1. Chapter 9 Common-Emitter Amplifiers
Pictures are redrawn (with some modifications) from Introductory Electronic Devices and Circuits By
Robert T. Paynter

2. Objectives
Describe AV, AI, AP associated with the three amplifier configurations.
Describe the input/output voltage and current phase relationships.
Calculate the ac emitter resistance of a transistor.
Discuss two roles of the capacitors in the circuits.
Derive the ac equivalent circuit for a given amplifier.
Explain how the voltage gain drift due to temperature occurs.
Discuss the relationship between the load resistance and voltage gain of a CE amplifier.

3. Objectives (Cont.) Calculate Zin(base) and Zin for a CE amplifier
Discuss the effects of swamping on the ac characteristics of a CE amplifier
List and describe the four ac h-parameters

4. Fig 9.1 Common-emitter input-output phase relationship.

5. AC Emitter Resistance where r’e = ac emitter resistance
IE = the dc emitter current, found as VE / RE for example.

6. Fig 9.2 Example 9.1.

7. Fig 9.3 Graphical determination of ac emitter resistance.

8. Fig 9.4 The determination of ac beta.
hFE = dc beta
hfe = ac beta

9. AC Model of A BJT

10. Roles of Capacitors in Amplifiers
A coupling capacitor passes an ac signal from one amplifier to another, while providing dc isolation between the two.
A bypass capacitor is used to “short circuit” an ac signal to ground while not affecting the dc operation of the circuit.
The higher the freq., the lower the capacitor impedance.

11. Fig 9.5 Coupling capacitors in a multistage amplifer.

12. Fig 9.6a AC coupling.

13. Fig 9.6b DC isolation.

14. Fig 9.7 Capacitive vs. direct coupling.

15. Fig 9.8-9 Bypass capacitors.
For AC analysis
For DC analysis

16. Fig 9.10 Typical common-emitter amplifier signals.

17. Fig 9.11 Deriving the CE ac-equivalent circuit.
(b)
(c)

18. Fig 9.12a Example 9.2.

19. Fig 9.12b Example 9.2.

20. Fig 9.12c-d Example 9.2.
(c)
(d)

21. Voltage Gain of CE

22. Fig 9.13 Example 9.4. (1)
Transform the base circuit to its Thevenin equivalent.

23. Fig 9.13 Example 9.4. (2)

24. Fig 9.14 Example 9.5. (1)
Transform the base circuit to its Thevenin equivalent.

25. Fig 9.14 Example 9.5. (2)

26. CE Current Gain Ai is always less than hfe due to two factors:
The input ac current is divided between the transistor and the biasing network.
The output collector current is divided between the collector resistor and the load.

27. Power Gain
Example 9.7 The amplifier shown in Fig. 9.5 has values of Av = 45.3 and Ai = 20. Determine the power gain (Ap) of the amplifier and the output power when Pin = 80 mW.

28. The Effects of Loading
The lower the load resistance is, the lower the voltage gain.

29. Example 9.8
The load in Fig is open. Calculate the open-load voltage gain of the circuit.

30. Calculating Amp. Input Impedance

31. Fig 9.17 Example 9.9.

32. Calculating the Value of Ai

33. Example 9.10
Calculate the value of Ai for the circuit shown in Fig

34. Multistage Amp. Gain Calculations
Procedure:
Do dc analysis
Find r’e for each stage
Find rC for each stage
Using r’e and rC to find Av for each stage
Input impedance of next stage is the load of current stage.
(Zin of next stage is RL of current stage)

35. Fig 9.18 Example (1)
Determine Av of the 1st stage. Assume that r’e for the 1st stage is 19.8 W and r’e for the 2nd stage is found to be 17.4 W. For the 2nd stage, hfe is 200.
The input impedance of the 2nd stage:

36. Fig 9.18 Example (2)
Finally, Av for the 1st stage is found as

37. Example (1)
Determine the value of AvT for the amplifier in Figure 9.18.
rC for the 2nd stage can be found as
Av for the 2nd stage is found as

38. Fig 9.19 The swamped CE amplifier and its ac equivalent ckt.
Swamped amplifier is an amplifier that uses a partially bypassed emitter resistance to increase ac emitter resistance. Also referred to as a gain-stabilized amplifier.

39. Av of Swamped Amp.

40. Fig 9.20 Example (1)

41. Fig 9.20 Example (2)

42. Example 9.14
Determine the change in gain for the amplifier in Example 9.13 when r’e doubles in value.
Swamping improves the gain stability of a CE amplifier when rE >> r’e.

43. The Effect of Swamping on Zin

44. Fig 9.22 Gain stabilization.Av
-rC / r’e
-rC / (r’e+rE)
Zin(base)
hfer’e
hfe(r’e+rE)
Advantage
Higher values of Av than the swamped amplifier.
Relatively stable Av. Much smaller distortion.
Disadvantage
Relatively unstable values of Av.
Lower Av than the standard amplifier.

45. The Hybrid Equivalent Model
Hybrid model is derived from two-port system.

46. Six Circuit-Parameter Models for Two-Port Systems
Independent Variables
Dependent Variables
Circuit Parameters
I1, I2
V1, V2
Impedance Z
Admittance Y
V1, I2
I1, V2
Inverse Hybrid g
Hybrid h
V2, I2
V1, I1
Transmission T
Inverse Transmission T’

47. Equations for Hybrid Model
Let V1 = Vi, I1 = Ii, V2 = Vo, and I2 = Io. Then

48. Equivalent Circuit for Hybrid Model

49. h-Parameters
h11 = hi = Input Resistance h12 = hr = Reverse Transfer Voltage Ratio h21 = hf = Forward Transfer Current Ratio h22 = ho = Output Admittance

50. h-Parameters for CE Amp.
hie = the base input impedance
hfe = the base-to-collector current gain
hoe = the output admittance
hre = the reverse voltage feedback ratio

51. Hybrid Model for CE Configuration
May be neglected.

52. h-parameters of 2N3904

53. Hybrid Model without hre and hoe

54. Determining h-Parameter Values
Use geometric means if given max. and min. values.
See examples 9.18 and 9.19.

55. Summary AC concepts Roles of capacitors in amplifiers
Common-emitter ac equivalent circuit
Amplifier gain
Gain and impedance calculations
Swamped amplifiers
h-parameters