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# {2} - BJT Transistor Modeling CHAPTER 5 Topic objectives At...

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BJT Transistor Modeling BJT Transistor Modeling CHAPTER 5 CHAPTER 5

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2 Topic objectives Topic objectives At the end of the course you will be able to At the end of the course you will be able to Understand about the small signal analysis of  Understand about the small signal analysis of  circuit network using r circuit network using r e e  model and hybrid   model and hybrid  equivalent model equivalent model Understand the relationship between those  Understand the relationship between those  two available model for small signal analysis two available model for small signal analysis
3 To begin analyze of small-signal AC response of BJT amplifier the knowledge of modeling the transistor is important. The input signal will determine whether it’s a small signal (AC) or large signal (DC) analysis. The goal when modeling small-signal behavior is to make of a transistor that work for small-signal enough to “keep things linear” (i.e.: not distort too much) [3] There are two models commonly used in the small signal analysis: a) r e model b) hybrid equivalent model INTRODUCTION: TRANSISTOR INTRODUCTION: TRANSISTOR MODELING MODELING

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4 How does the amplification be How does the amplification be done? done? Conservation; output power  Conservation; output power  of a system cannot be large  of a system cannot be large  than its input and the  than its input and the  efficiency cannot be greater  efficiency cannot be greater  than 1 than 1 The input dc plays the  The input dc plays the  important role for the  important role for the  amplification to contribute its  amplification to contribute its  level to the ac domain where  level to the ac domain where  the conversion will become  the conversion will become  as  as  η η =P =P o(ac) o(ac) /P /P i(dc) i(dc)
5 Disadvantages Disadvantages R R e e  model  model Fails to account the output impedance level of  Fails to account the output impedance level of  device and feedback effect from output to  device and feedback effect from output to  input input Hybrid equivalent model Hybrid equivalent model Limited to specified operating condition in  Limited to specified operating condition in  order to obtain accurate result order to obtain accurate result

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6 V S V CC C 1 C 2 C 3 + - V o R S V i + - R E R C R 1 R 2 V S + - V o R S V i + - R C R 1 R 2 I/p coupling capacitor s/c Large values Block DC and pass AC signal Bypass capacitor s/c Large values DC supply “0” potential Voltage-divider configuration under AC analysis Redraw the voltage-divider configuration after removing dc supply and insert s/c for the capacitors O/p coupling capacitor s/c Large values Block DC and pass AC signal
7 V S R S R2 R1

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{2} - BJT Transistor Modeling CHAPTER 5 Topic objectives At...

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