Darlington BJT

A Darlington bipolar junction transistor (BJT) contains two BJT devices in a special configuration named after its inventor, Sidney Darlington.  The two devices are connected in such a way that their gains are multiplied, creating a component with an effective gain much higher than could be achieved with a single device.   A Darlington BJT can be created with two discrete transistors, but is available as a single component with two BJTs in a package.  They are commonly used in circuits such as operational amplifiers where a very high gain is desired.