Semiconductor Circuit Building

A Darling pair is made up of two bipolar transistors, either two NPN transistors or two PNP transistors which are connected in such a way so that it boosts a weak signal. 

The first transistor the circuit encounters increases the voltage, which is then given to the second transistor which increases the current. 

Combining these two together create a much stronger overall current compared to a single transistor by itself, with the final output coming from the collector of the second transistor.