1. Chapter 8 Introduction to Amplifiers
Pictures are redrawn (with some modifications) from Introductory Electronic Devices and Circuits By
Robert T. Paynter

2. Objectives List the three fundamental ac properties of amplifiers
Describe the general model of a voltage amplifier
Discuss how i/p and o/p impedance effect on the effective gain of the circuit
Study the three BJT amplifier configurations
Discuss the various amplifier operation classes
Understand the concept of dB

3. Fig 8.1 Typical amplifiers.
JFET Amplifier
BJT Amplifier
Op-Amp Based Amplifier

4. Fig 8.2 General amplifier models.

5. Table 8.1 Gain symbols. Type of Gain Symbol Relation Voltage Av
Current Ai
Power Ap

6. Fig 8.3 Example 8.1.
The symbol shown in Fig. 8.3 is a generic symbol for an amplifier. Calculate the voltage gain for the amplifier represented in the figure.

7. Fig 8.4 Voltage amplifier model.

8. Fig 8.8 Combined effects of the input and output circuits.

9. Voltage Amplifier Characteristics
Ideal:
Any value of voltage gain (can be infinite if needed)
Infinite input impedance
Zero output impedance
Practical:
Certain value of gain (cannot reach infinity).
High input impedance
Low output impedance

10. BJT Amplifier Configurations
Common-emitter (CE) amplifier
Common-collector (CC) amplifier
Common-base (CB) amplifier

11. Table 8.2 Property ranges. Property Low Midrange High Gain < 100
>1000
Impedance
<1kW
1kW-10kW
>10kW

12. Fig 8.13 Common-emitter (CE) amplifier.
Midrange values of voltage and current gain.
High power gain
Midrange input impedance
Midrange output impedance

13. Fig 8.14 Common-collector (CC) amplifier.
Midrange current gain.
Extremely low voltage gain
High input impedance
Low output impedance

14. Fig 8.15 Common-base (CB) amplifier.
Midrange voltage gain
Extremely low current gain (slightly less than 1)
Low input impedance
High output impedance

15. Table 8.3 A comparison of CE, CC, and CB circuit characteristics.
Type Av Ai Ap
Zin
Zout CE
Midrange
High CC
< 1
@ Ai
Low CB
@ Av

16. BJT Terminal Connections
Type
Emitter
Base
Collector CE
Common
Input
Output CC CB

17. Amplifier Classifications
Class A – low distortion, high loss
Class B – some distortion, lower loss
Class C – high distortion, lowest loss
Others
Classes D, E, G, H, T

18. Class A Amplifiers
Conduction: Transistor conducts during 360 deg. of ac input.
Maximum theoretical eff.: 25%
Distortion: Little (subject to nonlinear distortion.)

19. Class B Amplifiers
Conduction: Each transistor conducts for 180 deg. of ac input.
Maximum theoretical eff.: 78.5%
Distortion: Little. Crossover distortion is most common.

20. Class C (Tuned) Amplifiers
Conduction: Each transistor conducts for less than 180 deg. of ac input.
Maximum theoretical eff.: 99%
Distortion: Mild to severe.

21. Decibels (dB) dB Value Ap 3 2 -3 1 / 2 6 4 -6 1 / 4 12 16 -12 1 / 1620
100
-20
1 / 100

22. Fig 8.25 dB gains are additive.

23. The dBm Reference
dB Voltage Gain

24. Summary Amplifier properties BJT amplifier configurations
Amplifier classifications
Decibels