Darlington Pair Explained | The Darlington Pair as a Switch

Summary

This video from the 'All About Electronics' YouTube channel delves into the Darlington Pair, a transistor configuration known for its high current gain, often referred to as super β transistor. It explains how the Darlington Pair, created by connecting the emitter of one transistor to the base of another, achieves a current gain approximately equal to the product of the individual transistor gains (β1*β2). The video explores its applications in high current requirements like audio amplifiers, display drivers, and motor control, and its use as a high-performance emitter follower. It also touches on the Darlington Pair's limitations, such as higher voltage drops and power dissipation, and sets the stage for the next video, which will focus on its use as an amplifier.

Takeaways

• 😀 The Darlington Pair is a transistor configuration known for its high current gain, often referred to as super β transistor.
• 🔗 In a Darlington Pair, the emitter of the first transistor is connected to the base of the second transistor, and their collectors are connected together.
• 🔢 The overall current gain (βD) of a Darlington Pair is the product of the current gains (β1 and β2) of the individual transistors.
• 🎚️ Darlington Pairs are used in applications requiring high current, such as audio amplifiers, display drivers, motor or solenoid control, and as high-performance emitter followers.
• 🔗 When used as an amplifier, a Darlington Pair is essentially a cascaded connection of two common collector amplifiers.
• 🔌 Commercially available Darlington Pairs come in a single package with internal connections between the two transistors.
• 🔋 The current gain of a Darlington Pair is approximately equal to the product of the individual transistor gains, β1*β2, due to the high product value compared to the individual gains.
• 🔄 Darlington Pairs are efficient as switches, allowing high current loads to be controlled with a small base current, making them suitable for microcontroller applications.
• ⚠️ Darlington Pairs have some disadvantages, including a larger voltage drop across the base-emitter junction and a higher saturation voltage, leading to higher power dissipation.
• ⏲️ They are not suitable for fast switching applications due to limited bandwidth and low switching speeds, but are adequate for controlling certain devices.

Q & A

What is a Darlington Pair in electronics?

A Darlington Pair is a transistor configuration that provides a very high current gain (β) by connecting the emitter of one transistor to the base of another transistor. This configuration is also known as a super β transistor.

How is the overall current gain (βD) of a Darlington Pair calculated?

The overall current gain (βD) of a Darlington Pair is calculated by multiplying the current gains (β1 and β2) of the individual transistors (Q1 and Q2) in the pair, i.e., βD = β1 * β2.

What are some applications of Darlington Pairs due to their high current gain?

Darlington Pairs can be used in applications requiring high current, such as audio amplifiers, display drivers, switches for motor or solenoid control, and as high-performance emitter followers or voltage followers.

How is a Darlington Pair used as an amplifier?

When used as an amplifier, a Darlington Pair can be considered as a cascaded connection of two common collector amplifiers. The input is applied at the base, and the output is measured at the emitter terminal.

What is the significance of the base current (IB2) being equal to the emitter current (IE1) in a Darlington Pair?

In a Darlington Pair, the base current (IB2) of the second transistor is equal to the emitter current (IE1) of the first transistor because the emitter of the first transistor is connected to the base of the second transistor.

How does the commercial availability of Darlington Pairs in a single package affect their usage?

Commercially available Darlington Pairs in a single package simplify their integration into circuits, as the two transistors are internally connected, reducing the complexity of external connections.

What is the relationship between the base current (IBD), collector current (ICD), and emitter current (IED) of a Darlington Pair?

In a Darlington Pair, the base current (IBD) is equal to the base current of the first transistor (IB1). The collector current (ICD) is the sum of the collector currents of both transistors (IC1 + IC2), and the emitter current (IED) is the sum of the emitter currents of both transistors.

Why is the current gain (βD) of a Darlington Pair approximately equal to the product of the individual transistor gains (β1*β2)?

The current gain (βD) of a Darlington Pair is approximately equal to the product of the individual transistor gains (β1*β2) because the product is usually much larger than the sum of the individual gains, making it the dominant term in the total current gain calculation.

How does the Darlington Pair facilitate driving high current loads with a small base current?

The Darlington Pair allows driving high current loads with a small base current due to its high current gain. For example, with a βD of 10000, only a few milliamps of base current are needed to drive several amperes through the load.

What are some disadvantages of using a Darlington Pair?

Disadvantages of using a Darlington Pair include a larger voltage drop across the base-emitter junctions, higher saturation voltage (VCE(sat)), increased power dissipation, limited bandwidth, and lower switching speeds compared to single transistors.

Outlines

00:00

📡 Introduction to Darlington Pair

This paragraph introduces the Darlington Pair, a transistor configuration known for its high current gain (β), which can also be referred to as a super β transistor. It explains that the Darlington Pair consists of two transistors connected in such a way that the emitter of the first transistor is connected to the base of the second transistor, and both collectors are connected together. The overall current gain of the Darlington Pair is the product of the current gains of the individual transistors (β1*β2). This high current gain makes it suitable for applications requiring high current, such as audio amplifiers, display drivers, and as a switch for motor or solenoid control. Additionally, due to its high β, it can be used as a high-performance emitter follower or voltage follower. The paragraph also discusses the Darlington Pair's configuration as a cascaded connection of two common collector amplifiers and sets the stage for deriving the expression for β in the next video.

05:10

🔍 Derivation of Darlington Pair's Current Gain

In this paragraph, the focus is on deriving the expression for the current gain (β) of a Darlington Pair. It starts by defining the current gains of the individual transistors (β1 and β2) and the currents involved (IB1, IC1, IB2, IC2). The paragraph explains that the base current of the second transistor (IB2) is equal to the emitter current of the first transistor (IE1), which is also equal to (β1 + 1) times the base current of the first transistor (IB1). The collector current of the second transistor is then expressed as β2 times IB2, which is a function of β1 and IB1. The paragraph goes on to describe how commercially available Darlington Pairs are packaged as a single unit with three terminals (base, collector, and emitter) and how the overall current gain (βD) can be approximated as the product of β1 and β2, given their typically high values. An example is provided to illustrate the calculation of βD and to emphasize the high current gain advantage of the Darlington Pair.

10:12

🔌 Applications and Limitations of Darlington Pair

This paragraph discusses the practical applications and limitations of the Darlington Pair. It highlights the Darlington Pair's common use as a switch, especially in controlling high current loads with a small base current, using a microcontroller as an example. The paragraph contrasts the requirements for driving a load with a single transistor versus a Darlington Pair, demonstrating how the latter allows for the control of heavy loads with a minimal base current. However, it also points out the disadvantages of the Darlington Pair, such as a larger voltage drop across the base-emitter junction compared to a single transistor and a higher saturation voltage (VCE(sat)), which can lead to higher power dissipation. As a result, Darlington Pairs often require heat sinks. Additionally, the paragraph mentions the limited bandwidth and low switching speed of the Darlington Pair, making it unsuitable for very fast switching applications. The paragraph concludes with an invitation for questions and suggestions and a prompt to like and subscribe for more educational content.

Keywords

💡Darlington Pair

A Darlington Pair is a configuration of two bipolar junction transistors (BJTs) connected in such a way that the emitter of the first transistor is connected to the base of the second transistor, with their collectors tied together. This configuration is highlighted in the video as it provides a very high current gain (β), which is the product of the current gains of the individual transistors (β1*β2). The video explains that this high gain makes the Darlington Pair suitable for applications requiring high current, such as audio amplifiers and motor control. The Darlington Pair is also referred to as a 'super β transistor' due to this high gain.

💡Current Gain (β)

Current gain, often denoted as β (beta), is a measure of how much current is amplified by a transistor. In the context of the video, the Darlington Pair is described as having a very high β because it combines the current gains of two transistors. The video uses the formula βD = β1*β2 to illustrate how the overall current gain is calculated. This high current gain is crucial for the Darlington Pair's ability to control high current loads with a small input current, as demonstrated in the example of driving a 5A load with a microcontroller that can only provide 20mA.

💡Emitter Follower

An emitter follower, also known as a voltage follower, is a type of amplifier configuration where the input is applied to the base and the output is taken from the emitter. The video mentions that the Darlington Pair, when used as an amplifier, can be considered as a cascaded connection of two common collector amplifiers, which are essentially emitter followers. This configuration is beneficial for applications where a high input impedance and low output impedance are required.

💡Common Collector Amplifier

A common collector amplifier is a type of transistor amplifier configuration where the collector terminal of the transistor is common to both the input and output circuits. The video explains that the Darlington Pair can be thought of as two common collector amplifiers connected in cascade, with the emitter of the first transistor connected to the base of the second, forming a high-gain amplifier. This configuration is beneficial for applications where a high current gain is needed.

💡Base Current

Base current (IB) is the current flowing into the base of a transistor, which controls the flow of a much larger current through the collector and emitter. In the video, the base current is discussed in the context of driving a Darlington Pair into saturation. The video provides an example where a base current of 250mA is required to drive a 5A load with a transistor having a β of 100. This concept is crucial for understanding how a small base current can control a much larger load current in a Darlington Pair configuration.

💡Collector Current

Collector current (IC) is the current that flows out of the collector terminal of a transistor. In the video, the collector current is discussed in relation to the Darlington Pair's ability to handle high current loads. The video explains that the overall collector current (ICD) of a Darlington Pair is the sum of the collector currents of the individual transistors (IC1 + IC2), which is used to derive the overall current gain of the pair.

💡Saturation Voltage (VCE(sat))

Saturation voltage (VCE(sat)) is the minimum voltage required across the collector and emitter terminals of a transistor to keep it in saturation, where it can carry the maximum current. The video points out that the Darlington Pair has a larger VCE(sat) compared to a single transistor, which can lead to higher power dissipation. This is an important consideration when using Darlington Pairs, as it may require additional cooling solutions like heat sinks.

💡Power Dissipation

Power dissipation refers to the amount of power a component generates as heat during operation. The video discusses how the Darlington Pair's high current gain and larger VCE(sat) can lead to significant power dissipation, especially when used as a switch for high current loads. The video provides a calculation example where a 5A current at 1V VCE(sat) results in 5W of power dissipation, highlighting the need for heat sinks to manage this heat.

💡Switch

In the context of the video, a switch refers to the use of a Darlington Pair to control the flow of current to a load, such as a lamp or motor. The video demonstrates how a Darlington Pair can be used as a switch to control a high current load with a small base current from a microcontroller. This application is a primary advantage of Darlington Pairs, as it allows for the control of heavy loads with minimal input current.

💡Microcontroller

A microcontroller is a small computer on a single integrated circuit that can be used to control electronic devices. In the video, a microcontroller is mentioned as a device that can be used to control a high current load using a Darlington Pair as a switch. The video provides an example where a microcontroller with a limited current output can still drive a 5A load by using a Darlington Pair with a high current gain.

Highlights

Darlington Pair is a transistor configuration known for its high current gain.

It is also referred to as a super β transistor due to its high β value.

The configuration connects the emitter of one transistor to the base of another.

The overall current gain (βD) is the product of the individual transistor gains (β1*β2).

Darlington Pairs are used in applications requiring high current, such as audio amplifiers and display drivers.

They can act as switches for motor or solenoid control due to their high current driving capability.

High β value makes them suitable for high-performance emitter followers or voltage followers.

When used as an amplifier, Darlington Pairs are considered as cascaded common collector amplifiers.

The input is applied at the base, and the output is taken across the emitter terminal in this configuration.

The derivation of the Darlington Pair's current gain (βD) is detailed, showing it as the sum of individual transistor gains.

Commercially available Darlington Pairs come in a single package with internal connections.

The Darlington Pair's high current gain is approximated as the product of the individual transistor gains.

The Darlington Pair is effective as a switch, even with a small base current, due to its high gain.

It allows microcontrollers to drive heavy loads that they wouldn't be able to handle with a single transistor.

The Darlington Pair has a larger voltage drop across the base-emitter junction compared to a single transistor.

It also has a larger saturation voltage (VCE), which can lead to higher power dissipation.

The Darlington Pair may require a heat sink due to its high power dissipation.

It has limited bandwidth and low switching speed, making it unsuitable for very fast switching applications.

The video concludes with a teaser for the next video, which will cover the use of Darlington Pairs as amplifiers.

Transcripts

Hey friends, welcome to the YouTube channel ALL ABOUT ELECTRONICS.

So, in this video, we will learn about the Darlington Pair.

So, this Darlington Pair is a very popular transistor configuration which provides very

high value of β.

And that’s why sometimes it is also known as super β transistor.

So, in this configuration, the emitter of one transistor is connected to the base of

the second transistor.

And the collector of both transistors is connected together.

So, if β1 and β2 are the current gains of this transistor Q1 and Q2 then the overall

current gain of this Darlington Pair, let's say it is equal to βD, then it can be given

as β1*β2.

So, because of its high current gain, this Darlington Pair can be used in the applications

where the high current is required.

For example, it can be used in the audio amplifiers or in the display drivers, and it can be used

as a switch for the motor or the solenoid control.

Apart from that, because of its high value of β, it can also be used as a high-performance

emitter follower or the voltage follower.

And in fact, when this Darlington Pair is used as an amplifier, then it can be considered

as the cascaded connection of the two common collector amplifiers.

So, this is the first common collector amplifier, and this is the second one.

Because as you are aware, in the common collector configuration, the input is applied at the

base terminal while the output is taken across the emitter terminal.

So, here the emitter of the first transistor is connected to the base of the second transistor.

That means here, the output of the first amplifier is connected to the input of the second amplifier.

And here, the output is taken across the emitter of the second transistor.

So, basically, it is the cascaded connection of the two common collector amplifiers.

So, this is how this Darlington Pair can be used as an amplifier.

Where the input is applied at the base terminal, while the output is measured at the emitter

terminal.

But we will discuss more about it in the next video.

But here, first of all, let's derive the expression of β for this Darlington Pair.

So, let's say, this β1 and β2 are the current gain of this transistor Q1 and Q2.

And let's also denote the current for each transistor.

So, here this will be the base current, that is IB1, while this will be the collector current.

That is IC1.

And this current will be equal to IE1.

Similarly, this current is IB2, while this current is IC2, while the current through

the emitter is equal to IE2.

And here, all these currents are the DC currents.

So, as you can see over here, clearly, this IB2 is equal to IE1, right !!

And we know that this emitter current can be given as (β +1 )*IB.

So, here this current IE1 = (β1 + 1)*IB1.

And that is equal to IB2 right !! So, now this collector current of the second

transistor can be given as β2*IB2 = β2 * (β1 + 1)*IB1.

On the other end, this collector current Ic1 can be given as β1*IB1.

Now, commercially, this Darlington Pair is available in the single package.

Where these two transistors are connected internally.

So, if we consider these two transistors as a single unit, then it will have three terminals.

That is the base, collector, and emitter.

And let's say, for this Darlington Pair, the base current is equal to IBD, while the collector

current and the emitter currents are ICD and IED respectively.

So, here this IBD = IB1, while this collector current ICD = IC1 +IC2.

That is the summation of these two currents right !!

So, this IC1 = β1*IB1, while this IC2 = β2* (β1 + 1)*IB1

That means ICD = IB1 * (β1 + β2 + β1*β2) And this IB1 is nothing but the IBD.

That means ICD / IBD, that is the current gain of this Darlington Pair is equal to β1

+ β2 +β1*β2.

So, this is the current gain or the value of β for the Darlington Pair.

Now, usually, the product of β1 and β2 is very high compared to β1 and β2.

So, approximately we can say that, this βD ≈ β1*β2.

For example, if β1 is 100 and β2 is 200, then β1*β2 will be equal to 20000 right

!! And if we see the total value of βD, then

it will be equal to 20300.

So, approximately we can say that the value of βD is equal to 20000.

And that is why usually, the current gain of this Darlington Pair is expressed as β1*β2.

Alright, so now let's understand how this high current gain is very useful in some applications.

Now, one of the most common applications of the Darlington Pair is to use it as a switch.

So, let's understand the usefulness of this Darlington Pair by understanding how it can

be used as a switch.

Let's say, using the microcontroller we just want to turn ON and OFF the particular lamp.

And for that, we are using the BJT as a switch.

So, let's say, here the required current through the load is 5A, while the β of the transistor

is equal to 100.

And the maximum current which can be delivered by the microcontroller is equal to 20 mA.

Now, here as we are using the BJT as a switch, so to draw a 5A current through the load,

the value of this Ic (sat) will be equal to 5A right !!

And to drive this transistor into saturation, the value of the base current should be at

least equal to Ic (sat) / β.

That is equal to 5A / 100.

That is equal to 50 mA.

And to ensure the saturation condition or for the deep saturation, let's say the base

current is 5 times this value.

That means the required value of the base current will be equal to 250 mA.

But clearly, you can see here, this microcontroller can only be able to provide 20mA,

And hence, it won't be able to drive this particular load.

But instead of the transistor, if we use the Darlington Pair, then using the same microcontroller

we can drive this particular load.

So, let's say, the β of this Darlington Pair is equal to 10000.

So, in this case, Ic (Sat) is still 5A, but now the required value of the base current

will be equal to Ic (sat) / βD.

That is equal to 0.5 mA.

And for the deep saturation, if we take the value of IB as 5 times this value, then the

required value of the base current will be equal to 2.5 mA.

So, just by the base current of 2.5 mA, we are now able to drive the load of 5 A.

And clearly, now this microcontroller can easily provide the required current.

So, using this Darlington Pair, this high current driving load can be easily controlled

even with very small base current.

And using the microcontroller also, we can drive such heavy load.

So, that is the very common application of the Darlington Pair.

But this Darlington Pair also has some disadvantages.

First is, the voltage drop across the base and emitter is larger compared to the normal

transistor.

Because in this case, as two transistors are connected together, so the overall voltage

drop will be a voltage drop across the two transistors.

That means here this VBED or the VBE of this Darlington Pair is equal to VBE1 + VBE2.

Where VBE1 and VBE2 are the voltage drop across the two transistors.

Then the second is, this saturation voltage VCE is larger compared to the normal transistors.

So, for the normal transistors, it is in the range of 0.2 to 0.3V.

But for the Darlington Pair, it could be even 1V or even more.

So, because of this large value of the VCE (sat), the power dissipation across this Darlington

Pair will be very high.

For example, if this Darlington Pair is used as a switch and if it draws 5A current with

value of VCE (sat) = 1V, then the power dissipation across this Darlington Pair will be equal

to 5W.

And that is why, because of this high power dissipation, this Darlington Pair is often

used with the heat sink.

Then the limited bandwidth and the low switching speed is another disadvantage of this Darlington

pair.

So, this Darlington Pair can not be used for very fast switching, but it can be used as

a switch to control some devices.

So, these are some limitations of this Darlington pair.

So, I hope in this video, you understood what is Darlington pair, and how it can be used

as a switch in the high current applications.

So, in the next video, we will see that, how this Darlington pair can be used as an amplifier.

So, if you have any questions or suggestions, do let me know here in the comment section

below.

If you like this video, hit the like button and subscribe to the channel for more such

videos.