5.3 The Common Collector Amplifier

Chapter 5.3 The Common Collector Amplifier

Physics Lecture Notes – Phys 395 Electronics Book
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Physics Lecture Notes – Phys 395 Electronics Book

  • 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.