EEE118: Lecture 11 Review Review EEE118: Electronic Devices and Circuits Introduced the idea of a dynamic resistance or small signal Lecture XI resistance. Compared the voltage source model and th´ evenin model of a James E Green diode. Department of Electronic Engineering Considered how capacitors can be used to block quiescent University of Sheffield conditions (DC) but pass signals (AC). j.e.green@sheffield.ac.uk Introduced the idea of a small signal equivalent circuit - How the signal “sees” the circuit. Introduced the bipolar transistor. Briefly discussed two numbering systems for active devices. 2/ 22 1/ 22 EEE118: Lecture 11 EEE118: Lecture 11 Review BJT Modes of Operation Outline BJT Modes of Operation 1 Review There are four possible modes of operation where each of the two 2 BJT Modes of Operation junctions is either forward or reverse biased. 3 Characteristics V CB Input Characteristics Output Characteristics Reverse Saturation Transfer, Mutual or Transconductance ( g m ) Characteristics Active (Switch) 4 Large Signal Model of a BJT (in backwards...) “On” State 5 ZTX653 Large Signal Parameters β ≈ 1 6 Large Signal Model of a MOSFET V BE 7 IFR510 Characteristics 8 Switches Forward 9 Switch Types Active Mechanical Switches Off (Amplifier) Electro-Mechanical Switches 10 Review β ≈ 25 − 1000 11 Bear 3/ 22 4/ 22 EEE118: Lecture 11 EEE118: Lecture 11 BJT Modes of Operation Characteristics Input Characteristics BJT Modes of Operation II Input Characteristics Shape of characteristics The forward active region provides amplification of voltage essentially governed by the diode and/or current (both means power amplification ( P = IV )). equation when V CE ≈ 0. I B In the saturation region the transistor appears like a switch � � � � q V BE I B = I S exp − 1 k T + which is turned on. V CE V BE Increasing V CE with constant − + In the ‘off’ region the transistor appears like a switch which is V BE V BE decreases the base width, − turned off. slightly decreasing the The reverse active region is used when the BE and CB recombination of minority carriers junctions are accidentally exchanged (transistor in the circuit 10 1 V in the base region. Note, the 34 V backwards). Performance is poor c.f forward active region as 8 67 V base current is incidental, it is Increasing V CE 100 V I B [mA] transistor designers adjust doping densities and region widths 6 the base emitter voltage that is to optimise performance in other regions. 4 controlling the transistor. But Note: some transistors are designed for amplification (linear) use others 2 like the compressor from Lecture are designed for switching use. All transistors can perform both functions 0 9, V BE will rise to whatever is 550 600 650 700 750 800 850 900 but the design of “switching” transistors is optimised for switching V BE [mV] necessary to admit the desired applications. Likewise for “amplifier transistors”. current I B 5/ 22 6/ 22
EEE118: Lecture 11 EEE118: Lecture 11 Characteristics Characteristics Output Characteristics Transfer, Mutual or Transconductance ( g m ) Characteristics Output Characteristics Transfer Characteristics The transfer characteristic relates A family of curves showing effect the controlling voltage ( V BE ) to on the output V CE and I C as a I C I C the controlled parameter I C . V BE function of the input V BE (or + + is related to I C for a BJT by V CE V CE I B ). When V CE is small the − − � � � � + q V BE + V BE I C = I S exp − 1 and V BE transistor is in saturation both k T − − by square law expressions for BE and CB junctions forward FETs (see handouts). This biased (transistor switched “on”) 140 Increasing V BE expression holds over many 160 (left of graph). While V BE is too 575 mV 120 1 V 625 mV 140 34 V Increasing V CE orders of magnitude while the 100 650 mV small to cause I C to rise above 120 67 V 662.5 mV I C [mA] I C [mA] 100 V 80 relationship between base current 100 675 mV the leakage current level, the 686.5 mV 80 60 and collector current changes 693 mV transistor is off ( y ≈ 0 on the 60 700 mV 40 considerably ( h FE not constant). 40 graph). Forward active region is 20 20 See Horowitz and Hill, second 0 indicated by nearly parallel 0 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 1 . 4 575 600 625 650 675 700 Ed. pp 79 - 81 section 2.10 for characteristics. V CE [V] V BE [V] full details. 7/ 22 8/ 22 EEE118: Lecture 11 EEE118: Lecture 11 Large Signal Model of a BJT Large Signal Model of a BJT Large signal BJT model V BE is the controlling variable and I C is the controlled Large signals deal with active devices moving through large variable and I B is incidental (MOSFETs have no equivalent, non-linear regions of their characteristics. A suitable large signal I G ). However, in the large signal model V BE is fixed at 0.7 V. model for the forward active region of a BJT is to replace the base How can it be the controlling variable if it is fixed? emitter junction with a 0.7 V source (it is a diode after all...) and In switching applications we want the transistor to change to replace the reverse biased collector base junction with a current from conducting to non-conducting (and back) as quickly as source where the current is controlled by the base current (the two possible. The load (whatever the collector is connected to) are linked by the large signal current gain h FE 1 ) I C = h FE I B . 2 defines the maximum value of I C . B C The designer ensures that the input to the transistor I B (whatever the base is connected to) is able to supply whatever I C = h FE I B + 0.7 V base current ( I B ) is required to cause the transistor to switch. I C ≈ β F I B − This makes the exact value of V BE and I B less important. In this way the dependence of the circuit operation on h FE is E lessened. This is desirable because h FE varies a great deal 1 note capital F and capital E means large signal parameters even between transistors of the same type. 2 h FE h ybrid model F orward, E mitter common. Also “Ebers-Moll transistor model”, Millman and Grabel second ed. pp. 87 - 114. 9/ 22 10/ 22 EEE118: Lecture 11 EEE118: Lecture 11 Large Signal Model of a BJT ZTX653 Large Signal Parameters Large signal BJT model: Saturation (i.e. Switched “On”) Finding a Value for h FE , V CE ( sat ) , V BE ( sat ) A large signal model for the saturation mode of the BJT is two voltage sources representing the saturation voltage between the The following parameters of the Large Signal Model may be found base and emitter and the collector and emitter. These are given on on a transistor datasheet. switching transistor datasheets (e.g. ZTX653 note not Pro E or h FE - the large signal (capital subscript letters) forward active JEDEC) or the exam question... mode relationship between I C and I B . C B V CE ( sat ) the saturation voltage between collector and emitter. I B I C + + V BE ( sat ) the saturation voltage between base and emitter V BE ( sat ) V CE ( sat ) − − All of these parameters are functions of I C . For the purposes of this course h FE = 100 may be assumed if no value is given. The E dependence of V CE ( sat ) and V BE ( sat ) have also be ignored, but in real design situations (project work etc.) these dependencies In saturation I C = h FE I B does not apply . A model for the should not be forgotten. transistor when it is off is open circuit between B, C and E. In practice very small leakage currents flow, which are defined on the transistor datasheet. 11/ 22 12/ 22
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