Darlington Transistor or Darlington Pair: Working along with Applications


Darlington Transistor: What is it? (Darlington Pair) – The Darlington transistor is a cascaded of two Bipolar junction transistor – BJT or field effect transistor – FET that increased current switching for a given base current. Below the figures show the NPN or PNP Darlington transistor configuration. In this article we understand the faction circuit diagram and application of Darlington pair.


The Darlington transistor is invented by Sidney Darlington. These transistors are special arrangement of two NPN or PNP transistor. These can make by bipolar transistor or field effect transistor. The emitter of transistor TR1 is connected to base of transistor TR2 to produce a high current gain. Therefore, where the more current gain required we use Darlington.

The Darlington transistor pair is cascaded of two transistors in a single package with the Base, Emitter and Collector connecting leads. It is available in a wide variety of case styles and voltage ratings in both PNP and NPN.

What is a Darlington Transistor?

A Darlington transistor is also called Darlington pair. It is an electronics component that is cascading of two transistors. The cascading of transistor in that way it allows a very high amount of currant gain. The high current gain is achieved by compounding of amplifier, whereby the current is amplified by the first transistor and further amplified by the second transistor.

This transistor was invented by Sidney Darlington in 1953. This transistor behaves like single unit of transistor because it only one emitter, collector and base terminal. If the current gain of a transistors is β1 and β2, the overall current gain of Darlington pair is product of β1β2. This current gain product (β1β2) of this transistor is very high compared to normal transistor. Therefore, another name is “Super Beta Transistor”.

Basic Darlington Transistor Configuration


The basic configuration of Darlington transistor is show in figure. In the figure the collector of both terminals is common, the emitter terminal of TR1 is connected to the base of TR2. Thorough this configuration we achieve β multiplication because for a Base current ib, the collector current is β*ib where the current gain is greater than one, or unity and this is defined as:


But the base current (IB2) is equal to transistor TR1 emitter current and of TR1 is connected to the base of TR2. Therefore:


Then substituting in the first equation:



  • β1→ Gains of TR1
  • β2→ Gains of TR1

The overall currant gain of Darlington transistor are the current gain of first transistor is multiplied with the current gain of second transistor. In other words, the pair of bipolar transistor makes Darlington transistor with high value of β and consequently a high input resistance.

PNP and NPN Darlington Transistor symbol

The Darlington transistor pair constructs by PNP and it is called PNP Darlington pair. Darlington is made of NPN transistor. It is called NPN Darlington Transistor. The connection diagram of both PNP and NPN Darlington is as shown in the below figure.


In both type of connection the collector term is common terminal. In PNP type transistor, the base current is given to emitter of the second transistor and NPN type transistor the emitter current given to base of the second transistor.

Working of a Darlington Transistor

Darlington acts as a single transistor with high current gain property. The high current gain property is use in microcontroller or a sensor to run a larger load. Below the figure shows the Darlington circuit which are made of two transistors.


A basic property of BJT transistor is providing current gain but when high current gain required. Hence we form Darlington circuit. When applied small amount of current to the base of first transistor that it plays a major role in the flow of the current from the collector. The output is connected to next transistor this process again continues and current will more amplify.

TIP120 Darlington pair Transistor

The TIP120 is an NPN Darlington pair with the high current gain 1000. The current gain is a ratio of output current to input current. This IC is uses at that point where interface with high current load with arduino and microcontroller. It is three pin configurations.

  • PIN-1 → Base terminal (b).
  • PIN-2 → Collector terminal (C).
  • PIN-3 → Emitter terminal (E).

The base terminal is use for “ON” and “OFF” the transistor. The current flow through collector is normally connected with the load. And the current flows through Pin-3 (Emitter) are normally connected to the ground.

TIP120 has 5A collector current. The maximum value of peak current is 8 but it is not design for 8 amp it is just for stand during short period. TIP120 circuit is shown below.


Applications of Darlington pair 

The application of Darlington pair where switching and high current gain is requires. Some of the applications are high and low side switching, audio, sensor amplifier, light-sensitive applications photodarlingtons etc. some of the specified applications for Darlington pair is given below.

Darlington Transistor as Amplifier

If the transistor use as amplifier the output is taken from collector terminal of the output. This output is suit for power amplifier because power or voltage amplifiers have the load resistance at the output is very low to have high current flow, transistors must drive high load currents. The single transistor cannot fulfill this requirement. To meet high current requirement the Darlington pair is use which offers high current gain.


The circuit diagram shows the Darlington pair which high current gain. The current gain of amplifier is product of individual gain. This configuration offers high current gain to load.

Advantages of Darlington Pair

  1. The advantage of Darlington pair over single transistor amplifier is given below.
  2. It has high current gain over single stage amplifier.
  3. It has high input impedance that it can alter a high impedance input or source to low impedance load.
  4. It is a single packing of two transistors
  5. It is easy circuit configuration because it uses few components.
  6. The photo Darlington pair produces less noise as compare with photodiode.

Disadvantages of Darlington pair

  1. Its switching speed is low
  2. It has a Limited bandwidth
  3. The base emitter voltage are high, it is two time of single standard transistor.
  4. It has High power dissipation due to high saturation voltage.
  5. Leakage current of Darlington is high because the leakage current of first transistor are further amplify by second transistor.
  6. The Darlington pair introduces a phase shift at a certain frequency in the negative feedback circuit.

Also read:- Binary Multiplication, Passive Band Pass RC Filter.

People also ask about Darlington pair

What does a Darlington transistor do?

The configuration of Darlington transistor is pair of two transistors and it act as a single transistor amplifier with high current gain. This means low amount of currant is uses for microcontroller or a sensor to run a larger load.

What is Darlington NPN transistor?
The Darlington pair transistor consist of two PNP or NPN transistor with back to back connection. It has a common collector with single package. The emitter current of the first transistor is the base current of second transistor.

What is another name for a Darlington transistor?

The Darlington transistor is also known as “Darlington pair” or “super-alpha circuit”, it is paring of two NPN or PNP with common collector. The emitter current of first transistor is the base current of second transistor.

How many transistors are there in Darlington pair?

The Darlington pair consist of two PNP or NPN.

What is the advantage of a Darlington pair?

Benefits or advantages of Darlington transistor:
➨It offers very high current gain as comparison with 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.

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