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Lecture 10 Bipolar Junction Transistor (BJT)
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Outline Continue BJT Continue DC analysis
More examples Introduction to AC signal analysis BJT
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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
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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
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Example (2) Determine the dc level of IB and VC for the BJT circuit
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Solution For the dc mode, the capacitor assumes
the open-circuit equivalence and RB =R1+R2 BJT
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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
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Solution Using Exact Analysis Method BJT
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Solution (cont’d) Return to the example: BJT
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Solution (cont’d) BJT
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Solution (cont’d) Using Approximate Analysis Method
The condition that will define whether the approximate approach can be applied BJT
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Solution (cont’d) Return to the example: Testing: BJT
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Solution (cont’d) Now, compare between obtained results by the exact and approximate methods BJT
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Example (4) Determine the voltage VCB and the current IB for the common-base configuration as shown in figure BJT
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Solution Applying Kirchhoff’s voltage law to the input circuit yields
Applying Kirchhoff’s voltage law to the output circuit gives BJT
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Example (5) Given the device characteristics as shown in figure, determine VCC, RB, and RC for the fixed bias configuration BJT
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Solution From the load line BJT
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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
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BJT AC Analysis BJT
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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 rac = 26 mV/ID, where ID is the dc current through the diode at the Q (quiescent) point One disadvantage to this model is its sensitivity to the DC level. This model is designed for specific circuit conditions BJT
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Common-Base Configuration
Input impedance: Output impedance: Voltage gain: Current gain: Forward-biased junction RL BJT
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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
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Solution BJT
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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
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