Lecture 2 - Lecture 2 Introduction to electronic analog...

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Lecture 2: Introduction to electronic analog circuits 361-1-3661 1 Our main aim in the two next lectures is to build all the possible practical circuits (amplifiers) by using a BJT transistor and a resistor. (We use the resistor to translate the output current of the circuit into voltage; otherwise the circuit will not be able to provide a voltage gain.) We then analyze and compare the circuits' small-signals gains to understand for what applications they can be suitable. We are particularly interested in the applications where there is a need to amplify power and dc signals. In this lecture, we develop all the models for the transistors ± as we did this for the diode ± and then will build and analyze ± with the help of these small-signal models ± all the possible single-transistor amplifiers. 2.1. BJT transistor: symbol, physical structure, analytical model, and graphical characteristics The symbols of the npn and pnp BJT transistors and the physical structure of the npn transistor are given in Fig. 1. We will analyze in the lectures only npn transistors. The only difference between the npn and pnp transistors is in their static states: the static state of the pnp transistors is reverse to that of the npn ones because of their opposite structures. There will be no difference in the small-signal behavior and models. The circuits analyzed in home exercises, the lab, and the exam will comprise both npn and pnp transistors. In analog circuits, the operating point of transistors is usually defined in active (linear) region, where the emitter junction is forward biased and the collector junction is reverse biased. Thus, the emitter injects the electrons into the base, and the collector collects them. The amount of the injected electrons is controlled by the emitter-base voltage, v BE (or base-to-emitter current, i B ). The collector collects almost all the electrons from the base if its potential is sufficiently high: is greater or equal to that of the base. The base is very thin and the electrons prefer entering the collector ± even its potential equals that of the base ± and not the base, because the resistance that they see looking into the base is much greater than that they see looking into the collector. To define the operating point of the transistor in active region, we ground the emitter and bias the transistor junctions with a current and voltage source as shown in Fig. 1. A single transistor circuit (with no other components, except independent sources) with grounded emitter is called the common-emitter configuration. Although we develop all the models of the transistor for the common-emitter E T E B V V Bo e n / ' B C p n p+ n++ C p Eo p Co i C E B C E n p p n n W i B v CE i B i R Injection Extraction V' CE > V CE i E n Bo B W' V CE = V BE V CE > V BE + 180 mV I' B = 0.5 I B ; I' C = 0.5 I C r B B' e 180/26 = 1015.44 * * * V CE = 0 Fig. 1. Symbol of the n-p-n and p-n-p BJT transistors and the physical structure of the npn
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This note was uploaded on 01/14/2012 for the course EE 361-1-3711 taught by Professor Prof.eugenepaperno during the Fall '11 term at Ben-Gurion University.

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Lecture 2 - Lecture 2 Introduction to electronic analog...

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