Darlington Connection in Transistors

Named after its inventor Sidney Darlington, the Darlington transistor is a special arrangement of two standard interconnected NPN or PNP bipolar transistors (BJT). The emitter of one transistor is to connect to the base of the other to produce a more precise transistor with a much larger current gain, which is useful in applications where current amplification or switching is required.

Darlington transistor pairs can be made from two individually connected bipolar transistors or from a single device commercially produced in a single package as standard. Base, emitter and collector connector connectors and a wide range of chassis styles and voltage (and current) ratings are available in both NPN and PNP versions.

In addition to being used as an amplifier as we see as a switch tutorial in our transistor, the bipolar connection transistor (bjt) can be made to work as an on-off switch as shown.

Bipolar Transistor Working as a Key

Darlington
Bipolar Transistor Working as a Key

When the base terminal of the NPN transistor is grounded (0 volts), zero current flows to the base before IB = 0. Since the base terminal is grounded, no current flows from the collector to the emitter terminals. Therefore, the non-conductive NPN transistor is changed to "off" (interrupt). Now, if we shift the base terminal according to emitter using a voltage source greater than 0.7 volts, the movement of the transistor causes a much larger current to flow than the transistor between the collector and emitter terminals. The transistor is now said to be "open" (conductor). If we operate the transistor between these two cutting and transmission modes, the transistor can be made to work as an electronic switch.

However, the base terminal of transistors must be changed from zero to a positive value much larger than 0.7 volts for the transistor to function fully. A higher voltage causes an increased base current, Ib flows into the device. This causes the collector current Ic to be large, while causing Voltage Drop along the collector and emitter terminals. Vce shrinks. Next, we can see that a smaller current flowing into the base terminal can cause a much larger current to flow between the collector and emitter.

The ratio of collector current to base current (β) is known as the current gain of the transistor. A typical β value for a standard bipolar transistor can range from 50 to 200 and can vary even between transistors of the same part number. In some cases where the current gain of a single transistor is too low to drive a direct load, one way to increase earnings is to use a Darlington pair.

A Darlington transistor configuration, also known as the "Darlington pair" or "super alpha circuit", consists of two interconnected NPN or PNP transistors. Thus, the emitter current of the first transistor TR1 becomes the base current of the second transistor TR2. The transistor TR1 is then connected as a emitter tracker and tr2 as a common emitter amplifier, as shown below.

Also note that in this Darlington pair configuration, the collector current of the slave or control transistor is "in-phase" with that of tr1's main switching transistor TR2.

Basic Configuration of Darlington Transistor

Darlington
Darlington Connection in Transistors

Using the NPN Darlington pair as an example, the collectors of the two transistors are connected. Tr1's emitter drives tr2's base. This configuration reaches the multiplication of the β because the collector current for a base current is β*ib. Here the current gain is greater than or unified, and this is defined as follows:

Darlington

However, the IB2 base current is equal to the transistor tr1 emitter current. In other words, tr1 is 1 since the emiter is connected to the tr2 base. Therefore:

Darlington

Then our first equation will be as follows.

Darlington

Here, β1 and β2 are the gains of individual transistors.

As the current gains of the two transistors increase, it means that the second transistor is given the gain of the first transistor, which is multiplied by the gain. In other words, a pair of bipolar transistors assembled to make a single Pair of Darlington transistors can be considered a single transistor with a very high β value and therefore high input resistance.

Darlington Transistor Example

The two NPN transistors are connected in the form of a Darlington pair to replace the 12V 75W halogen lamp. If the forward current gain of the first transistor is 25 and the forward current gain (Beta) of the second transistor is 80. Calculate the maximum base current required to pass the drop, full lamp between the two transistors, ignoring any voltage.

Darlington

Using the equation above, the basic current is given as follows:

Darlington

Then we can see that only a very small base current of 3.0 ma, such as those provided by a digital logic gate or the output port of a microcontroller, can be used to change the 75 Watt lamp to "on" and "off".

If two identical bipolar transistors are used to make a single Darlington device, β1 is equal to β2 and the overall current gain will be given as follows:

Darlington

Usually the value of β2 is much larger than 2β, in which case it can be ignored to simplify the mathematics a little.

Darlington Transistor Applications

Darlington

The base of the Darlington transistor is sensitive enough to respond to small input current coming directly from a switch or through a TTL or 5V CMOS logic gate. The maximum collector current for any Darlington pair is the same as ic(max), the main switching transistor TR2, so it can be used to operate relays, DC motors, solenoids and lamps, etc.

One of the main drawbacks of a Darlington transistor pair is the minimum voltage drop between the base and emitter when fully saturated. Unlike a single transistor, which has a saturated voltage drop of 0.3 v to 0.7 v when fully turned on, a Darlington device has a base emitter voltage drop of 1.2 V instead of 0.6 V, twice the base emitter voltage drop. This high base emitter voltage drop means that the darlington transistor may be hotter than a normal bipolar transistor for a certain load current and therefore requires a good cooling system.

To overcome the slow response, increased voltage drop and thermal disadvantages of a standard Darlington transistor Device, complementary NPN and PNP transistors can be used in the same cascading arrangement to produce another type of Darlington transistor called Sziklai configuration.

Sziklai Transistor Pair

Named after Hungarian inventor George Sziklai, the Sziklai Darlington pair is a complementary or composite Darlington device consisting of separate NPN and PNP complementary transistors connected to each other, as shown below.

Sziklai Darlington Transistor Configuration

Darlington

We can see that the base emitter voltage drop of the Sziklai Device is equal to the diode drop of a single transistor in the signal pathway. However, the Sziklai configuration cannot be satisfied with a decrease of less than a diode, that is, 0.7 V instead of the normal 0.2 V.

Also, as with the darlington couple, the Sziklai pair have slower response times than a single transistor. Sziklai dual complementary transistors are widely used in push-pull and EU-class audio amplifier output stages, which allow only the polarity of an output transistor. Both Darlington and Sziklai transistor pairs are available in both NPN and PNP configurations.