Darlington transistor

The transistor darlington is the combination of two Transistor S bipolar in the same way standard (both NPN or both PNP), resulting in a hybrid component which still has characteristics of transistor. These 2 transistors are often integrated in the same case. The profit while running of the darlington is equal to the product of the profits of each transistor. The assembly is the following: the collectors are common and, correspond to the collector of the darlington. The transmitter of the transistor of order is connected to the base of the transistor of exit. The base of the transistor of order and the transmitter of the transistor of exit correspond respectively to the base and the transmitter of the darlington.

Advantages

Large profit: Profit of the first transistor multiplied by the profit of the second (1000 to 20000).

Disadvantages

the threshold of conduction VBE from which the darlington starts to lead is doubled compared to a simple transistor, the current of order crosses the junction bases/transmitting first transistor then the junction bases/transmitting second, therefore the VBE of the darlington is the addition of the two VBE.
the drop voltage VCEsat of the darlington (typically 1,5V) is higher than that of a simple transistor bipolar (typically 0,5V), which increases appreciably the losses by Joule effect, in particular in the applications of power.

Remarks

not to confuse this assembly with the assembly Cascode.
There exists another combination associating a transistor NPN and a PNP which multiplies also the profit called Paire of Sziklai.
Darlington is supplanted more and more by the Field-effect transistor, including in electronics of power. This one profits a current from grid quasi no one (from where an in theory infinite profit while running!) as of a voltage drop VDSsat = RDSon * ID in general lower than that of Darlington.

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