Transistors and Transistor Biasing 1 Transistor - 羿ரான்殿ஸ்ட쏍 ❖ 3 terminal device – 2 back-to-back p-n junctions ❖ NPN Transistor - p-type sandwiched between two n-type semiconductors ❖ PNP Transistor - n-type sandwiched between two p-type semiconductors 2 Emitter(E) உமி폍ப்பான்- emits (supplies) charges - always forward biased Collector(C) ஏ쟍பான்- collects charges- always reverse biased Base(B) அ羿வாய் -Middle sections which forms 2PN junctions- forward biased 3 Doping and Size Emitter is heavily Collector (C) is Base is lightly doped doped (inject large moderately doped no. of electrons) • Base is thin • Emitter is moderate • Collector (C) is size wider than E and B ❖ Since the base is thin, most carriers from emitter injected into the collector 4 Transistor Symbol Conventional current (arrow) is opposite to electron flow 5 ❖ Emitter diode is always Forward Biased -믁ன்ன ா埍கு சார்ꯁ ❖ Collector diode is always Reversed Biased -பின்ன ா埍கு母 சார்ꯁ ❖ EB junction is Forward Biased (FB)- low resistance- குறைந்த மின்தறை ❖ CB junction is Reversed Biased (RB) - High resistance- அதிக மின்தறை ❖ Transistor transfers signal from low resistance to high resistance ❖ ‘Trans’ means transfers; ‘istor’ means family of resistors 6 Working of NPN Transistor EB - forward biased - 믁ன்ன ா埍கு சார்ꯁ –VEB CB - reversed biased - பின்ன ா埍கு母 சார்ꯁ–VCB VEB < VCB EB junction - EB சந்தி (heavily doped) - ejects more electrons Majority charge carriers பப쏁ம்பான்றம கைத்திகள் from emitter move towards the base - emitter current IE (100%) 7 Working of NPN Transistor (cont..) •The electrons enter into the base (lightly doped) •Combine with the few holes - constitutes the base current IB (5%) •Reversed bias potential of the collector is high •Attracts the electrons reaching collector (95%) •Emitter current is the sum of the collector or the base current IE=IB+IC 8 Transistor connections Diode - 2 terminal device 1 terminal –input உள்ள ீ翁 2 terminal- output பவளியீ翁 One battery –needed to give biasing Transistor -3 terminal device 1 terminal – input 2 terminal - output 3 terminal - common (பபா鏁) for both input and output Input applied between 1st terminal and common terminal Output is taken between 2nd terminal and common terminal Two batteries needed- one in input side; another in output side 9 Common base configuration - பபா鏁 அ羿வாய் அறமப்ꯁ Base --Common terminal ❖ E and B Forward Biased ❖ C is Reversed Biased 10 Common base configuration 11 Common Emitter configuration - பபா鏁 உமிழ்ப்பான் அறமப்ꯁ ❖ E and B - Forward Biased ❖ C is Reversed Biased 12 Common Emitter configuration 13 Common Collector configuration - பபா鏁 ஏற்பான் அறமப்ꯁ 14 Common Collector configuration 15 16 Common Base Configuration (CB) • Base terminal is common for both input and output of the transistor •Emitter –Base junction is forward biased •Collector –Base junction is reverse biased •VCB is kept constant •Input current = Emitter current IE •Output current = collector current IC 17 CB- Current Amplification Factor (மின்ன ா翍ைபப쏁埍ககாரணி) (α) Current amplification factor = Ratio of output current to the input current ✓The ratio of change in collector current (ΔIC) to the change in emitter current (ΔIE) when collector voltage VCB is kept constant, is called as Current amplification factor. ✓It is denoted by α (less than 1) ✓α=ΔIC / ΔIE at constant VCB 18 CB- ஏற்பான்மின்ன ா翍ைத்திற்கா னகாறவ Expression for Collector current in CB mode •Current at C = Part of emitter current + some amount of base current IB (which flows through the base terminal due to electron hole recombination) (மின்鏁கள்-鏁றள ம쟁னசர்埍றக). •The emitter current that reaches the collector terminal is αIE ( α=IC / IE ) •As collector-base junction is reverse biased, there is another current which flows is due to minority charge carriers (சி쟁பான்றம கைத்திகள்) This is the leakage current கசிퟁ மின்ன ா翍ைம் - Ileakage • This is due to minority charge carriers and hence very small •Total collector current ( )= IC=αIE+Ileakage ஏற்பான்மின்ன ா翍ைம் 19 Expression for Collector Current in CB mode If the emitter-base voltage VEB = 0, IB =0 there flows a small leakage current ICBO (collector to base current with emitter open) The collector current therefore can be expressed as IC=α IE+ICBO (IE=IC+IB) IC=α(IC+IB)+ICBO IC(1−α)=α IB+ICBO IC = IB+ ICBO IC=βIB+(β+1)ICBO --Equation for collector current The value of collector current depends on base current and leakage current along with the current amplification factor of that transistor in use. 20 CE- ஏற்பான்மின்ன ா翍ைத்திற்கா னகாறவ Expression for Collector current in CE mode Emitter – base --- forward biased Collector is reverse biased Input current =base current IB Output current = collector current IC 21 CE -Current Amplification factor (மின்ன ா翍ைபப쏁埍ககாரணி) (β) β= Output current/ Input current The ratio of change in collector current IC to the change in base current IB is known as base current amplification factor(β) β=ΔIC / ΔIB IB =5% of the emitter current β is greater than 20. β = 20 to 500 22 Relation between α and β (α , β 埍கா பதாைர்ꯁ) 23 Relation between α and β β= α/(1−α) β(1- α)=α β-αβ=α If β= 98 what is α? β=α+ αβ β=α(1+ β) β/(1+β)=α 24 PNP transistors CB CE CC 25 PNP transistors Common Base Common Emitter Common Collector 26 Expression for Collector Current in CE mode IE=IB+IC IC=αIE+ICBO IC=α(IB+IC)+ICBO IC(1−α)=αIB+ICBO IC = IB+ ICBO If the base-emitter voltage VBE = 0, base circuit is open, i.e. IB = 0, there flows a small leakage current, which can be termed as ICEO (collector to emitter current with base open) 27 Expression for Collector Current in CE mode CB- ஏற்பான் மின்ன ா翍ைத்திற்கா னகாறவ The collector emitter current with base open is ICEO ICEO=[1/(1−α)]ICBO Substituting the value of this in the previous equation, we get IC=[α/(1−α)]IB+[1/(1−α)]ICBO IC =[α/(1−α)]IB+ICEO Since β = IC=βIB+ICEO This is the equation for collector current 28 Transistor Characteristics in Common emitter (CE) mode பபா鏁 உமிழ்ப்பான் - 羿ரான்சிைர் சிைப்பியல் 29 Input Characteristic Curve - உள்ள ீ翁 சிைப்பியல் Graph between – VBE (X axis) and IB (y axis) VCE = constant VBE is varied and IB is measured Repeated for different constant VCE =2V, 6V, 10V Family of curves are drawn Curve is similar to a forward diode characteristics IB increases with the increases in VBE - Sharp increase Input resistance of the CE is comparatively higher that of CB 30 Input Characteristic Curve -உள்ள ீ翁 சிைப்பியல் Input Resistance(~100 ohms): Ratio of change in base-emitter voltage VBE to the change in base current ∆IB at constant collector-emitter voltage VCE , 31 Output Characteristic Curve - பவளியீ翁 சிைப்பியல் VCE (X axis) and IC (y axis) IB = constant; VCE is varied and IC is measured • Repeated for different constant IB = 20,30,40,50,60 μA •Upto Knee region : (0-1V); IC increases with VCE . This value of VCE up to which collector current IC changes with VCE is called the Knee Voltage •Above Knee region (transistors are operated in this region) • IC ~ constant ; for a particular VCE, IC ~ βIB (because β=IC / IB) •IC is independent of VCE ; depletion layer gets wider •Small increase in IC, because collector captures electrons before recombination in base area •Cut off Region: A small current IC (is not zero), equal to ICEO (due to minority carriers) flows The output resistance of CE is less than CB 32 Output Characteristic Curve - பவளியீ翁 சிைப்பியல் Output Resistance (~50k ohm): The ratio of change in collector-emitter voltage VCE to the collector current IC at a constant base current IB 33 Transfer Characteristics for CE Transistor CE 毁ற்ைில் பரிமாற்쟁 சிைப்பியல் •The variation of output current in accordance with the input current, keeping the output voltage constant. IC and IB increase almost linearly •The variation of IC with IB keeping VCE as a constant. β=ΔIC / ΔIB •Current Amplification Factor (β) is the ratio of change in the collector current (IC) to the change in base current (IB) when the collector-emitter voltage (VCE) is kept constant. 34 DC Load Line - ப쿁埍னகா翁 •To determine collector current Ic for various collector emitter voltage VcE •Can be determined from output characteristics •Convenient method- Load line method 35 Load Line - ப쿁埍னகா翁 •Maximum possible collector current (IC) is a point on the Y-axis - Saturation point (பதவி翍翁 ꯁள்ளி) (A) •The maximum possible collector emitter voltage VCE is a point on the X-axis- Cutoff point (பவ翍翁 ꯁள்ளி)(B) •A line is drawn joining these two points - Load line •This is called so as it symbolizes the output at the load. 36 •The load line is drawn by joining the saturation (பதவி翍翁 ꯁள்ளி) and cut off (பவ翍翁 ꯁள்ளி) points • The region that lies between these two is the linear region. A transistor acts as a good amplifier in this linear region •DC load line is drawn only when DC biasing is given to the transistor, but no input signal is applied, then such a load line is called as DC load line •No amplification as the signal is absent 37 . The value of collector emitter voltage VCE=VCC−ICRC (Y=mX) VCC and RC are fixed values First degree equation - a straight line on the output characteristics.