This configuration is called as Darlington transistor configuration. A Darlington transistor is a back to back connection of two transistors that comes as a complete package with three leads base, emitter and collector as equivalent to the single transistor. The pair of bipolar transistors provides a very high current gain as compared with single standard transistor as mentioned above.
Consider the NPN configuration of Darlington transistor. In this a very small base current causes a large emitter current to flow which is then applied to the base of the next transistor.
Amplified current in the first transistor is again amplified by the current gain of the second transistor. Therefore, the emitter current of the second transistor is very large which is high enough to drive high loads.
So the overall current gain is This value is very high as compared with a single transistor so this high current gain gives a high load current. Generally, to turn ON the transistor, the base input voltage must be greater than 0. Since the two transistors are used in this configuration therefore the base voltage must be greater than 1.
Similarly, the current gain of next transistor,. But the base current of the second transistor,. Substituting in the above equation,. In the above relation individual gains are neglected and the total equation is approximated as.
Consider the following circuit where Darlington pair is used to switch the load that is rated at 12V and 80W. The current gains of first and second transistors are given as 50 and 60 respectively. So, the base current required to make the lamp fully ON is calculated as follows.
From the above calculation, it is clear that with small base current we can switch the larger lamp loads. This small base input voltage can be applied from any microcontroller output or any digital logic circuits.
Darlington transistors are mainly used in switching and amplification applications for delivering a very high DC current gain. Some of the key applications are high and low side switches, sensor amplifiers and audio amplifiers. For light-sensitive applications photodarlingtons are used. Let us look at the operation of a Darlington transistor for a specific application.
As stated earlier this equals the input current x the gain of the transistor hFE. If it is not possible to increase the input current then the gain of the transistor will need to be increased.
This can be achieved by using a Darlington Pair. Therefore this will allow a very low input current to switch a much bigger load current. Base Activation Voltage Normally to turn on a transistor the base input voltage of the transistor will need to be greater than 0. As two transistors are used in a Darlington Pair this value is doubled. Therefore the base voltage will need to be greater than 0. It is also worth noting that the voltage drop across collector and emitter pins of the Darlington Pair when the turn on will be around 0.
You may be interested in other similar resources: How A Transistor Works. How to identify components. See it in a real project We have two electronic kits which utilise a Darlington pair to allow an output current of up to half an Amp to be controlled: Kitronik Light Activated Switch Kitronik Heat Activated Switch Download a pdf version of this page here Learn more about the author read more » If you found this article helpful and you would like to receive product updates and free electronic resources from us then sign up to our newsletter here.
Kitronik's suggested additional learning. How A Transistor Works. Tags: Electronic principles , Understanding components , Understanding principles.
Carl Salmans 24 February at am I am glad that someone is keeping up with the basics in electronics. Rob Haywood 16 December at pm A Darlington Pair is actually to increase current not voltage, a voltage drop across a Darlington Pair is normal of around 0. Ubochi Chinomso 15 December at am I have been trying to build a power bank using Darlington pair transistors, my aim is to increase a supply voltage of 4. Kitronik Newsletter Sign up now to be the first to know about the latest products and resources!
Your email. Follow us. Latest Resources. No Quibble Refunds If you don't like the product for whatever reason, send it back packaged and unused for a full refund. This makes Darlington transistor pairs very easy to use and incorporate into a new electronic design. The Darlington pair circuit configuration is quite distinctive. It normally consists of two transistors, although in theory it can contain more.
The emitter of the input transistor is connected directly to the base of the second. Both collectors are connected together. In this way the base current from the first transistor enters the base of the second. This results in a very high level of current gain. The overall current gain of the Darlington pair is the product of the two individual transistors:. Although the Darlington pair circuit is often used in its basic format, it is often seen with a bypass resistor between the base and emitter connections of the final transistor.
The bypass resistor is included to aid the switch-off process. Without the resistor in place, there is no discharge path for any charge held in the capacitor formed by the base emitter junction. Including it enables the charge stored in this capacitor to dissipate and this aids a faster turn-off.
It is good design to include this resistor, but if speed is not an issue then the resistor can be omitted. However, unless cost and component count are key drivers within the circuit design, then it is wise to include this component. Determining the resistor value is not an exact science. Smaller resistors will give a faster turn-off, but if they are made too small then a large proportion of the drive current for the second transistor passes through the resistor and gain is lost.
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