The common-base amplifier

Chapter 4.7 The common-base amplifier

Lessons In Electric Circuits Volume III – Semiconductors Book
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Lessons In Electric Circuits Volume III – Semiconductors Book

  • 212CHAPTER 4. BIPOLAR JUNCTION TRANSISTORSRloadVinVoutVout = Vin - 1.4+-+-0.7 V+-0.7 VFigure 4.51:Darlington pair based common-collector amplifier loses two VBE diode drops.sidering any power supplies.• The common-collector amplifier is also known as an emitter-follower.• The output voltage on a common-collector amplifier will be in phase with the input volt-age, making the common-collector a non-invertingamplifier circuit.• The current gain of a common-collector amplifier is equal to β plus 1. The voltage gain isapproximately equal to 1 (in practice, just a little bit less).• A Darlington pairis a pair of transistors “piggybacked” on one another so that the emitterof one feeds current to the base of the other in common-collector form. The result is anoverall current gain equal to the product (multiplication) of their individual common-collector current gains (β plus 1).4.7The common-base amplifierThe final transistor amplifier configuration (Figure 222,4.52) we need to study is the common-base.This configuration is more complex than the other two, and is less common due to its strangeoperating characteristics.It is called the common-baseconfiguration because (DC power source aside), the signalsource and the load share the base of the transistor as a common connection point shown inFigure 222,4.53.Perhaps the most striking characteristic of this configuration is that the input signal sourcemust carry the full emitter current of the transistor, as indicated by the heavy arrows in thefirst illustration. As we know, the emitter current is greater than any other current in thetransistor, being the sum of base and collector currents. In the last two amplifier configura-tions, the signal source was connected to the base lead of the transistor, thus handling the leastcurrent possible.