2 TransistorA transistor is a semiconductor device used to amplify and switch electronic signals.A transistor is made up of three layers – an ‘n’ layer sandwiched between two ‘p’ layers or a ‘p’ layer between two ‘n’ layers. Doping of each layer is different and that is what is responsible for the operation(amplification).
3 The BJT – Bipolar Junction Transistor The Two Types of BJT Transistors:N P NnpEBCCross SectionSchematic SymbolP N PpnEBCCross SectionSchematic SymbolCollector is moderately dopedBase is lightly dopedEmitter is heavily dopedClick to view NPN TransistorClick to view PNP Transistor
5 DC and DC = Common-base current gain = IC = IC IB IE = Common-emitter current gain = Common-base current gain = IC = ICIB IEThe relationships between the two parameters are: = = Note: and are sometimes referred to as dc and dc because the relationships being dealt with in the BJT are DC.
6 Modes of Operation Active: Most important mode of operation Central to amplifier operationEmitter –Base junction Forward biased and Collector –base Reverse BiasedSaturation:Barrier potential of the junctions cancel each other out causing a virtual shortIdeal transistor behaves like an closed switchBoth junction are Forward biasedCutoff:Current reduced to zeroIdeal transistor behaves like an open switchBoth junction are Reverse biased
7 Three Types of BJT Configurations Biasing the transistor refers to applying voltage to get the transistor to achieve certain operating conditions.Common-Base Biasing (CB)input = VEB & IEoutput = VCB & ICCommon-Emitter Biasing (CE)input = VBE & IBoutput = VCE & ICCommon-Collector Biasing (CC)input = VBC & IBoutput = VEC & IE
8 BJT Transconductance Curve Typical NPN TransistorCollector Current:Transconductance:(slope of the curve)gm = IC / VBEIES = The reverse saturation current of the B-E Junction.VT = kT/q = 26 mVT=300K) = the emission coefficientand is usuallyIC8 mA6 mA4 mA2 mAVBE0.7 V
9 Common-Base Circuit Diagram: NPN Transistor The Table Below lists assumptions that can be made for the attributes of the common-base biased circuit in the different regions of operation. Given for a Silicon NPN transistor.Region of OperationICVCEVBEVCBC-B BiasE-B BiasActiveβIB=VBE+VCE0.7V0VRev.Fwd.SaturationMax-0.7V<VCE<0CutoffNone/Rev.
10 Common-Base input characteristics Input characteristics for the CB configuration gives relation between the input quantities, input voltage VEB and input current IE for fixed VCB valuesThe input circuit in CB configuration involves the emitter-base diode, which is forward biased in active region. Therefore, the relationship between VEB and IE is nothing but the forward characteristics of a diode
11 Common-Base input characteristics In the above characteristics, VCB = Open represents the characteristics of the forward biased emitterWith increase of VCB, the curves shift downwards i.e., we get the same IE with less VEB. This is because, from the early effect increases the IE increases with VEB held constant
12 Common-Base out put characteristics Although the Common-Base configuration is not the most common biasing type, it is often helpful in the understanding of how the BJT works.Emitter-Current Curves
13 Common-Emitter Circuit Diagram Collector-Current Curves VCE IC IC +_Active RegionVCCIBIBRegion of OperationDescriptionActiveSmall base current controls a large collector currentSaturationVCE(sat) V, VCE increases with ICCutoffAchieved by reducing IB to 0, Ideally, IC will also equal 0.VCESaturation RegionCutoff RegionIB = 0
14 Common Emitter Input Characteristics The input quantities for C.E. configuration are base current IB and base emitter voltage VBEThe input characteristics curves are in between IB and VBE for various values of collector to emitter voltage VCEIf VCE = 0 and if the base-emitter junction is forward biased, the input characteristics is the same as the characteristics of forward biased diodeIf VCE is increased then VCB increasesBy applying KVL around the transistor
15 Common Emitter Input Characteristics Increase in VCB leads to decrease in effective base width WB| due to early effect, resulting in decrease of recombination and consequently, decrease in base current due to recombination.
16 Common Emitter output Characteristics The output quantities in C.E. configuration are IC and VCE the o/p characteristics gives a relationship between IC and VCE with base current IB as a parameter.This family of curves may be divided into three regions those are active region, saturation region and cutoff region.
17 Common-CollectorIt is often called an emitter follower since its output is taken from the emitter resistor.Is useful as an impedance matching device since its input impedance is much higher than its output impedance.It is also termed a "buffer" for this reason and is used in digital circuits with basic gates.
18 Common-CollectorThe Common-collector biasing circuit is basically equivalent to the common-emitter biased circuit except instead of looking at IC as a function of VCE and IB we are looking at IE.Also, since 1, and = IC/IE that means IC IEEmitter-Current CurvesVCEIEActive RegionIBSaturation RegionCutoff RegionIB = 0
19 Common collector input Characteristics As VCB increases according to early effect base width decreases and IB decreases.
20 Common collector output Characteristics The common-collector circuit is basically same as the common-emitter, with the exception that the load resistor is in the emitter circuit, the output characteristics are similar to that of CE configuration. It is because
21 Transistor as amplifier Transistor amplifies current as well as voltage and is a current operated device.The CE configuration is widely used as it amplifies current and voltage unlike the other configurations.Click to view Image
22 COMPARISION of CB, CE AND CC PARAMETERS Input impedance(Ri)Output Impedance(Ro)D.C current gainVoltage gainVery LargeModerateApplicationsFor High FrequencyFor Audio frequencyFor impedance matchingPhase relationship between i/p and o/pIn- phaseOut-of phase