5.1.2 Basic Circuit Configurations

Chapter 5.1.2 Basic Circuit Configurations

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

  • CHAPTER 5. TRANSISTOR CIRCUITS83NPNVCBVEBICIBIC >IBCEB++CBEVCE >VBEVCBVCEVBEFigure 5.3: a) NPN transistor biased for operation and b) voltage levels developed withinthe biased semiconductor.the collector and constitute almost all the collector current IC. IC is slightly less than IEand we may write α = IC/IE, where from above α =0.95 to 0.99.The behaviour of a transistor can be summarized by the characteristic curves shown infigure 5.4. Each curve starts from zero in a nonlinear fashion, rises smoothly, then rounds aknee to enter a region of essentially constant IC. This flat region corresponds to the conditionwhere the depletion region at the base-emitter junction has essentially disappeared. To beuseful as a linear amplifier, the transistor must be operated exclusively in the flat region,where the collector current is determined by the base current.A small current flow into the base controls a much larger current flow into the collector.We can writeIC = βIB = hFEIB,(5.1)where β is the DC current gain and hFE is called the static forward-current transfer ratio.From the previous definition of α and the conservation of charge, IE = IC + IB,wehaveβ =α1− α.(5.2)For α =0.99 we have β = 99 and the transistor is a current amplifying device.5.1.2Basic Circuit ConfigurationsIn any transistor circuit design you must supply a DC bias current and voltage to operate inthe linear region of the characteristic curve. The DC operating point is defined by the valuesof IB,IC,VBE and VCE. You must also obtain the proper AC operation.The transistor is a three-terminal device that we will use to form a four-terminal circuit.Small voltage changes in the base-emitter junction will produce large current changes in