Darlington Pair Transistors: Advantages and Disadvantages

This page discusses the advantages and disadvantages of Darlington Pair transistors. It highlights the benefits and drawbacks associated with using Darlington transistors.

What is a Darlington Pair?

Introduction: The Darlington pair, invented by US Engineer Sidney Darlington in 1953, is a transistor configuration that utilizes two standard BJT (Bipolar Junction Transistor) transistors connected together as a pair.

There are two common configurations: one using two NPN transistors and the other using two PNP transistors. In the Darlington configuration, the emitter of one transistor is connected to the base of the other transistor, and the collectors of the two transistors are also connected together.

Darlington Pair

A variation known as the Sziklai Darlington pair (or push-pull configuration) uses a cross configuration, pairing a PNP transistor with an NPN transistor, or vice versa.

The following equation is used to calculate the overall current gain of the Darlington pair:

Darlington Pair Current Gain

The following equation can be used to calculate the collector current from the base current (and vice versa) if the current gains of both individual transistors are known:

Collector Current of Darlington Pair

Applications

Darlington pairs are used in various applications, including:

• Audio amplifiers
• Current sensing in skin-contact applications
• Water level indicators (as a switch)
• Touch-sensitive buttons (connected to a metal piece)

Examples:

• NPN Darlington pairs: TIP120, TIP121, TIP122, BC517
• PNP Darlington pairs: BC516, BC878, TIP125
• Darlington Transistor ICs: ULN2002A, ULN2003A, ULN2004A

Benefits or Advantages of Darlington Transistors

The following are the benefits or advantages of using Darlington transistors:

• High Current Gain: Offers a very high current gain compared to a single transistor.
• High Input Impedance: Provides a very high input impedance.
• Simple Circuit Design: Uses few components, enabling easy circuit designs.
• Amplification Capability: Can amplify signals to a greater extent.
• Power Application Driving: Allows designers to drive higher power applications with a small current source (a few mA).
• Current Sensitivity: Very sensitive to current.
• Low Noise (Photo-Darlington): Photo-Darlington pairs introduce less noise than photo-transistors with external amplifiers.
• Reduced Heat (Sziklai): Sziklai Darlington pairs, using the cross configuration, produce less heat.
• Faster Response (Sziklai): Sziklai Darlington pairs offer faster response times.

Drawbacks or Disadvantages of Darlington Transistors

The following are the drawbacks or disadvantages of using Darlington transistors:

• Voltage Drop: There is a voltage drop across the base (“B”) and emitter (“E”) terminals when the Darlington pair is in saturation (fully ON) region.
• Lower Switching Speed: Switching speed is slower.
• Limited Bandwidth: Bandwidth is limited.
• Phase Shift: Introduces phase shift at certain frequencies in negative feedback circuits.
• High Power Dissipation: Offers high power dissipation due to high saturation voltage.
• Higher Leakage Current: The leakage current of the first transistor is amplified by the second transistor, resulting in a higher overall leakage current for the Darlington pair.