Lecture 4Voltage-Dependent Current SourceVoltage-Dependent Current Source with Input ResistanceExponential Voltage-Dependent Current SourceReverse-Biased PN Junction as a Current SourceBJT Structure and Circuit SymbolNPN BJT Operation (Qualitative)Base CurrentBJT DesignCarrier Transport in the Base RegionDiffusion Example ReduxCollector CurrentEmitter CurrentSummary of BJT CurrentsParallel Combination of TransistorsSimple BJT Amplifier ConfigurationBJT as a Constant Current SourceConstraint on Load ResistanceBJT I-V CharacteristicsExampleBJT Large Signal ModelBJT vs. Back-to-Back DiodesEE105 Fall 2007 Lecture 4, Slide 1 Prof. Liu, UC BerkeleyLecture 4OUTLINE•Bipolar Junction Transistor (BJT)–General considerations–Structure–Operation in active mode–Large-signal model and I-V characteristicsReading: Chapter 4.1-4.4.2EE105 Fall 2007 Lecture 4, Slide 2 Prof. Liu, UC BerkeleyVoltage-Dependent Current Source•A voltage-dependent current source can act as an amplifier.•If KRL is greater than 1, then the signal is amplified.LinoutVKRVVA EE105 Fall 2007 Lecture 4, Slide 3 Prof. Liu, UC BerkeleyVoltage-Dependent Current Source with Input Resistance•The magnitude of amplification is independent of the input resistance rin.EE105 Fall 2007 Lecture 4, Slide 4 Prof. Liu, UC BerkeleyExponential Voltage-Dependent Current Source•Ideally, a bipolar junction transistor (BJT) can be modeled as a three-terminal exponential voltage-dependent current source:EE105 Fall 2007 Lecture 4, Slide 5 Prof. Liu, UC BerkeleyReverse-Biased PN Junction as a Current Source•PN junction diode current is ~independent of the reverse-bias voltage. It depends only on the rate at which minority carriers are introduced into the depletion region. We can increase the reverse current by injecting minority carriers near to the depletion region.EE105 Fall 2007 Lecture 4, Slide 6 Prof. Liu, UC BerkeleyBJT Structure and Circuit Symbol•A bipolar junction transistor consists of 2 PN junctions that form a sandwich of three doped semiconductor regions. The outer two regions are doped the same type; the middle region is doped the opposite type.EE105 Fall 2007 Lecture 4, Slide 7 Prof. Liu, UC BerkeleyNPN BJT Operation (Qualitative)In the forward active mode of operation:•The collector junction is reverse biased.•The emitter junction is forward biased.BCIIcurrent gain:EE105 Fall 2007 Lecture 4, Slide 8 Prof. Liu, UC BerkeleyBase Current•The base current consists of two components: 1) Injection of holes into the emitter, and 2) Recombination of holes with electrons injected from the emitter. BCIIEE105 Fall 2007 Lecture 4, Slide 9 Prof. Liu, UC BerkeleyBJT Design•Important features of a well-designed BJT (large ):–Injected minority carriers do not recombine in the quasi-neutral base region.–Emitter current is comprised almost entirely of carriers injected into the base (rather than carriers injected into the emitter).EE105 Fall 2007 Lecture 4, Slide 10 Prof. Liu, UC BerkeleyCarrier Transport in the Base Region•Since the width of the quasi-neutral base region (WB = x2-x1) is much smaller than the minority-carrier diffusion length, very few of the carriers injected (from the emitter) into the base recombine before they reach the collector-junction depletion region. Minority-carrier diffusion current is ~constant in the quasi-neutral base •The minority-carrier concentration at the edges of the collector-junction depletion region are ~0.EE105 Fall 2007 Lecture 4, Slide 11 Prof. Liu, UC BerkeleyDiffusion Example Redux•Non-linear concentration profile varying diffusion currentLNqDdxdpqDJppdiffp ,ddppdiffpLxLNqDdxdpqDJexp ,•Linear concentration profile constant diffusion currentdLxNp expLxNp 1EE105 Fall 2007 Lecture 4, Slide 12 Prof. Liu, UC BerkeleyCollector Current•The equation above shows that the BJT is indeed a voltage-dependent current source; thus it can be used as an amplifier. BBinESTBESCTBEBBinECWNnqDAIVVIIVVWNnqDAI22 whereexp1expEE105 Fall 2007 Lecture 4, Slide 13 Prof. Liu, UC BerkeleyEmitter Current•Applying Kirchhoff’s Current Law to the BJT, we can easily find the emitter current. 11CBCEIIIIEE105 Fall 2007 Lecture 4, Slide 14 Prof. Liu, UC BerkeleySummary of BJT Currents1exp1exp1expTBESETBESBTBESCVVIIVVIIVVIIEE105 Fall 2007 Lecture 4, Slide 15 Prof. Liu, UC BerkeleyParallel Combination of Transistors•When two transistors are connected in parallel and have the same terminal voltages, they can be considered as a single transistor with twice the emitter area.EE105 Fall 2007 Lecture 4, Slide 16 Prof. Liu, UC BerkeleySimple BJT Amplifier Configuration•Although the BJT converts an input voltage signal to an output current signal, an (amplified) output voltage signal can be obtained by connecting a “load” resistor (with resistance RL) at the output and allowing the controlled current to pass through it.EE105 Fall 2007 Lecture 4, Slide 17 Prof. Liu, UC BerkeleyBJT as a Constant Current Source•Ideally, the collector current does not depend on the collector-to-emitter voltage. This property allows the BJT to behave as a constant current source when its base-to-emitter voltage is fixed.EE105 Fall 2007 Lecture 4, Slide 18 Prof. Liu, UC BerkeleyConstraint on Load Resistance•If RL is too large, then VX can drop to below ~0.8V so that the collector junction is forward biased. In this case, the BJT is no longer operating in the active mode, and so There exists a maximum tolerable load resistance.BCIIEE105 Fall 2007 Lecture 4, Slide 19 Prof. Liu, UC BerkeleyBJT I-V CharacteristicsEE105 Fall 2007 Lecture 4, Slide 20 Prof. Liu, UC BerkeleyExampleEE105 Fall 2007 Lecture 4, Slide 21 Prof. Liu, UC BerkeleyBJT Large Signal Model•A diode is placed between the base and emitter terminals, and a voltage-controlled current source is placed between the collector and emitter terminals.EE105 Fall 2007 Lecture 4, Slide 22 Prof. Liu, UC BerkeleyBJT vs. Back-to-Back Diodes•Figure (b) presents a wrong way of modeling the
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