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MALVINO

SIXTH EDITION. MALVINO. Electronic. PRINCIPLES. Transistor Biasing. Chapter 8. Voltage divider bias. {. +V CC. R C. R 1. R 1 and R 2 form a voltage divider. R 2. R E. +V CC. Divider analysis:. R 1. R 2. V BB =. V CC. +V BB. R 1 + R 2. R 2. ASSUMPTION: The base

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MALVINO

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  1. SIXTH EDITION MALVINO Electronic PRINCIPLES

  2. Transistor Biasing Chapter 8

  3. Voltage divider bias { +VCC RC R1 R1 and R2 form a voltage divider R2 RE

  4. +VCC Divider analysis: R1 R2 VBB = VCC +VBB R1 + R2 R2 ASSUMPTION: The base current is normally much smaller than the divider current.

  5. +VCC VBB - VBE RC IE = RE VBB RE Now the circuit can be viewed this way: To complete the analysis: IC @ IE VC = VCC - ICRC VCE = VC - VE

  6. 1. Calculate the base voltage using the voltage divider equation. 2. Subtract 0.7 V to get the emitter voltage. 3. Divide by emitter resistance to get the emitter current. 4. Determine the drop across the collector resistor. 5. Calculate the collector voltage by subtracting. 6. Calculate the collector-emitter voltage by subtracting. The six-step process

  7. RTH = R1 R2 Is the divider a stiff source? { +VCC RC R1 Find the Thevenin resistance. R2 RE

  8. A Thevenin model of the bias circuit: +VCC RC RTH VBB RE

  9. RTH RIN VBB The 100:1 rule applied to the bias circuit: +VCC RC RTH < 0.01 RIN When the rule is met, the divider is stiff. RE

  10. < 0.1 bdcRE R1 R2 R1 R2 IE = VBB - VBE bdc RE + Sometimes a firm divider is chosen. +VCC RC R1 A closer approximation: R2 RE

  11. VEE - 0.7 V IE = RE 2 V - 0.7 V IE = = 1.3 mA 1 kW Two-supply stiff emitter bias: 3.6 kW 10 V Assume 0 V 2.7 kW 1 kW 2 V

  12. Find the voltages: 3.6 kW 10 V VC = 10 V - (1.3 mA)(3.6 kW) = 5.32 V VCE = 5.32 V - (-0.7 V) = 6.02 V 2.7 kW 1 kW 2 V

  13. +VCC RC Base bias: RB • The least predictable • Q point moves with replacement • Q point moves with temperature • Not practical

  14. +VCC RC Emitter-feedback bias: RB • Better than base bias • Q point still moves • Not popular RE

  15. +VCC Collector-feedback bias: RC RB • Better than emitter-feedback bias • Q point still moves • Limited application

  16. +VCC Collector- and emitter -feedback bias: RC RB • Better than emitter-feedback bias • Not as good as voltage-divider bias • Limited application RE

  17. Two-supply emitter bias: • Very stable • Requires 2 supplies

  18. +VCC Voltage divider bias: RC R1 • Very stable • Requires 1 supply • The most popular R2 RE

  19. IC IC IB IB IE IE PNP Transistors Electron flow Conventional flow

  20. -VCC PNP Biasing with a negative supply RC R1 R2 RE

  21. +VEE PNP Biasing with a positive supply RE R2 R1 RC

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