CHAPTER 5. TRANSISTOR CIRCUITS945.3The Common Collector AmpliﬁerThe common collector ampliﬁer is also called the emitter follower ampliﬁer because theoutput voltage signal at the emitter is approximately equal to the voltage signal input onthe base. The ampliﬁer’s voltage gain is always less than unity, but it has a large currentgain and is normally used to match a high-impedance source to a low-impedance load: theampliﬁer has a large input impedance and a small output impedance.A typical common collector ampliﬁer is shown in ﬁgure 5.12.REvEVcciBvBFigure 5.12: Basic common collector ampliﬁer.The voltage gain can be written asvEvB=hfeREhie + hfeRE.(5.48)The gain is thus in phase and slightly less than unity. The output impedance of the CCampliﬁer can be substantially less than the output impedance of the driving signal source.5.4The Common Base AmpliﬁerThe common base ampliﬁer is also known as the grounded base ampliﬁer. This ampliﬁer canproduce a voltage gain but generates no current gain between the input and the output sig-nals. It is normally characterized by a very small input impedance and an output impedancelike the common emitter ampliﬁer. Because the input and output currents are of similar size,the stray capacitance of the transistor is of less signiﬁcance than for the common emitterampliﬁer. The common base ampliﬁer is often used at high frequencies where it providesmore voltage ampliﬁcation than the other one-transistor circuits.A common base circuit is shown in ﬁgure 5.13. Above the corner frequency the capacitorbetween base and ground on the circuit provides an eﬀective AC ground at the transistor’sbase.The voltage gain is given byvCvE=hfeRChie,(5.49)which is the same as the CE ampliﬁer except for the lack of voltage inversion.