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Transistor Circuit DC Bias Part 1

Transistor Circuit DC Bias Part 1. ENGI 242. DC Biasing Circuits. Fixed-Bias Circuit Emitter-Stabilized Bias Circuit Collector-Emitter Loop Voltage Divider Bias Circuit DC Bias with Voltage Feedback Miscellaneous Bias Circuits. Maximum Power Curve. Fixed-Bias Circuit.

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Transistor Circuit DC Bias Part 1

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  1. Transistor Circuit DC BiasPart 1 ENGI 242

  2. DC Biasing Circuits • Fixed-Bias Circuit • Emitter-Stabilized Bias Circuit • Collector-Emitter Loop • Voltage Divider Bias Circuit • DC Bias with Voltage Feedback • Miscellaneous Bias Circuits ENGI 242

  3. Maximum Power Curve ENGI 242

  4. Fixed-Bias Circuit ENGI 242

  5. DC Equivalent circuit ENGI 242

  6. Base-Emitter (Input) Loop Using Kirchoff’s voltage law: – VCC + IBRB + VBE = 0 Solving for IB: ENGI 242

  7. Collector-Emitter (Output) Loop Since: IC =  IB Using Kirchoff’s voltage law: VCE – VCC – IC RC Because: VCE = VC - VE Since VE = 0V, then: VC = VCE Also: VBE = VB - VE with VE = 0V, then: VB = VBE ENGI 242

  8. BJT Saturation Regions When the transistor is operating in the Saturation Region, the transistor is conducting at maximum collector current (based on the resistances in the output circuit, not the spec sheet value) such that: ENGI 242

  9. Determining Icsat ENGI 242

  10. Determining Icsat for the fixed-bias configuration ENGI 242

  11. Load Line Analysis ENGI 242

  12. Load Line Analysis • The end points of the line are : ICsat and VCEcutoff • For load line analysis, use VCE = 0 for ICSAT, and IC = 0 for VCEcutoff • ICsat: • VCEcutoff: • Where IB intersects with the load line we have the Q point • Q-point is the particular operating point: • Value of RB • Sets the value of IB • Where IB and Load Line intersect • Sets the values of VCE and IC. ENGI 242

  13. Circuit values effect Q-point ENGI 242

  14. Circuit values effect Q-point (continued) ENGI 242

  15. Circuit values effect Q-point (continued) ENGI 242

  16. DC Fixed Bias Circuit Example ENGI 242

  17. Load-line analysis ENGI 242

  18. Fixed-bias load line ENGI 242

  19. Movement of Q-point with increasing levels of IB ENGI 242

  20. Effect of RC on the load line and Q-point ENGI 242

  21. Effect of VCC on the load line and Q-point ENGI 242

  22. Example ENGI 242

  23. Emitter Stabilized Bias

  24. Emitter-Stabilized Bias Circuit Adding a resistor to the emitter circuit (between the emitter lead and ground) stabilizes the bias circuit ENGI 242

  25. Improved Bias Stability • The addition of RE to the Emitter improves the stability of a transistor • Stability refers to a bias circuit in which the currents and voltages will remain fairly constant for a wide range of temperatures and transistor forward current gain () • The temperature surrounding the transistor circuit is not always constant • Therefore, the transistor  is not a constant value ENGI 242

  26. Base-Emitter Loop ENGI 242

  27. Equivalent Network ENGI 242

  28. Reflected Input impedance of RE ENGI 242

  29. Base-Emitter Loop Applying Kirchoffs voltage law: - VCC + IB RB + VBE +IE RE = 0 Since: IE = ( + 1) IB We can write: - VCC + IB RB + VBE + ( + 1) IBRE = 0 Grouping terms and solving for IB: Or we could solve for IE with: ENGI 242

  30. Collector-Emitter Loop ENGI 242

  31. Collector-Emitter Loop Applying Kirchoff’s voltage law: - VCC + IC RC + VCE + IE RE = 0 Assuming that IE IC and solving for VCE: IC =VCC – VCE – (RE + RC) Solve for VE: VE = IE RE Solve for VC: VC = VCC - IC RC or VC = VCE + IE RE Solve for VB: VB = VCC - IB RB or VB = VBE + IE RE ENGI 242

  32. Transistor Saturation At saturation, VCE is at a minimum We will find the value VCEsat = 0.2V For load line analysis, we use VCE = 0 To solve for ICSAT, use the output KVL equation: ENGI 242

  33. Load Line Analysis The load line end points can be calculated: At cutoff: At saturation: ENGI 242

  34. Emitter Stabilized Bias Circuit Example ENGI 242

  35. Load Line For The Emitter-bias Configuration. ENGI 242

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