Now in this post you will learn about this amazing BJT device which is called Darlington Transistor. It got this name after genius engineer Sidney Darlington who first gave this idea, so we remember him.
What Is Inside Darlington Transistor
Basically this device is like special combo of two normal NPN or PNP bipolar junction transistors that are connected in clever way.
So what happens is that the Emitter of one transistor connects directly to the Base of the other, and this makes whole setup super sensitive, and it gives much bigger current gain than single transistor.
This thing is very useful when you need to amplify current or when you want to do switching action, so you can use it in many projects.
How Darlington Pair Can Be Made
Now if you are thinking how these Darlington pairs are made, then it can happen in two ways. One way is that you take two separate BJTs and connect them together. Another way is that you can find it ready-made as single device which is already packaged.
This single package has the normal three pins like Base, Emitter, and Collector, and you can buy them in many different styles and ratings for voltage and current, and you can get both NPN and PNP type.
Darlington As Switch And Amplifier
Remember then when we talked about transistors as switches? Just like that tutorial on Transistor as a Switch, these Darlington BJTs can also do same ON and OFF switching. At the same time these can be used as amplifiers also, so they are very versatile.
Why Darlington Is Special
So in simple way, Darlington Transistor is like having two transistors working together so that we get huge boost in current gain just from tiny input base current. That means you can control very big loads by giving very small input base voltage and current, like from microcontrollers or sensors. That makes whole thing super efficient.
Other Special Types
Plus there are also special types like complementary Sziklai Darlington transistor. In that type we mix one NPN and one PNP transistor together, and that gives even better efficiency in some cases.
How a BJT Works Like a Switch
Now let us revise how the NPN transistor works as a switch, and we talk mainly about its base terminal being grounded.
So suppose we ground the base terminal to 0 volts. That means the base current or Ib is basically zero.
Since the base is grounded, then there is no way for current to flow from collector to emitter. So now this NPN transistor is in non-conducting state which we call OFF, or in technical language it is in the cut-off region.
Now if you take a voltage source and then apply a positive voltage greater than 0.7 volts to the base terminal, that means we are forward biasing it with respect to emitter and then the BJT gets activated.
This forward biasing allows a much larger current to flow through from collector to emitter. At this moment, we say the transistor is switched ON and now it is conducting.
So if we keep toggling between these two states, cut-off which is OFF and conduction which is ON, then we can use this NPN transistor like an electronic switch. It is like we have a light switch that we can turn ON and OFF by only changing the voltage at the base.
But for the above NPN BJT to conduct fully, we need to switch the base terminal between zero volts and some positive voltage which must be at least 0.7 volts.
When we increase that voltage, then it pushes more base current or Ib into the transistor. This extra base current leads to a big increase in collector current or Ic, while at same time the voltage drop across collector and emitter terminals which we call Vce, becomes smaller.
Now what is interesting here is that just a little current going into base can make a much larger current flow between collector and emitter.
Understanding Current Gain Of Transistor
Now let us talk about current gain of transistor. This is represented by the ratio of collector current to base current, and that ratio is called β.
For a typical bipolar transistor, we can expect β to be somewhere between 50 and 200. But even transistors that have same part number can show different values of β.
Now sometimes when I am working with amplifier circuits, then I may find that a single transistor does not give enough current gain to drive a load directly. So what I do is, I set up a Darlington pair. This setup uses two transistors and gives me much higher gain, so that it becomes easy to control bigger loads without problem.
How Darlington Pair Works
A Darlington BJT configuration, also called a Darlington pair or a super-alpha circuit, has two transistors. These two can be NPN type or PNP type, and they are connected in such a way that the emitter current of first transistor TR1 goes as base current for second transistor TR2.
In this setup, we configure TR1 like an emitter follower, while TR2 works like a common emitter amplifier, just like we see in the diagram.
It is also important to see that in this Darlington pair configuration, the collector current of first transistor TR1 which is often called the slave or control transistor, flows in-phase with collector current of second transistor TR2, which acts like the master switching transistor.
Analyzing a Typical Darlington Transistor Configuration
Now let us see NPN Darlington pair arrangement. In this we see that the collectors of the two transistors are connected together and the emitter of the first transistor TR1 goes to drive the base of the second transistor TR2, so the chain becomes like that.
Now this kind of connection allows one special thing, and that thing is called β multiplication, and this is a phenomenon which makes the current gain very high so we can use it to boost current a lot.
So let us take simple example. Let us say we have one base current, and we call that ib. Then in this case the collector current we can show as: IC = β * ib. Here β is the current gain, and this β is always more than one or more than unity, so the current gain becomes bigger.
This relationship can be mathematically defined as follows:
IC = IC1 + IC2
IC = β1 * IB + β2 * IB
In this context, it is important to note that the base current IB2 of TR2 is equivalent to the emitter current IE1 of TR1.
This equivalence arises because the emitter of TR1 is directly connected to the base of TR2.
So, we can express this relationship as:
IB2 = IE1 = IC1 + IB
= β1 * IB + IB
= (β1 + 1) * IB
Next, further substituting this in the first equation gives us:
IC = β1 * IB + β2 * (β1 + 1) * IB
IC = β1 * IB + β2 * β1 * IB + β2 * IB
IC = (β1 + (β2 * β1) + β2) * IB
Now let us see that β1 and β2 are showing the gains of those individual transistors, and we must remember this point.
So what this tells us is that if we want to calculate the total current β then we simply take the gain of first transistor and multiply it by the gain of the second transistor and we get the answer.
How They Combine
It is good because the current gains of these two transistors actually combine together in this multiplication process, so you can see how they add in effect.
Darlington Pair Concept
To put in more simple way, when we configure a couple of bipolar transistors to create a Darlington transistor pair then we can think of them as working like only one single transistor, and that is easy to understand.
Result Of Combination
So this combined setup finally gives us a super high value of β, and that means we also get very high input resistance so this is very useful in many cases.
Solving a Darlington Pair Transistor Problem #1
Now we have this setup where we connect two NPN transistors together, and that becomes a Darlington Pair. The purpose of this configuration is that we want to control a 12V, 100W halogen lamp.
Current Gain Of First And Second BJT
The first transistor has forward current gain of 30. The second BJT has forward current gain or Beta of 100.
Ignoring Voltage Drops
For our calculation we are going to ignore any voltage drops that may happen across these two transistors, because we want to keep the calculation very simple.
Maximum Base Current Requirement
Now we want to calculate the maximum base current that we will need for switching this halogen lamp fully ON.
It is clear that the current used by the lamp has to be equal to the Collector current of the second BJT. So we can use the following formula:
IC = ILAMP
ILAMP = P/V = 100/12 = 8.33 Amps
By referring to our earlier derived β formulas, we can calculate the base current of the first BJT as:
IC = (β1 + (β2 * β1) + β2) * IB
IB = IC/(β1 + (β2 * β1) + β2)
= 8.33/((30 + (100 * 30) + 100) = 0.00266 Amps or 2.66 mA
So, we can see that we only need a very tiny base current of just 2.66 mA to get that 100 watt lamp to turn “ON” and “OFF.” This small base current can easily be provided by something like a digital logic gate or even the output port of a micro-controller. It is pretty impressive how we can control such a powerful lamp with just a negligible amount of of input current.
Now let us talk about what happens when we use two identical bipolar transistors to create a single Darlington device. In this case we find that β1 is equal to β2 which means that both the transistors have the same gain.
Because β1 is equal to β2 the total current gain for this setup can be calculated as follows:
IC = (β1 + (β2 * β1) + β2) * IB
∴ IC = (β2 + 2β) * IB
Typically we find that the value of β2 is way higher than 2β. Because of this big difference we can actually ignore the 2β part to make our calculations a bit simpler.
So when we are looking at two identical transistors that are set up as a Darlington pair, we can simply use a final equation that reflects this configuration, as given below:
IC = (β2 * IB)
So when we are looking at two identical transistors, we can see that β2 essentially takes the place of β. This means that they work together like one big transistor that has a really impressive gain.
You can actually find Darlington transistor pairs that have current gains that go beyond one thousand and can handle maximum collector currents of several amperes.
A couple of great examples of this are the NPN BJT TIP122 and its PNP complementary, the TIP127. Now one of the best things about using this kind of setup is that the switching transistor becomes super sensitive.
This is because it only needs a tiny base current to control a much larger load current. For example, the typical gain for a Darlington configuration is over 1,000, but if we were just using a standard single transistor stage, we would get a gain of just around 50 to 200.
So if we have a Darlington pair with a gain of 1,000:1 we could control an output current of 1 ampere in the collector-emitter circuit using just a miniscule input base current of only 1 mA.
This makes Darlington transistors absolutely ideal for interfacing with relays, lamps, and motors when we are working with sources such as low-power microcontrollers, computers, or logic controllers.
Understanding Applications of Darlington Transistor
Now when we talk about Darlington transistor, one good thing is that its base is very sensitive so we can use very small current at the base.
This means that it can respond easily to small input currents and these currents can come from a switch, or directly from TTL, or from 5V CMOS logic gate.
Now when we look at maximum collector current Ic(max), then it is equal to the main switching transistor in the second part of Darlington pair, and this is normally called TR2.
So this feature allows us to control many devices like relays, DC motors, solenoids, and lamps without much problem, and we can do it easily by using a Darlington BJT pair.
But now let us also see the drawback. One major drawback of Darlington transistor pair is that it has low voltage drop between base and emitter when fully saturated, but this drop is not as low as single BJT.
If we compare it to single BJT, then normally single BJT shows saturated voltage drop between 0.3V and 0.7V when it is in full ON mode. But a Darlington component shows higher base emitter voltage drop of almost 1.2V instead of only 0.6V.
This happens because this base emitter voltage drop includes combined drops from base emitter diodes of both transistors in the pair.
Depending on how much current flows through the transistors, this combined drop can be anywhere between 0.6V and 1.5V.
Now when we say that Darlington transistor has high base emitter voltage drop, what we are really saying is that this Darlington device becomes more hot compared to normal single bipolar transistor when we pass same load current.
So because of this extra heat, it is very important for us to use good heat sink, so that we can keep the device cool.
Now also remember that Darlington transistors are not fast switching devices in the transistor family. They show slower ON OFF response time, because it takes more time for the slave transistor TR1 to drive the master transistor TR2 fully ON or fully OFF.
How We Solve The Drawbacks
But you do not need to worry, because we can solve this problem also. There is a way to reduce slow response, extra voltage drop and heating problem of Darlington transistor.
We can use complementary NPN and PNP transistors in similar cascaded arrangement, and this gives us another type of Darlington transistor which is called Sziklai Configuration.
What Is Sziklai Transistor Pair
Now let us understand the Sziklai Transistor Pair which is also called Sziklai Darlington Pair. This was named after Hungarian inventor George Sziklai.
This is a complementary or compound device and it uses one NPN and one PNP transistor paired together in special arrangement, like we see in the diagram.
The good thing about this NPN PNP pairing is that it works similar to Darlington transistor, but with one nice benefit. It only needs base emitter voltage of around 0.6V to turn ON, so this is more efficient.
And just like standard Darlington, the Sziklai configuration also increases current gain effectively. If we use transistors with same gain value, then total current gain will be β². But if we use two transistors with different gain values then total gain will be product of those two gains, so this becomes flexible and useful option.
Understanding the Sziklai Darlington Transistor Configuration
Now in the diagram above, we can see that the base-emitter voltage drop in the Sziklai device is same like the diode drop that we see in a single transistor in the signal path.
But we have to note one important limitation. The Sziklai configuration cannot reach saturation below a full diode drop. This drop is usually around 0.7 volts. It is different from other configurations where saturation can go near 0.2 volts.
Also same like the Darlington pair, the Sziklai pair gives slower response time compared to a single transistor. So this can affect performance in some applications.
The complementary transistors that make the Sziklai pair are many times used in push-pull and class AB audio amplifier output stages. This allows us to work with only one polarity of output transistor.
Now let us also mention that both Darlington and Sziklai transistor pairs are available in NPN and PNP configurations. So this gives us flexibility in many circuit designs.
Understanding Darlington Transistor As Integrated Circuits
Now in most electronics projects, we just want the controlling circuit to turn DC output voltage or current ON or OFF directly. This is fine for output devices like LEDs or displays that only need few milliamps to run at low DC voltage. So we can drive them straight from logic gate.
But sometimes, as we saw earlier, we need more power to run a DC motor or such things. Then an ordinary logic gate or microcontroller cannot give enough current. So then we need some extra circuit to do that work.
One good option is the ULN2003 Darlington transistor chip. This Darlington family has ULN2002A, ULN2003A, and ULN2004A. All of these are high voltage and high current Darlington arrays. In each chip, there are seven open collector Darlington pairs inside one IC.
Each channel of the array can give 500mA and can also take peak current up to 600mA. So it is perfect for driving small motors, lamps, or even gates and bases of high-power semiconductors.
Also it has suppression diodes for inductive loads, and the inputs are placed opposite to the outputs. So this makes wiring and PCB layout easy for us.
The ULN2003A Darlington Transistor Array
Now let us look at the ULN2003A Darlington Transistor Array. This is a low cost unipolar Darlington transistor array. It is very efficient and uses low power. So it becomes very useful for driving many loads like solenoids, relays, DC motors, LED displays, or filament lamps.
The ULN2003A has seven Darlington transistor pairs. On the left side it has input pins and on the right side it has output pins. So it is just like we see in the diagram.
Now we talk about ULN2003A Darlington driver. This is one integrated circuit which is very efficient. We say this because it has extremely high input impedance and also big current gain. So you and me can drive it directly using TTL logic gates or +5V CMOS logic gates, and it will work.
Since sometimes we need other logic, then we can use other versions. For example, if you need +15V CMOS logic, then you must use ULN2004A. But if you need to switch high voltages up to 100V, then you must use SN75468 Darlington array, because that is suitable.
Input And Output Behavior
Now let us see how pins behave. When an input pin from 1 to 7 becomes HIGH, then the corresponding output becomes LOW and that means it sinks current.
But when the input is LOW, then the output goes to high impedance state. This OFF state blocks the load current and also reduces leakage current, so the device works with more efficiency.
Pin Connections
Now we must connect pins properly. Pin 8 which is GND must go to ground or 0 volts of the load. Pin 9 which is Vcc must go to the power supply of the load.
Any load must be put between +Vcc and one output pin from 10 to 16. But if we drive inductive loads like motor, relay, or solenoid, then it is very important that pin 9 goes directly to Vcc, otherwise it will not work well.
Current Handling
ULN2003A can switch up to 500mA or 0.5A per channel. But if you need more current, then you can parallel inputs and outputs. For example, if we connect input 1 with input 2, and output 16 with output 15, then both will combine and allow more current for the load.
Conclusion About Darlington
Now let us see why Darlington transistor is strong. This device is different because its current and voltage ratings are much higher compared to normal small signal transistors. So we can use it in many electronic applications.
When we check high power NPN or PNP transistors, then we see that DC current gain is low, sometimes only 20 or less. But small signal switching transistors usually have higher gain. Because of this low gain in high power type, we need large base current to drive one load, and this makes them less efficient.
So engineers use Darlington arrangement. This means two transistors are joined back to back. One transistor carries the main current, and the small one works like a switch to provide preamplified base current.
This system allows us to use small base current for controlling large load current. Since the current gains of both transistors multiply, we can think of it like one single transistor which has very high current gain β and also high input resistance.
Now apart from normal PNP and NPN Darlington pairs, we also have complementary Sziklai Darlington. That one uses NPN and PNP together in one pair, and this helps to improve efficiency.
Finally, we also have Darlington arrays like ULN2003A. With these arrays we can safely control high power loads like lamp, solenoid, and motor using microprocessor or microcontroller. This is very useful in robotics and mechatronics where we want accurate control.
Sources: