Darlington Transistor: Switch and configuration

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Transistor basics Transistor operation  Transistor characteristics  Transistor configurations Transistor oscillator circuit common emitter amplifier

Darlington transistor consists of two bipolar transistors is connected in such a way that the current amplified by the first transistor is amplified further by the second one. In our previous articles, we describe how transistor acts as an oscillator, switch and an amplifier. Here we are going to explain you Darlington transistor switch, configuration and applications in detail. Have a look over it.

This transistor was invented by Bell laboratories engineer, Sidney Darlington in 1953. He introduced the idea of having two or three transistors on a single chip sharing a collector. The emitter of the first transistor is connected to the base of the second one to produce a more sensitive transistor with a much larger current gain.

Darlington transistor image

                                       Darlington transistor

Darlington transistor configuration

Darlington transistor configuration is also known as Darlington pair or super alpha circuit. Into this, the two NPN transistors or PNP transistors are connected together in such a way so that the emitter current of the first transistor T1 become the base current of the second transistor T2. Then the transistor T1 is connected as an emitter follower and transistor T2 as a common emitter amplifier.

PNP Darlington Transistor image

                PNP Darlington Transistor

NPN darlington Transistor image

                NPN Darlington Transistor

Let’s take an example of NPN Darlington pair amplifier as shown in the figure above, the collectors of two transistors are connected together and the emitter of T1 drives the base of T2. Hence, this configuration achieves the β multiplication for the base current (Ib) and the collector current (Ic).

Calculation of Collector current:

Collector current is calculated in terms of gains of the individual transistor. As we know,

Ic = Ic1 + Ic2

Ic =   β1 Ib + β2 Ib2

As shown, the base current, IB2 is equal to transistor T1 emitter current, IE1 as the emitter of T1 is connected to the base of T2. Therefore:

 Ib2 = Ie1 = Ic1 + Ib = β1 Ib  + Ib

 Ib2 = (β1 + 1) Ib

Put this value in the above equation:

Ic =   β1 Ib + β2 1 + 1) Ib

Ic =   β1 Ib + β2 β1 Ib + β2 Ib

Ic = (β1 + (β2 β1) + β2 ) Ib

where β1 and β2 are the gains of the individual transistors.

Darlington Transistor switch

The Darlington transistor switch is used to directly switch the load current or voltage, sometimes the Dc current gain of the bipolar transistors is too low. To overcome this problem or to maximize the signal gain, the two transistors are connected in a Darlington configuration where the amplification factor is the product of the two individual transistors.

Darlington Transistor switch image

    Darlington transistor switch circuit                                       Diagram

This transistor simply contains the two bipolar NPN and PNP type transistors connected together so that the current gain of the first transistor is multiplied with that of the current gain of the second transistor to produce a device which acts like a single transistor switch. The overall product gain of the transistor device is the product of the two individual gains of the transistor which is defined as:

β total    =  β1 x β2

Darlington transistors with high β values and high collector currents are possible as compared to a single transistor switch. For example, if the first input transistor has a current gain of 150 and the second transistor has a current gain of 50 then the total current gain of the circuit will be 150 x 50 = 7500.

Darlington Transistor applications

Darlington transistor is used in various applications where a high gain is required at a low frequency. Following of the applications are:

  • Light and touch sensors
  • Controlling of motors
  • Power Regulators
  • Display Drivers
  • Controlling of solenoids
  • Audio amplifiers output stages

Darlington transistors are the most demandable transistors of an electronic industry. Hope you all like this article. For any suggestions please comment below. We always appreciate your suggestions.

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Gupta Himanshi


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