Darlington Transistor

The Darlington transistor is a circuit configuration consisting of a pair of bipolar junction transistors (BJT). This configuration is such that the emitter of the first transistor drives the base of the second, known as emitter-follower. This has the effect of providing a total gain that is a product of the individual gain.

Invented in 1953 by Sidney Darlington, the invention has undergone much refinement and today incorporated into integrated circuits (IC) such as the ULN2803, and many single transistor packages.

Making a Darlington Transistor

A Darlington circuit configuration may be made from either PNP or NPN transistors, however, NPN is the more commonly used for switching loads in common emitter circuits.

Here is a pair of NPN transistors in a standard Darlington arrangement. This type of arrangement is very sensitive to input signals due to their high gain. It makes a good touch switch, because simply touching the base junction will cause the transistor to conduct.

Darlington Pair Gain Calculation

Each transistor has a rated DC gain known as hFE, and the total gain is the product of this gain. The formula for the total gain is as follows.

hFE (total) = hFE of transistor 1 × hFE of transistor 2

A BC548 has a minimum hFE value of 110. Hence, in the Darlington configuration, the total gain is 12100.

With such a high gain of 12100, a very small current at the base of the transistor saturates it, and the transistor switches fully ON and conducts. As you can see, a pair of BC548 transistors in this configuration can easily switch small loads such as a relay coil and small DC motors. Unfortunately, the BC548 has an absolute maximum collector rating of 500 mA, which is usable up to 300 mA in practice. However, the BC639 is also an NPN transistor, and it has a collector rating of 1 A. Hence, a Darlington configuration using a pair of these transistors could switch loads with a higher current requirement.

NPN Darlington Transistor Symbol

This is the component symbol. As you can see, it consists of a pair of transistors encapsulated within a single package. It is interesting to note that the leads that come out of the package are marked "Base", "Emitter", and "Collector".

It is possible to buy a component in a single package that consists of a Darlington pair internally. The BCX38A/B/C is just such a transistor. It is a Darlington device intended to switch high power loads. It has a current gain of 10000, and a maximum collector current rating of 800 mA.

Darlington Array IC

The diagram above shows a ULN2803 Darlington Array IC with eight such transistors. These types of transistors packaged within a single IC provide designers with a cost effective means to control multiple devices. This IC is useful when you need to control multiple devices, to switch a maximum of eight independent loads. Typically used to control robot cars, they are easy to use and cheap.

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