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Transcript

Lecture 10 Bipolar Junction Transistor (BJT) BJT 1-1 Outline Continue BJT Continue DC analysis • More examples Introduction to AC signal analysis BJT 1-2 Example (1) A bipolar transistor having IS = 5×10-16 A is biased in the forward active region with VBE =750 mV. If the current gain (or β) varies from 50 to 200 due to manufacturing variations, calculate the minimum and maximum terminal currents of the device. BJT 1-3 Solution For a given VBE, the collector current remains independent of β The base current varies from IC/200 to IC/50: the emitter current experiences only a small variation because (β+ 1)/β is near unity for large β: BJT 1-4 Example (2) Determine the dc level of IB and VC for the BJT circuit BJT 1-5 Solution For the dc mode, the capacitor assumes the open-circuit equivalence and RB =R1+R2 BJT 1-6 Example (3) Determine the dc bias voltage VCE and the current IC for the voltage-divider configuration shown in the figure. (use exact and approximate methods) BJT 1-7 Solution Using Exact Analysis Method BJT 1-8 Solution (cont’d) Return to the example: BJT 1-9 Solution (cont’d) BJT 1-10 Solution (cont’d) Using Approximate Analysis Method The condition that will define whether the approximate approach can be applied BJT 1-11 Solution (cont’d) Return to the example: Testing: BJT 1-12 Solution (cont’d) Now, compare between obtained results by the exact and approximate methods BJT 1-13 Example (4) Determine the voltage VCB and the current IB for the common-base configuration as shown in figure BJT 1-14 Solution Applying Kirchhoff’s voltage law to the input circuit yields Applying Kirchhoff’s voltage law to the output circuit gives BJT 1-15 Example (5) Given the device characteristics as shown in figure, determine VCC, RB, and RC for the fixed bias configuration BJT 1-16 Solution From the load line BJT 1-17 BJT Transistor Modeling A model is an equivalent circuit that represents the AC characteristics of the transistor A model uses circuit elements that approximate the behavior of the transistor There are two models commonly used in small signal AC analysis of a transistor: re model Hybrid equivalent model BJT 1-18 BJT AC Analysis BJT 1-19 The re Transistor Model BJTs are basically current-controlled devices; therefore the re model uses a diode and a current source to duplicate the behavior of the transistor Recall (from lecture 5): the ac resistance of a diode can be determined by the equation r = 26 mV/ID, where ID is the dc current through the diode at the Q (quiescent) point ac One disadvantage to this model is its sensitivity to the DC level. This model is designed for specific circuit conditions BJT 1-20 Common-Base Configuration I c I e re 26 mV Ie Input impedance: Zi re Output impedance: Zo Voltage gain: AV Forwardbiased junction I e R L R L I e re re Current gain: RL Ai 1 BJT 1-21 Example For a common-base configuration, as shown in figure, with IE = 4 mA, α = 0.98, and an ac signal of 2 mV applied between the base and emitter terminals: (a) Determine the input impedance. (b) Calculate the voltage gain if a load of 0.56 kΩ is connected to the output terminals. (c) Find the output impedance and current gain. BJT 1-22 Solution BJT 1-23 Lecture Summary Covered material Continue BJT DC analysis • More examples Introduction to AC signal analysis Material to be covered next lecture Continue BJT analysis with AC signal BJT 1-24