We can change this relationship further with IC 1 + IB 1Ĭhanging the IC1 as we did previously, we get β 1IB 1 + IB 1 IB 1 (β 1 + 1) IB2 is controlled by the emitter current of T1, which is IE1.

Let’s see how the current gain is the multiplication of the two transistor’s current gain. In this situation, the current gain is unity or greater than one. Now as discussed above, we get the Collector current β * IB 1 So, the total current gain ( β ) is achieve, when the collector current is β * IB as hFE = fFE 1 * hFE 2Īs two transistors collector is connected together, Total Collector current (IC) = IC1 + IC2 So, the base current IB1, which is controlling the T1 is controlling the current flow at T2’s base. The first transistor T1 providing the required base current (IB2) to the second transistor T2’s base. The two NPN transistors T1 and T2 are connected together in an order where, T1 and T2’s collectors are connected. In the above image, two NPN transistors created a NPN Darlington configuration. The overall current gain of the Darlington pair will be- Current gain (hFE) = First transistor gain (hFE 1) * Second transistor gain (hFE 2) In the below image we can see two PNP or two NPN transistors are connected together. Simple Lie Detector Circuit using Transistorsĭarlington Transistor Pair Current Gain Calculation:.Generating Tones by Tapping Fingers using Arduino.Here are few projects where we used the Darlington Transistors: It is used for class AB amplifier or where the Push-Pull topologies are needed. This configuration produces less heat and has advantages about response time. In the above image the Sziklai Darlington pairs are shown. This type of cross configuration is called as Sziklai Darlington pair configuration or Push-Pull configuration. However, the configuration shown in the above image, uses either two PNP or two NPN, there are other Darlington configuration or the cross configuration is also available, where a PNP is used with NPN, or a NPN is used with PNP. The Darlington pair is perfect for the application where high current gain is needed. But in some cases the application needs more current gain that a single BJT transistor couldn’t provide. In typical BJT transistor the current gain is limited depending on the transistor specification.

The ratio of base current and collector current is called transistor’s current gain which is denoted as Beta ( β). This collector current is directly proportional with the base current. When we provide enough current to the base, the BJT enters in saturation mode and the current flows from collector to emitter. We need to provide current to the base which controls the collector current. In this configuration, the pair or the Darlington transistor produces much higher gain and large amplification capabilities.Ī normal BJT transistor ( NPN or PNP) can operate between two states, ON and OFF. This configuration is used for both NPN and PNP Darlington transistors. The first transistor’s emitter is directly connected across the base of other transistor, also the collector of the two transistors are connected together.

We can see NPN Darlington consist two NPN transistors, and PNP Darlington consist two PNP transistors. On the left side it is NPN Darlington and on the other side it is PNP Darlington. In the below images, two type of Darlington transistor are shown. If we see the Darlington Transistor’s symbol we can clearly see how two transistors are connected. Darlington transistor connected in a configuration where one of transistor’s emitter provides biased current to the other transistor’s base.ĭarlington Transistor Pair and its Configuration: Darlington transistor is invented in 1953, by a US electrical engineer and inventor, Sidney Darlington.ĭarlington transistor uses two standard BJT (Bi-polar junction transistor) transistors which are connected together.

Darlington transistor symbol