CHAPTER 5. TRANSISTOR CIRCUITS1025.10.1Coupling Between Single Transistor StagesQuite often the single-transistor ampliﬁer discussed in the previous lectures does not provideenough gain for an application, or more often, it does not combine gain with the desired inputand output impedance characteristics. Perhaps the most obvious solution (but generally notthe best) is to connect several single-transistor ampliﬁers, or stages, in tandem one afterthe other. Because of DC biasing considerations, it is usually not practical to connect theoutput of one stage directly to the input of another; some kind of coupling device must beused that permits a change in the DC level between two stages.A possible circuit features capacitor coupling between single-transistor stages. The inter-stage coupling capacitor allows the following transistor to be DC biased without concern forthe DC component of the driving signal. Thus the DC analysis of the two transistor stagesis separated into two distinct problems.5.10.2Darlington and Sziklai ConnectionsTwo 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 h2fe, providinghigh ampliﬁcation in a single stage.The arrangement of two NPN transistors shown in ﬁgure 5.20a is known as the Darlingtonconnection. The three unconnected terminals, the base of Q1 and the collector and emitterof Q2, behave like a single transistor with current gain hfe1hfe2 and an overall base to emitterDC voltage drop of 2VPN . Two transistors in the Darlington connection can be purchasedin 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 alsoknown as the complementary Darlington. The combination again results in a three-terminaldevice that behaves like a single high-current-gain transistor. The overall circuit behaveslike a transistor of the same polarity as Q1.