5 Transistor Circuits

Chapter 5 Transistor Circuits

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

  • Chapter 5Transistor CircuitsThe circuits we have encountered so far are passive and dissipate power. Even a transformerthat is capable of giving a voltage gain to a circuit is not an active element. Active elementsin a circuit increase the power by controlling or modulating the flow of energy or power froman additional power supply into the circuit.Transistors are active circuit elements and are typically made from silicon or germaniumand come in two types. The bipolar transistor controls the current by varying the numberof charge carriers. The field effect transistor (FET) varies the current by varying the shapeof the conducting volume.Before starting we will define some notation.The voltages that are with respect toground are indicated by a single subscript. Voltages with repeated letters are power supplyvoltages. And voltages between two terminals are indicated by a double subscript.5.1Bipolar Junction TransistorsBy placing two PN junctions together we can create a bipolar transistor. In a PNP transistorthe majority charge carriers are holes and germanium is favoured for these devices. Siliconis best for NPN transistors where the majority charge carriers are electrons.The thin and lightly doped central region is known as the base (B) and has majoritycharge carriers of opposite polarity to those in the surrounding material. The two outerregions are known as the emitter (E) and the collector (C). Under the proper operatingconditions the emitter will emit or inject majority charge carriers into the base region, andbecause the base is very thin, most will ultimately reach the collector. The emitter is highlydoped to reduce resistance. The collector is lightly doped to reduce the junction capacitanceof the collector-base junction.The schematic circuit symbols for bipolar transistors are shown in figure 5.1. The arrowson the schematic symbols indicate the direction of both IB and IC. The collector is usuallyat a higher voltage than the emitter. The emitter-base junction is forward biased while thecollector-base junction is reversed biased.81