TRANSISTOR - INTRODUCTIONBIPOLAR JUNCTION TRANSISTOR (BJT)

INTRODUCTIONBeside diodes, the most popular semiconductor devices is transistors. Eg: Bipolar Junction Transistor (BJT)Transistors are more complex and can be used in many waysMost important Application: As an Amplifier and as a switch Amplification can make weak signal strong (make low sound signal louder), in general, it provides a function called Gain

TRANSISTOR STRUCTURE BJT is bipolar because both holes (+) and electrons (-) will take part in the current flow through the deviceN-type regions contains free electrons (negative carriers)P-type regions contains free holes (positive carriers)Two types of BJTNPN transistorPNP transistorThe transistor regions are:Emitter (E) send the carriers into the base region and then on to the collectorBase (B) acts as control region. It can allow none,some or many carriers to flow Collector (C) collects the carriers

PPPNN NPNP AND NPN TRANSISTOR STRUCTUREIB(A)IB(A)Ic(mA)IE(mA)IE(mA)IC(mA)Arrow shows the current flows

NPNNPN Transistor Structure

TRANSISTOR CONFIGURATIONTransistor configuration is a connection of transistor to get variety operation.3 types of configuration:Common Collector.Common Base.Common Emitter

COMMON-COLLECTOR CONFIGURATIONThe input signal is applied to the base terminal and the output is taken from the emitter terminal. Collector terminal is common to the input and output of the circuitInput Terminals BCOutput Terminals EC

COMMON-BASE CONFIGURATIONBase terminal is a common point for input and output.Input Terminals EBOutput Terminals CB Not widely used as an amplifier because the relation between input current gain (IE) and output current gain (IC) is approximately 1, Means no amplification of current is there.

COMMON-EMITTER CONFIGURATIONEmitter terminal is common for input and output circuitInput BEOutput CEMostly applied in practical amplifier circuits, since it provides good voltage, current and power gain.In this IC=IBBeta is around in the range of 50-400 So due to high Beta , smallar value of IBWill result in larger value of IC .

NPNNPN Transistor BiasCBENo current flows.

NPNNPN Transistor BiasCBECurrent flows.

When both junctionsare biased....NPNNPN Transistor BiasCBENote that IB is smallerthan IE or IC.ICIBIE

NPNCBEAlthough IB is smallerit controls IE and IC.ICIBIENote: when the switch opens, all currents go to zero.Gain is something smallcontrolling something large(IB is small).

NPCBEIC = 99 mAIB = 1 mAIE = 100 mAb = ICIBThe current gain frombase to collector is called b.99 mA1 mA= 99

NPCBEIC = 99 mAIB = 1 mAIE = 100 mAIE = IB + IC99 mA= 1 mA + = 100 mAKirchhoffscurrent law:

CBEIC = 99 mAIB = 1 mAIE = 100 mAIn a PNP transistor,holes flow fromemitter to collector.Notice the PNPbias voltages.

Memory aid: NPNmeans Not Pointing iN.NPN Schematic Symbol

PNP Schematic Symbol

RECALL: NPN AND PNP BIASFundamental operation of pnp transistor and npn transistor is similar except for: role of electron and hole, voltage bias polarity, and Current direction

I-V CHARACTERISTICS FOR CE CONFIGURATION : INPUT CHARACTERISTICSInput characteristic: input current (IB) against input voltage (VBE) for several output voltage (VCE)From the graph IB = 0 A VBE < 0.7V (Si)IB = value VBE > 0.7V (Si)The transistor turned on whenVBE = 0.7V

I-V CHARACTERISTICS FOR CE CONFIGURATION : OUTPUT CHARACTERISTICSOutput characteristic: output current (IC) against output voltage (VCE) for several input current (IB)3 operating regions:Saturation regionCut-off regionActive region

I-V CHARACTERISTICS FOR CE CONFIGURATION : OUTPUT CHARACTERISTICSSaturation region in which both junctions are forward-biased and IC increase linearly with VCE

Cut-off region where both junctions are reverse-biased, the IB is very small, and essentially no IC flows, IC is essentially zero with increasing VCE

Active region in which the transistor can act as a linear amplifier, where the BE junction is forward-biased and BC junction is reverse-biased. IC increases drastically although only small changes of IB. Saturation and cut-off regions areas where the transistor can operate as a switch

Active region area where transistor operates as an amplifier

CURRENT RELATIONSHIPSRelations between IC and IE : = IC IEValue of usually 0.9998 to 0.9999, 1Relations between IC and IB : = IC @ IC = IB IBValue of usually in range of 50 400The equation, IE =IC + IB can also written in IC = IBIE = IB + IB => IE = ( + 1)IBThe current gain factor , and is: = @ = . + 1 - 1