darlington Transistor Pair
Darlington Transistor Pair

This transistor acts as a single transistor with high current gain. It has three terminals namely base, emitter and collector. These are equal to the terminals of a standard individual transistor.To turn on this transistor, it should be 0.7V across both the BE terminals that are connected in series in the Darlington pair. So it needs 1.4V to turn on.

Darlington transistor pairs are available in whole packages, but you can make up your own from two transistors. In Darlington transistor pair, the primary transistor is a low power type, but normally secondary transistor will need to be high power. The max collector current for the primary transistor is similar to the secondary transistor.

Darlington Transistor Pair Circuit

The circuit configuration of Darlington transistor is useful in numerous applications within electrical and electronic circuits. There are many advantages by using Darlington transistor pair circuit when we compare with other form of transistor circuits.

The circuit of the Darlington pair may be used in the form of discrete components, but there are also various integrated circuit forms frequently named as a Darlington transistor that may also be used. The components of this transistor may be attained in a variety of forms including those for high power applications where the levels of current of many amps may be essential.

Darlington Transistor Pair Circuit
Darlington Transistor Pair Circuit

The basic circuit of the Darlington can be formed by taking input emitter terminal of the transistor and connecting it to the base terminal of the second transistor, and collector terminals of these transistors can be connected together. The circuit of this transistor can be used as a single transistor that is used in a variety of circuits but mostly as an emitter follower.

When the transistor is used in a new electronics design, it is required to switch for the fact that it has a high frequency and greater phase shift than only when one transistor is used. When making a Darlington pair, the o/p transistor is necessary to be able to switch high levels of current. High power transistors normally have lower levels of current gain than the varieties of a small signal. This means that frequently the input device is a small signal high gain variety, while the o/p transistor is a high power device with an inherently lower current gain.

Darlington Transistor Pair Applications

The applications of Darlington transistor pair involve where a high gain is required at a low frequency like Power regulators, Audio amplifier output stages, Display drivers, Motor controllers, Touch and light sensors and solenoid control.

Darlington Transistor Pair based Rain Alarm

The circuit diagram of rain alarm using a Darlington transistor pair (BC547 transistor) is shown below. The rain alarm circuit is built with the following active components such as sensor using two screws fitted over a plastic strip, Darlington transistor pair, piezo buzz, 9v battery, 0.22uF, 10K resistor. The configuration of this circuit is in the form of a standard Darlington transistor pair. These transistors are mainly used to increase the capacity of current amplification enormously. When water drops or rain drops fell on the sensor, then the base of the transistor will connect to the positive supply to activate the alarm. Then finally it generates an alarm.

Darlington Transistor Pair based Rain Alarm
Darlington Transistor Pair based Rain Alarm

Advantages and Disadvantages of Darlington Transistor Pair

The Darlington transistor pair has many advantages and disadvantages depending on its use. They are


  • The current gain of this transistor is high
  • The input impedance of this circuit is high
  • These are widely available in a single package
  • The circuit configuration is easy and very convenient


  • Switching speed is slow
  • Narrow Bandwidth
  • Base emitter voltage is high
  • Saturation voltage is high which can lead to high levels of power dissipation in certain applications

Thus, this is all about the Darlington transistor pair and its working. We hope that you have got a better understanding of this concept. Furthermore, any queries regarding this concept or JFET transistor, please give your feedback by commenting in the comment section below.Here is a question for you, What is the main function of Darlington Transistor?

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