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.