Presentation on theme: "Transistors and transistor circuits"— Presentation transcript:
1 Transistors and transistor circuits Lecture (9, 10)Transistorsand transistor circuits
2 IntroductionTransistors are three terminal devices that replaced vacuum tubes.They are solid state devices that are used for…AmplificationSwitchingDetecting LightThe three terminals are the…Emitter, Base, Collector (BJT)Source, Gate, Drain (FET)
3 The BJTBipolar Junction Transistors are made with n-type and p-type semiconductors.There are two types: npn and pnp.Circuit SymbolsTransistor Analogy
4 The three layers of the sandwich are conventionally called the Collector, Base, and Emitter. A Bipolar Transistor essentially consists of a pair of PN Junction Diodes that are joined back-to-back.This forms a sort of a sandwich where one kind of semiconductor is placed in between two others.There are therefore two kinds of Bipolar sandwich, the NPN and PNP varieties.
5 The BJT – Bipolar Junction Transistor The Two Types of BJT Transistors:npnpnpnpnpnpECECCCCross SectionCross SectionBBBBSchematic SymbolSchematic SymbolEECollector doping is usually ~ 106Base doping is slightly higher ~ 107 – 108Emitter doping is much higher ~ 1015
6 BJT Relationships - Equations IEICIEIC-VCE++VEC-ECEC--++VBEVBCIBVEBVCBIB++--BBnpnIE = IB + ICVCE = -VBC + VBEpnpIE = IB + ICVEC = VEB - VCBNote: The equations seen above are for the transistor, not the circuit.
7 Some of the basic properties exhibited by a Bipolar Transistor are immediately recognizable as being diode-like. However, when the 'filling' of the sandwich is fairly thin some interesting effects become possible that allow us to use the Transistor as an amplifier or a switch. To see how the Bipolar Transistor works we can concentrate on the NPN variety.
11 What are the primary uses of transistors? Switching (Logic Gates)Amplification (Op-Amps)
12 Architecture of BJTsThe bipolar junction transistor (BJT) is constructed with three doped semiconductor regions separated by two pn junctionsRegions are called emitter, base and collector
13 Architecture of BJTs There are two types of BJTs, the npn and pnp The two junctions are termed the base-emitter junction and the base-collector junctionThe term bipolar refers to the use of both holes and electrons as charge carriers in the transistor structureIn order for the transistor to operate properly, the two junctions must have the correct dc bias voltagesthe base-emitter (BE) junction is forward biased(>=0.7V for Si, >=0.3V for Ge)the base-collector (BC) junction is reverse biased
16 Operation of BJTs BJT will operates in one of following four region Cutoff region (for digital circuit)Saturation region (for digital circuit)Linear (active) region (to be an amplifier)Breakdown region (always be a disaster)
18 DC Analysis of BJTs Transistor Currents: IE = IC + IB alpha (DC) IC = DCIEbeta (DC)IC = DCIBDC typically has a value between 20 and 200
19 Transistor Equations Alpha Factor Current Gain Kirchhoff’s Current Rule
20 DC Analysis of BJTs DC voltages for the biased transistor: Collector voltageVC = VCC - ICRCBase voltageVB = VE + VBEfor silicon transistors, VBE = 0.7 Vfor germanium transistors, VBE = 0.3 V
21 DC and DC = Common-emitter current gain = Common-base current gainThe 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.
22 BJT Example Using Common-Base NPN Circuit Configuration C Given: IB = 50 A , IC = 1 mAFind: IE , , and Solution:IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA= IC / IB = 1 mA / 0.05 mA = 20 = IC / IE = 1 mA / 1.05 mA = could also be calculated using the value of with the formula from the previous slide.ICVCB+_IBBVBEIE+_E
23 BJT Transconductance Curve Typical NPN Transistor 1 Collector Current:IC = IES eVBE/VTTransconductance:(slope of the curve)gm = IC / VBEIES = The reverse saturation currentof the B-E Junction.VT = kT/q = 26 mV T=300K) = the emission coefficient and isusually ~1IC8 mA6 mA4 mA2 mAVBE0.7 V
24 Modes of Operation Active: Saturation: Cutoff: Most important mode of operationCentral to amplifier operationThe region where current curves are practically flatSaturation:Barrier potential of the junctions cancel each other out causing a virtual shortCurrent reduced to zeroIdeal transistor behaves like an open switchCutoff:* Note: There is also a mode of operation called inverse active, but it is rarely used.
25 Three Types of BJT Biasing 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
26 Common-BaseAlthough 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 CurvesICActive RegionIESaturation RegionCutoffIE = 0VCB
27 Orange = Actual IC (IC’) The Early Effect (Early Voltage)ICNote: Common-Emitter ConfigurationIB-VAVCEGreen = Ideal ICOrange = Actual IC (IC’)
28 Early Effect ExampleGiven: The common-emitter circuit below with IB = 25A, VCC = 15V, = 100 and VA = 80.Find: a) The ideal collector currentb) The actual collector currentCircuit Diagrama)ICVCE+_VCCIBb)
29 Breakdown Voltage The maximum voltage that the BJT can withstand. BVCEO = The breakdown voltage for a common-emitter biased circuit. This breakdown voltage usually ranges from ~ Volts.BVCBO = The breakdown voltage for a common-base biased circuit. This breakdown voltage is usually much higher than BVCEO and has a minimum value of ~60 Volts.Breakdown Voltage is Determined By:The Base WidthMaterial Being UsedDoping LevelsBiasing Voltage
30 BJT as an amplifierClass A AmplifiersClass B Amplifiers
31 BJT Class A AmplifiersIn a class A amplifier, the transistor conducts for the full cycle of the input signal (360°)used in low-power applicationsThe transistor is operated in the active region, between saturation and cutoffsaturation is when both junctions are forward biasedthe transistor is in cutoff when IB = 0The load line is drawn on the collector curves between saturation and cutoff
33 BJT Class A Amplifiers Three biasing mode for class A amplifiers common-emitter (CE) amplifiercommon-collector (CC) amplifiercommon-base (CB) amplifier
34 BJT Class A Amplifiers A common-emitter (CE) amplifier capacitors are used for coupling ac without disturbing dc levels
35 BJT Class A Amplifiers A common-collector (CC) amplifier The 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 voltage gain ~ 1, but current gain is greater than 1 and = IC/IE that means IC~IEEmitter-Current Curves
36 BJT Class A Amplifiers The third configuration is the common-base (CB) the base is the grounded (common) terminalthe input signal is applied to the emitteroutput signal is taken off the collectoroutput is in-phase with the inputvoltage gain is greater than 1current gain is always less than 1
37 BJT Class B AmplifiersWhen an amplifier is biased such that it operates in the linear region for 180° of the input cycle and is in cutoff for 180°, it is a class B amplifierA class B amplifier is more efficient than a class AIn order to get a linear reproduction of the input waveform, the class B amplifier is configured in a push-pull arrangementThe transistors in a class B amplifier must be biased above cutoff to eliminate crossover distortion
39 The BJT as a SwitchWhen used as an electronic switch, a transistor normally is operated alternately in cutoff and saturationA transistor is in cutoff when the base-emitter junction is not forward-biased. VCE is approximately equal to VCCWhen the base-emitter junction is forward-biased and there is enough base current to produce a maximum collector current, the transistor is saturated