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Lecture no 2 to 5 THE BASIC BJT AMPLIFIER CONFIGURATIONS

Lecture no 2 to 5 THE BASIC BJT AMPLIFIER CONFIGURATIONS. Prepared by Engr:Sarfaraz Khan Turk Lecturer at IBT LUMHS Jamshoro. Basic BJT Amplifiers Circuits. Single-Stage BJT Amplifiers. Key Words : Common-Emitter Amplifier Graphical Analysis Small-Signal Models Analysis

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Lecture no 2 to 5 THE BASIC BJT AMPLIFIER CONFIGURATIONS

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  1. Lecture no 2 to 5THE BASIC BJT AMPLIFIER CONFIGURATIONS Prepared by Engr:Sarfaraz Khan Turk Lecturer at IBT LUMHS Jamshoro

  2. Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers Key Words: Common-Emitter Amplifier Graphical Analysis Small-Signal Models Analysis Common-Collector Amplifier Common-Base Amplifier

  3. Basic BJT Amplifiers Circuits DC + small signal Single-Stage BJT Amplifiers C-E Amplifiers To operate as an amplifier, the BJT must be biased to operate in active mode and then superimpose a small voltage signal vbeto the base. coupling capacitor (only passes ac signals)

  4. Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers C-E Amplifiers

  5. Basic BJT Amplifiers Circuits vBE=vi+VBE Single-Stage BJT Amplifiers C-E Amplifiers Apply a small signal input voltage and see ib

  6. Basic BJT Amplifiers Circuits • vi = 0 IB、IC、VCE vCE=vce+VCE iC=ic+IC Single-Stage BJT Amplifiers C-E Amplifiers See how ibtranslates into vce. • vo out of phase with vi

  7. BJT Amplifiers Circuits Considering (all the capacitors are replaced by open circuits) Considering (all the capacitors are replaced by short circuits) Single-Stage BJT Amplifiers C-E Amplifiers

  8. Considering (all the capacitors are replaced by open circuits) Considering (all the capacitors are replaced by short circuits) Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers C-E Amplifiers

  9. Basic BJT Amplifiers Circuits VCC Single-Stage BJT Amplifiers Graphical Analysis • Can be useful to understand the operation of BJT • circuits. • • First, establish DC conditions by finding IB(or VBE) • • Second, figure out the DC operating point for IC Can get a feel for whether the BJT will stay in active region of operation – What happens if RCis larger or smaller?

  10. Basic BJT Amplifiers Circuits VCC Single-Stage BJT Amplifiers Graphical Analysis

  11. Basic BJT Amplifiers Circuits VCC Single-Stage BJT Amplifiers Graphical Analysis Q-point is centered on theac load line:

  12. Basic BJT Amplifiers Circuits VCC Single-Stage BJT Amplifiers Graphical Analysis Q-point closer to cutoff: Clipped at cutoff (cutoffdistortion)

  13. Basic BJT Amplifiers Circuits VCC Single-Stage BJT Amplifiers Graphical Analysis Q-point closer to saturation: Clipped at cutoff (saturationdistortion)

  14. Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers Graphical Analysis

  15. Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers Small-Signal Models Analysis Steps for using small-signal models 1. Determine the DC operating point of the BJT － in particular, the collector current 2. Calculate small-signal model parameters: rbe 3. Eliminate DC sources – replace voltage sources with short circuits and current sources with open circuits 4. Replace BJT with equivalent small-signal models 5. Analysis

  16. Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers Small-Signal Models Analysis Example 1 IC≈ βIB, IE= IC+ IB= (1+β)IB

  17. Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers Small-Signal Models Analysis Example 2

  18. Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers Small-Signal Models Analysis There are three basic configurations for single-stage BJT amplifiers: – Common-Emitter – Common-Base – Common-Collector

  19. Basic BJT Amplifiers Circuits Note : is slightly less than due to the voltage drop introduced by Single-Stage BJT Amplifiers Common-Collector Amplifier

  20. Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers Common-Collector Amplifier The last basic configuration is to tie the collector to a fixed voltage, drive an input signal into the base and observe the output at the emitter.

  21. Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers Common-Collector Amplifier Let’s find Av， Ai：

  22. << Rb >>1 Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers Common-Collector Amplifier Let’s find Av， Ai：

  23. Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers Common-Collector Amplifier Let’s find Ri：

  24. Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers Common-Collector Amplifier Let’s find Ro：

  25. Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers Common-Collector Amplifier

  26. Basic BJT Amplifiers Circuits >>1 Single-Stage BJT Amplifiers Common-Collector Amplifier • C-C amp characteristics: • Voltage gain is less than unity, but close (to unity) since β is large and rbeis small. • Also called an emitter follower since the emitter follows the input signal. • Input resistance is higher, output resistance is lower. • - Used for connecting a source with a large Rsto a load with low • resistance.

  27. Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers Common-Base Amplifier Ground the base and drive the input signal into the emitter

  28. Basic BJT Amplifiers Circuits Ri= For RL<<RC, Single-Stage BJT Amplifiers Common-Base Amplifier Ro≈RC

  29. Basic BJT Amplifiers Circuits ForRL<<RC, Ri= Single-Stage BJT Amplifiers Common-Base Amplifier Ro≈RC • CB amp characteristics: • current gain has little dependence on β • is non-inverting • most commonly used as a unity-gain current amplifier or current buffer and not • as a voltage amplifier: accepts an input signal current with low input resistance • and delivers a nearly equal current with high output impedance • most significant advantage is its excellent frequency response

  30. Summary for three types of diodes: Basic BJT Amplifiers Circuits Zout > Zin Zout < Zin Zout > Zin Vout > Vin Vout ≈ Vin Vout > Vin

  31. Basic BJT Amplifiers Circuits Frequency Response Key Words: Basic Concepts High-Frequency BJT Model Frequency Response of the CE Amplifier

  32. Basic BJT Amplifiers Circuits 1.0V 0.5V 0V -0.5V -1.0V 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0ms V(1) V(2) Time Frequency Response Basic Concepts

  33. Basic BJT Amplifiers Circuits 800mV 600mV 400mV 200mV 0V 0Hz 2KHz 4KHz 6KHz 8KHz 10KHz 12KHz 14KHz 16KHz 18KHz 20KHz V(2) V(1) Frequency Frequency Response Basic Concepts

  34. Basic BJT Amplifiers Circuits The drops of voltage gain(output/input) is mainly due to: 1、Increasing reactance of (at low f) 2、Parasitic capacitive elements of the network (at high f) 3、Dissappearance of changing current (for transformer coupled amp.) Lower cut off frequency Upper cut off frequency Frequency Response Basic Concepts

  35. Basic BJT Amplifiers Circuits C rbe rbe C' C C' Frequency Response High-Frequency BJT Model In BJTs, the PN junctions (EBJ and CBJ) also have capacitances associated with them

  36. Basic BJT Amplifiers Circuits rbe C' C' There are three capacitors in the circuit. At the mid frequency band, these are considered to be short circuits and internal capacitors and are considered to be open circuits. C' C', Frequency Response Frequency Response of the CE Amplifier

  37. Basic BJT Amplifiers Circuits ---is neglected Frequency Response Frequency Response of the CE Amplifier At low frequencies, C1, C2 are an open circuit and the gain is zero. Thus C1 has a high pass effect on the gain, i.e. it affects the lower cutoff frequency of the amplifier. 2 is the time constant for C2.

  38. Basic BJT Amplifiers Circuits ---is neglected Frequency Response Frequency Response of the CE Amplifier Capacitor Ce is an open circuit. The pole time constant is given by the resistance multiplied by Ce.

  39. Basic BJT Amplifiers Circuits In summary:the lower cut off frequency is determined by networkcapacitence. e.g. The higher cut off frequency is determined by the parasitic ferquency of the BJT. e.g. rbe C' C' At high frequencies, C1, C2 Ce are all short circuit. The frequency that dominates is the lowest pole frequency. The time constant is neglectedfor C' Frequency Response Frequency Response of the CE Amplifier

  40. Basic BJT Amplifiers Circuits rbe C' C' Frequency Response Frequency Response of the CE Amplifier

  41. Basic BJT Amplifiers Circuits rbe C' C' Frequency Response Frequency Response of the CE Amplifier

  42. Basic BJT Amplifiers Circuits decade decade 0 Frequency Response Frequency Response of the CE Amplifier

  43. For student References. • Chapter 9 amplifier fundamentals (9.1) (9.2) from the book Electronic devices, circuit and systems (Micheal M cirovic) • Chapter 8 introduction to amplifiers (8.1) (8.2) from the book introductory electronic devices and circuits by author (Robert T .paynter). • Wikipedia and world wide web

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