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Understanding BJT AC Amplifiers: Models and Configurations

Explore BJT AC response amplification and modeling with hybrid equivalent and re transistor models. Learn about common BJT configurations like CE, CB, voltage-divider bias, and more.

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Understanding BJT AC Amplifiers: Models and Configurations

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  1. Elektronica II Ch.5 BJT AC Analysis

  2. 5.1 Introduction • Ch.3 Transistor: basic construction, appearance, characteristics • Ch.4 Transistor: biasing • Ch.5 AC response BJT amplifier by reviewing the (small-signal) models re, hybrid π and hybrid equivalent

  3. 5.2 Amplification in the AC domain

  4. 5.3 BJT transistor modeling • hybrid equivalent model: specification sheets include parameters, defined for a specific set of operating conditions (Ic, Vce, f) • re model: important parameter determined by actual operating conditions • hybrid π model: high-frequency analysis

  5. 5.3 BJT transistor modeling (cont’d) AC equivalent model is obtained by: • Setting all dc sources to zero and replacing them by a short-circuit equivalent • Replacing all capacitors by short-circuit equivalent • Removing all elements bypassed by the short-circuit equivalents introduced by steps 1 and 2 • Redrawing the network in a more convenient and logical form Parameters: Vi, Ii, Zi, Vo, Io, Zo, Av, Ai

  6. 5.3 BJT transistor modeling (cont’d)

  7. 5.4 The re transistor model • Common-Base Configuration • Common-Emitter Configuration • Common-Collector Configuration (use model defined by CE configuration)

  8. 5.4 The re transistor model (cont’d)Common-Base Configuration

  9. 5.4 The re transistor model (cont’d)Common-Base Configuration • Typical values of Zi range from a few ohms to a maximum of about 50 Ω • Typical values of Zo are in the mega-ohm range • In general, the input impedance is relatively small and the output impedance quite high

  10. 5.4 The re transistor model (cont’d)CB-Amplifier

  11. 5.4 The re transistor model (cont’d)CB-Amplifier (example 5.11)

  12. 5.4 The re transistor model (cont’d)Common-Emitter Configuration

  13. 5.4 The re transistor model (cont’d)Common-Emitter Configuration • Typical values of Zi defined by βrerange from a few hundred ohms to the kilo-ohm range, with a maximum of about 6 kΩ to 7 kΩ • Typical values of Zo are in the range of 40 kΩ to 50 kΩ

  14. 5.4 The re transistor model (cont’d)CE-Amplifier: Fixed-bias

  15. 5.4 The re transistor model (cont’d)CE-Amplifier: Fixed-bias

  16. 5.4 The re transistor model (cont’d)CE-Amplifier: Fixed-bias

  17. 5.4 The re transistor model (cont’d) CE-Amplifier:Fixed-bias (example 5.4)

  18. 5.4 The re transistor model (cont’d)CE-Amplifier: Voltage-divider

  19. 5.4 The re transistor model (cont’d)CE-Amplifier: Voltage-divider

  20. 5.4 The re transistor model (cont’d) CE-Amplifier:Voltage-divider (Example 5.5)

  21. 5.4 The re transistor model (cont’d) CE-Amplifier:Voltage-divider (Example 5.8)

  22. 5.4 The re transistor model (cont’d) CE-Amplifier:Voltage-divider (Example 5.9)

  23. 5.4 The re transistor model (cont’d) Other configurations => 5.8 common-emitter fixed-bias: Zi, Zo, Av, phase relationship, example 5.4 => 5.9 voltage-divider bias: => Re bypassed, with ro : Zi, Zo, Av, phase relationship, example 5.5 en 5.9 => Re unbypassed, with ro : example 5.8 => 5.10 ce emmitter-bias: => Re unbypassed, without ro : Zi, Zo, Av, phase relationship => Re unbypassed, with ro: Zi, Zo, Av => bypassed => zie ce fixed-bias -> example 5.6 (unbypassed, with ro) en 5.7 (bypassed, with ro) => 5.11 emitter-follower: => without ro : Zi, Zo, Av, phase relationship => with ro : Zi, Zo, Av -> example 5.10: without en with ro => variaties: with voltage-divider biasing en with collector resistor Rc => 5.12 common-base configuration: Zi, Zo, Av, Ai: example 5.11 => 5.13 collector feedback: => without Re: => without ro: Zi, Zo, Av, phase relationship => with ro: Zi, Zo, Av -> example 5.12: without en with ro => with Re: exercise => 5.14 collector dc feedback: with ro: Zi, Zo, Av, phase relationship, example 5.13

  24. 5.15 Determining the current gain For each transistor configuration, the current gain can be determined directly from the voltage gain, the defined load and the input impedance

  25. 5.15 Determining the current gain (cont’d)

  26. 5.16 Effect of RL and RS Two approaches can be used: • By inserting the (re model) equivalent circuit and use methods of analysis to determine the quantities of interest • By defining a two-port equivalent model and use the parameters determined for the no-load situation

  27. 5.16 Effect of RL and RS (cont’d) 1st approach

  28. 5.16 Effect of RL and RS (cont’d)

  29. 5.16 Effect of RL and RS (cont’d)

  30. 5.16 Effect of RL and RS (cont’d) Fixed-bias

  31. 5.16 Effect of RL and RS (cont’d) Fixed-bias

  32. 5.16 Effect of RL and RS (cont’d) Fixed-bias: example 5.14 • Voltage divider • Emitter follower

  33. 5.17 2nd (Two-port systems) approach (effect of RL and RS)

  34. 5.17 Two-port systems approach (effect of RL and RS) (cont’d)

  35. 5.17 Two-port systems approach (effect of RL and RS) (cont’d)

  36. 5.17 Two-port systems approach (effect of RL and RS) (cont’d)

  37. 5.17 Two-port systems approach (effect of RL and RS) (cont’d) Example 5.15

  38. 5.18 Summary tables

  39. 5.18 Summary tables (cont’d)

  40. 5.19 Cascaded systems

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