1. ECE 333 Linear Electronics
Chapter 7 Transistor Amplifiers
How a MOSFET or BJT can be used to make an amplifier  linear amplification  model the linear operation Three basic ways  Practical circuits by discrete components

2. Introduction
The basic principles of using MOSFET and BJT in amplifier design are the same.
Active region
- MOSFET: saturation or pinch-off region)
- BJT: active mode

3. 7.1 Basic Principles 7.1.1 The basis for amplifier operation
The basic application of a transistor in amplifier design is that when the device is operated in the active region, a voltage-controlled current source is realized.

5. 7.1.2 Obtaining a voltage amplifier (NMOS and npn amplifiers)

7. 7.1.3 The voltage-transfer characteristics
VTC is non-linear:
For BJT:

8. 7.1.4 Obtaining Linear Amplification by Biasing the Transistor
A dc voltage VGS is selected to obtain operation at a point Q on the segment AB of the VTC
Q: bias point or dc operation point, or quiescent point
The signal to be amplified is vgs(t)

9. Figure 7.3 Biasing the MOSFET amplifier at a point Q located on the segment AB of the VTC.

10. Fig. 7.4 The MOSFET amplifier with a small time-varying signal vgs(t)

11. 7.1.5 The Small-Signal Voltage Gain
(* because at B point, VGS is largest for saturation)

12. Example 7.1
Solution: VGS=0.6V, VOV=0.2V

13. (b) The max negative swing at the drain is 0. 2V
(b) The max negative swing at the drain is 0.2V. The positive side is fine with 0.2V (0.6V is still less than VDD) Max More precise analysis

14. For BJT:

15. Example 7.2
(Read it by yourself)

16. 7.1.6 Determining the VTC by Graphical Analysis

18. 7.2 Small-Signal Operation and Models
7.2.1 The MOSFET Case

19. The signal current in the drain terminal
Small-signal condition:

20. If the small-signal condition is satisfied:
Voltage gain

21. Fig. 7.12

22. Separating the DC analysis and the signal analysis
Small-signal equivalent circuit models

23. With MOSFET channel modulation

24. The Transconductance gm

25. Example 7.3: small-signal voltage gain?

26. IG = 0 

29. Modeling the Body effect

30. 7.2.2 The BJT Case

31. Collector current and Transcoductance
If:

32. If:

34. The base current and the input resistance at the base
The emitter current and the input resistance at the emitter

35. Example 7.5: determine vo/vi . Known β=100

36. At the quiescent operating point
Since VC > VB  CBJ is reverse biased, the device is operating in the active mode

37. 2. Determine the small-signal model 3. Determine signal vbe and vo

38. Small signal at output
Voltage gain
* The voltage gain is small because RBB is much larger than rπ

42. 7. 3 Basic Configurations

43. 7.3.2 Characterizing Amplifiers
Output resistance
Overall voltage gain

44. 7.3.3 The common-source (CS) and common-emitter amplifiers

45. Common-emitter amplifier

46. 7.3.4 CS and CE amplifier with a Rs or Re
With load resistance:

47. 7.3.5 The common-Gate (CG) and the Common-Base (CB) Amplifiers

48. The source and emitter followers (common-drain or common-collector amplifiers)
(* because infinite Rin)

49. 7.4 Biasing
To establish in the drain (collector) a dc current that is predictable, and insensitive to variations in temperature and to large variations in parameter values between devices of the same type;
To locate the dc operating point in the active region and allow required output signal swing without the transistor leaving the active region.

50. The MOSFET case
- E.g., biasing by fixing VG and connecting a Rs

51. Example 7.11
Solution: design the resistance by distributing VDD into 3 equal part on RD, transistor VDS and RS
(each part = 5 V)

52. When Vt = 1.5 V

53. 7.5 Discrete-Circuit Amplifiers (self-reading)
A. A common-source (CS) amplifier

54. A common-source (CS) amplifier

55. B. A Common-Emitter (CE) Amplifier

56. C. A CE amplifier with an emitter resistance Re

57. C. A CE amplifier with an emitter resistance Re With Re must use T Model

58. D. A common-base (CB) amplifier

59. E. An emitter follower (output at emitter)
T model

60. F. The amplifier frequency response
Use CS or CE amplifier as example