Advantages of Darlington Pair | Disadvantages of Darlington transistor

This page covers advantages and disadvantages of Darlington Pair. It mentions benefits or advantages of Darlington transistor and drawbacks or disadvantages of Darlington transistor.

What is Darlington Pair ?

US Engineer Sidney Darlington has invented this transistor configuration in 1953. It uses two standard BJT transistors connected together to form pair as shown. There are two possible configurations viz. one using two NPN transistors and the other using two PNP transistors. In darlington configuration, one transistor's emitter is connected with other transistor's base and also collectors of the two transistors are connected.

Darlington Pair

There is one more configuration called cross configuration in which PNP is used with NPN and NPN is used with PNP transistor. It is called Sziklai Darlington pair or push pull configuration.

Following equation is used to calculate overall current gain of the Darlington pair.

Darlington Pair Current Gain

Followng equation is used to calculate collector current from base current and vice versa provided current gains of both individual transistors are available.

Collector Current of Darlington Pair

Applications :
• Audio amplifiers
• To pick up current in the skin
• Water level indicator in which it is used as switch
• To build touch sensitive button by connecting it to metal piece

Examples :
• NPN Darlington pairs include TIP120, TIP121, TIP122, BC517
• PNP Darlington pairs include BC516, BC878, and TIP125
• Darlington Transistor IC : ULN2002A, ULN2003A, ULN2004A

Benefits or advantages of Darlington transistor

Following are the benefits or advantages of Darlington transistor:
➨It offers very high current gain in comparison to single transistor.
➨It offers very high input impedance.
➨It uses few components and hence can be used for easy circuit designs.
➨It can amplify signal to larger extent.
➨It allows designers to drive more power applications by few mA of current source.
➨It is very sensitive to current.
➨Photo-Darlington pair introduces less noise than photo-transistor with external amplifier.
➨Sziklai Darlington pair which uses cross configuration of transistors produces less heat.
➨Sziklai Darlington pair offers less response time.

Drawbacks or disadvantages of Darlington transistor

Following are the drawbacks or disadvantages of Darlington transistor:
➨There is voltage drop across base ("B") and emitter ("E") terminals when Darlington pair is in saturation (i.e. Fully ON) region.
➨Switching speed is lower.
➨Bandwidth is limited.
➨This configuration introduces phase shift at certain frequencies in negative feedback circuit.
➨It offers high power dissipation due to high saturation voltage.
➨Leakage current of first transistor is amplified by the next transistor. Hence overall leakage current of Darlington pair is higher.

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