outlineIntroductionBipolar Junction Trnasistor (BJT)Construction of BJTOperation of TransistorsBJT configurations ● Common- base configuration ● Common-emmiter configuration ● Common-collector configuration
2.1 IntroductionThe invention of transistors (in 1947)marked the beginning of revolution inelectronics.The pace of the revolution wasaccelerated a decade later by thedevelopment of the integrated circuit(IC or silicon chip).Transistors are broadly classified intotwo types: Bipolar Junction Transistor(BJT) and Field Effect Transistor (FET).
2.2 Bipolar Junction Transistor (BJT)Construction: The transistor is a three-layer semiconductordevice consisting of either two n- and one p-type layers ofmaterial or two p- and one n-type layers of material. Theformer is called an npn transistor, while the latter is called apnp transistor.The term bipolar reflects the fact that holes and electronsparticipate in the injection process into the oppositelypolarized material. If only one carrier is employed (electron orhole), it is considered a unipolar device.
Construction of BJTIn the previous diagrams, the terminalshave been indicated by the capitalletters E for emitter, C for collector,and B for base.
2.2.2 Transistor OperationConsider the pnp trnasistor of previousfigure (Fig. (a) ) to describe the basicoperation of a BJT transistor.The operation of the npn transistor isexactly the same if the roles played bythe electron and hole areinterchanged.We investigate the two junctions ofpnp transistor (separately).
Forward-biased junction of apnp transistor (without base-to- collector bias). • The depletion region has been reduced in width due to the applied bias, resulting in a heavy flow of majority carriers from the p- to the n- type material. • It is similar to forward biased diode.
Reverse-biased junction of a pnp transistor (without base-to-emitter bias).• Flow of majority carriers is zero, resulting in only a minority-carrier flow.• Similar to reverse biased diode. The number of uncovered negative ions in the depletion region of the P- type material will increase due to the large number of “free” holes drawn to the negative potential of the applied voltage. The effect is a widening of the depletion region.
Operation of BJTIn general: ● One p-n junction of a transistor is reverse biased, while the other is forward biased. Both biasing potentials are applied (normal operation), which results in Majority and minority carrier flow of a pnp transistor, as follows.
Operation of BJTThe widths of the depletion regions, indicating clearly whichjunction is forward-biased and which is reverse-biased.A large number of majority carriers will diffuse across theforward-biased p-n junction into the n-type material.Since the sandwiched n-type material is very thin and has alow conductivity, a very small number of these carriers willtake this path of high resistance to the base terminal.
Operation of BJTThe larger number of these majority carriers will diffuse acrossthe reverse-biased junction into the p-type material connectedto the collector terminal.Applying Kirchhoff’s current law to the transistor of previousFig. : IE = IC + IBThe collector current, however, is comprised of twocomponents—the majority and minority carriers. The minoritycurrent component is called the leakage current and is giventhe symbol ICO (IC current with emitter terminal Open). Thecollector current is then given by IC = ICmajority + ICOminority
2.2.3 BJT ConfigurationsA) Common base configurationIn common base configuration, the base is common to both the input and output sides of the configuration.The arrow in the graphic symbol defines the direction of emitter current (conventional flow) through the device.
Common base configurationTo fully describe the behavior of a three-terminal device suchas the common base amplifiers of above figure, it requirestwo sets of characteristics—one for the driving point or inputparameters and the other for the output side. The input set for the common-base amplifier, as shown in Fig.below, relates an input current (IE) to an input voltage (VBE)for various levels of output voltage (VCB).
Common base configurationThe output set relates an outputcurrent (IC) to an output voltage (VCB)for various levels of input current (IE)as shown in following figure.
Common base configuration Click to edit Master text styles Second level ● Third level ● Fourth level ● Fifth level
Common base configurationThe output or collector set of characteristics has three basicregions of interest, as indicated in the previous figure - theactive, cutoff, and saturation regions.The active region is the region normally employed for linear(undistorted) amplifiers. In particular : In the active region thecollector-base junction is reverse-biased, while the base-emitter junction is forward-biased.
Common base configuration The curves clearly indicate that a firstapproximation to the relationshipbetween IE and IC in the active regionis given by IC ≈ IE .As inferred by its name, the cutoffregion is defined as that region wherethe collector current is 0 A. Inaddition, in cutoff region the collector-base and base-emitter junctions of atransistor are both reverse-biased.
Common base configurationThe ac alpha is formally called the common-base,short-circuit, amplification factor
B) Common emitter configurationIt is called the common-emitterconfiguration since the emitter iscommon or reference to both the inputand output terminals (in this casecommon to both the base and collectorterminals).Notation and symbols used with thecommon-emitter configuration (forboth npn and pnp transistors) areshown in the following figure.
Common emitter configurationEven though the transistor configuration has changed, thecurrent relations developed earlier for the common-baseconfiguration are still applicable . That is, IE = IC + IB and IC= ᾳIE.For the common-emitter configuration the outputcharacteristics are a plot of the output current (IC) versusoutput voltage (VCE) for a range of values of input current(IB). The input characteristics are a plot of the input current(IB) versus the input voltage (VBE) for a range of values ofoutput voltage (VCE). Both plots are shown in the followingfigure.
Common emitter configurationThe curves of IB are not as horizontal as those obtained for IEin the common-base configuration, indicating that thecollector-to-emitter voltage will influence the magnitude of thecollector current.Note on the collector characteristics of the previous Fig. thatIC is not equal to zero when IB is zero. For the common-baseconfiguration, when the input current IE was equal to zero, thecollector current was equal only to the reverse saturationcurrent ICO, so that the curve IE = 0 and the voltage axiswere, for all practical purposes, one.The reason for this difference in collector characteristics canbe derived as follows:
Common emitter configurationIf we consider the case IB = 0 A, andsubstitute a typical value of ᾳ such as0.996, the resulting collector current isthe following: