Two bipolar transistors in either the Darlington or Sziklai connection can be used as a single, high gain, transistor. These circuits have AC current gains on the order of , providing high amplification in a single stage.
The arrangement of two NPN transistors shown in figure 5.20a is known as the Darlington connection. The three unconnected terminals, the base of and the collector and emitter of , behave like a single transistor with current gain and an overall base to emitter DC voltage drop of . Two transistors in the Darlington connection can be purchased in a single three-terminal package.
A similar circuit result can be obtained with the Sziklai connection of figure 5.20b. Because it uses one transistor of each polarity (NPN and PNP), this connection is also known as the complementary Darlington. The combination again results in a three-terminal device that behaves like a single high-current-gain transistor. The overall circuit behaves like a transistor of the same polarity as .
Figure 5.20: a) The Darlington connection of two transistors to obtain higher current gain and b) the Sziklai connection of an NPN and PNP transistor to obtain the equivalent of a high-current-gain NPN.