NPN Transistor Darlington Configuration
In the Darlington configuration, this emitter follower has a pair of transistors. The emitter current of one transistor becomes the base current of the second in this arrangement. The Darlington configuration acts like one transistor with a beta which is the product of the betas of the two transistors.
Where high output currents are needed, they can be used. The Darlington configuration has a quite high input impedance. The resistor is commonly tied between the emitters to increase the speed switching because switching of the second transistor may be slow. Darlington pairs are available as single packages with resistor included.