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Unit 12 Transistors and Amplifiers UNIT 12 TRANSISTORS AND AMPLIFIERS Structure 12.1 Introduction Ol>jectives 12.2 Bipolar Junction Transistor (BJT) 12.2.1 Transislor Actroo 12.2.2 V-I(:haracteristics and Biasing 12.2.3 Transistor as a (lo~itrollzdSwitch 12.2.4 Transislor as an Amplifier 12.3 Field Effect Transisfor (FET) 12.3.1 Trnnsistor Action 12.3.2 V-lCh;lractenslics~sti:und Biasing 12.3.3 lians~storas a Co~itrolledSwitcli 12.3.4 Transistor as an Amplifier 12.4 Differential An~plifier 12.4.1 Bipolar Versioa ' 12.4.2 FET Version 12.5 CMOS Inverter 12.6 Sunlmary 12,1 INTRODUCTION In Unit 11, you were introduced to the characteristics and applicatiol~sof diodcs. I~ithe present Unit, you will learn about transistors. their classification, properties 'and applications. Diodes and transistors are the most basic of discrete electronic devlces and are used in many eleclronic circuils. Even though rnodem electronics largely employs integrated circuits (IC's), the latter in fact arc nothing but assemblies of a large number of tr,msistors and other circuit elcnlents, all fabricated in a single se~niconductorchip. The transistor derives its nanlc as a sl~ortform of fransfer resistor, which describes its action of transferring a signal current from a low resistaice input circuit to a high resistance output circuit in a particular configuration. Unlike a diode which is a two-terminal device, transistors are threc terminal devices with one tenninal being the illput tenrunal, a second one being the output tcrrninal imd tl~ethird one being thc common terminal for the input as well as the output. The input pair of terminals may have an input voltage or current as tile co~~trollingquantity 'and the output pair has its voltage or current controlled by Ule input variable. (Recall the discussion on controlled or dependent sourccs in Unit 1). If the input voltage controls he output current the parameter associated with his linear relationship is tcrn~edthe forwclrd transfer conductm~e,g,. lllere arc two types of transistors popularly available, one known as Bipolur Junction Transistor(BJT) and the other known as Field EBct Tri~nsistor(FET).The relationship between input voltage and output current in the case of BJT is exponential. The relationship of input voltage and output current in the case of FET is a square law. ale devices if operated in the proper regions can act as controlled switcl~esor amplifiers. Both these applications will be highlighted in this Unit. You will also learn about diffelential amplifiers and inverters, Objectives After a study of this unit, you should be able to * describe the input/output characteristics of the two types of transistors viz, BJTs and FETs, describe how to bias these transistors to make them work in the regions of interest, explain their use as controlled switel~esand amplifiers, identify these transistor amplifiers as controlled sources, ana describe the working of a differential anlplitier and a. CMOS iliverlt*- Analog Eledda 12.2 BIPOLAR JUNCTION TRANSISTOR (BJT) A bipolar transistor is a device with two p-n junctions as shown for example in Figure 12.l(a). This is one of the possible types of BJT known as pnp transistor. The other type is npn transistor. The symbol for apnp transistor is shown in Figure 12.l(b). Figure 12.1(c) is the symbol for an npn transistor. F~gure12.1 (4) . 1 he pnp tr~uslstor Figure 12.1 (b) : Syn~bolfor n pnp transistor C E F~gure12.13 (c) : Symbol for an npn transistor Consider the pnp transistor of Figure 12.l(a). One of the p regions Is heavily doped (p+). That region is called the emitter (E ). The other p region is called the collector (C ). The n region sandwiched between these two regions is known as base (B ). This region is made very thin. Wllen the EB junction (emitter-base junction) is forward biased, a large number of holes which are the majority carriers in the emitter region get injected into the base region and a small number of eleclro~lsfrom n region get injected into the emitter region constituting the total emitter current, I,. In order to get good transistor action, the major portion of I, should be made up of hole current. This is achieved by making the emitter region more heavily doped UMI the base region. This is said to increase the emitter efficiency. After the holes get injected into the base, the holes tend to recombine with electrons in the base. This loss of holes due to recombination c'm be reduced by inaking lhc base very thin. The ratio of the holes arriving at the BC junction to those injected is termed the base transport.fator. It must he kept close to one. The BC junction is reverse biased so that the field in the BC dcplction region easily collects the holes arriving at the junction. Thus, almost the entire cvrrent at the cnlitter is available at the collector as collector currect, Ic. Ic = a I,, where a is very close to 1. This is what is ternled as bipolar transistor action. Restating the same, when the emitter base junction of a BJT is forward biased and the collector base junction is reverse biased the collector current is very nearly the same as the emitter current. The is valid for VBC 2 0 i.e., as long as CB junction is reverse biased for a pnp transistor. Since IE = IB + Ic, IB = IE - Ic = IE(1-a) 30 i Bul Transistors and Amplifiers Ic = a Ip Therefore, I The action in an tipn transistor is similar, except that now all vollage polarities and current directions are reversed. Example 12.1 (a) Lct a = 0.99 for a BJT. Delermine P. Solution This indicates a typical value of P. Example 12.l(b) Eor the same transistor if Ihe emilter currenl is 1 mA,determine the base mid collector currents. Solution fE= 1 mA. Therefore Ic = 0.99 11iA and 12.2.2 V -I Characteristics and Biasing Consider the ttyn transistor of Figurc I 2.1 (c). From Uie diode equation in Unit 11, dcnoting I,therein as I,, in tlie present context 'and taking q = I, = I,, exp (V,, / VT) as EB junction is nonnally forward biased. Ic= al, = alEo[exp (VBE / VT)] (12.4) as long as BC j&ction remains reverse biased. Therefore, the colleclor current is expo~ientiallyrelated to Uie BE ti~rwardbills voltage. As it is necessary for the tr'msistor action to lake place that the EB junction is forward hiascd arid the collector base junclion is reverse biased, tllc transislor should renlain in this region throughoul its opralion. This region is hiown as 'Active region' for the uruisistor. If both the junctions arc forward biased, the Lmnsistor acts as a short or it is said to he in 'Saturation'. When botli the junctions are revcrsed biased the U;~11sistoris said to hc open or in lhc 'Cut off' region. 12.2.3 Transistor as a Controlled Switch Figure 12.2(a) shows an nyn transistor with emitter terminal as a comnon terminal between input and output, Base is taken as the input ternunal .and colleclor is taken as Ule output terminal. The reverse bias volt;~geis applied to the output Uirough Rc co~uiectedin scries with a bias supply voltage, Vcc. vi = vB,, iE = -Lrn[exp (vBE/ VT)] - vo = VCE, iC = ai, (12.6) Therefore, vo = Vcc - icRc -- In electronic circuit diagrams. the d.c. power supplies are often omitted for the sake of clarity. lo Figure 12.2(a), even if the dotted portion is omitted, it shoitld be taken that a voltage source of Vcc volts is connected between the terminal marked Vcc and the ground. 4lso all node voltages wherever spcified are to be -taken as referenced to ground. Figure 12.2 (a) : l'ransistor ~~iverterlswitch vo = Vcc - a Rc [exp (vi VT)I Figure 12.2(b) shows the output versus input characteristic of the transistor switcwinverter of Figure 12.2(a). l vcc Vr Vi Figurc 12.2 (b) : Inverter characteristic Region I vo = Vcc because ic - 0. Trarlsistor has not yet started conducting. Only when vi(vBE ) reaches V, called cut-in voltage (= 0.6 V for silicon), does a substantial amount of current start flowing through Rc. Till that point the Wansislor is 08. Region I1 : '4~~= V, 2 V, ,BE junction is sufficiently forward biased, BCjunction is reverse biased. Therefore transistor action lakes place and the tr,msistor is said to he in the aclive region. Eq.(12.8) depicts the output versus input relationship or transfer function. Transistor amplitiers are operated in the active region. Regi.01; I11 : As vi increases vo keeps decreasing. A point is reached when vo = vcE = vBE making vcB = 0 After this point the CB junctic~starts getting forward biased. Transistor action stops. Both junctions are forward biased and the transistor is in the salumtion region. Example 12.2 F~orat] inverter circuit with Vcc -- 5V and Rc = 1 k Q, sketch vo vs vi assuming that a silicon transistor is being used. Assume VT= 26 mV, a = 0.99 and a typical value . ofIEO.Say 10nA. Solution I Figure 12.3 : For solution to Example 12.2 Example 12.3 For the transistor inverter shown in Figure 12.4(a) sketch vo vs vi. For input voltage levels Vil = 5 V and Vi:! = 0 V and VT= 26 mV,determine the output voltages for a typical value of IEo if a for the transistor is 0.99.
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