1. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-1 Electronics Principles & Applications Eighth Edition Chapter 8 Large-Signal Amplifiers (student version) Charles A. Schuler ©2013

2. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-2 Amplifier Class Class A Class B Class AB Class C Class D INTRODUCTION

3. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-3 Dear Student: This presentation is arranged in segments. Each segment is preceded by a Concept Preview slide and is followed by a Concept Review slide. When you reach a Concept Review slide, you can return to the beginning of that segment by clicking on the Repeat Segment button. This will allow you to view that segment again, if you want to.

4. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-4 Concept Preview Efficiency is most important in power amplifiers. Poor efficiency means that much of the input power is converted to heat. A class A amplifier conducts for the entire signal cycle and has the lowest efficiency. A class B amplifier conducts for only half of the signal cycle. A class C amplifier conducts for less than half of the signal cycle. A class D amplifier switches between cutoff and saturation.

5. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-5 Power Amplifier P IN Efficiency = Input signal P OUT P IN Output signal HEAT = P IN - P OUT High efficiency means less heat.

6. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-6 Efficiency The dc power supplied to an amplifier is P IN = V CC x I DC Efficiency = P OUT /P IN x 100% The maximum efficiency for Class A amplifiers with a dc collector resistance and a separate load resistance is 25%. Class A is usually not acceptable when watts of power are required.

7. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-7 t ICIC t ICIC t ICIC t ICIC I SAT AB C D The major classes of amplifier operation

8. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-8 Class and efficiency quiz If P OUT = 100 W and P IN = 200 W, the efficiency is _________. 50% The efficiency of an ideal amplifier is __________. 100% When efficiency is poor, too much of the input is converted to ________. heat An amplifier that conducts for the entire cycle is operating Class _______. A An amplifier that conducts for half the cycle is operating Class _______. B

9. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-9 Concept Review Efficiency is most important in power amplifiers. Poor efficiency means that much of the input power is converted to heat. A class A amplifier conducts for the entire signal cycle and has the lowest efficiency. A class B amplifier conducts for only half of the signal cycle. A class C amplifier conducts for less than half of the signal cycle. A class D amplifier switches between cutoff and saturation. Repeat Segment

10. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-10 Concept Preview Class A amplifiers operate at the center of the load line and have a large quiescent current flow. Class B amplifiers operate at cutoff and have no quiescent current flow. Class B amplifiers are usually operated in push- pull configurations. Class B amplifiers have crossover distortion. Class AB reduces crossover distortion. Bridge amplifiers provide four times the output power and eliminate the output coupling capacitor.

11. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-11 A large-signal amplifier can also be called a power amplifier. This class A amplifier has a large quiescent collector current. C B E V CC = 18 V R L = 12  R B = 1.2 k  C  = 60 == V CC RBRB 18 V 1.2 k  = = 15 mA I C =  x I B = 60 x 15 mA = 0.9 A

12. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-12 0 24 6 8 1012 14 16 18 0.2 0.4 0.6 0.8 1.0 1.2 1.4 V CE in Volts I C in A 5 mA 0 mA 25 mA 20 mA  mA 10 mA I SAT = V CC RLRL 18 V 12  = = 1.5 A Q This is a Class A amplifier. P C = V CE x I C = 7.2 V x 0.9 A = 6.48 W

13. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-13 0 24 6 8 1012 14 16 18 0.2 0.4 0.6 0.8 1.0 1.2 1.4 V CE in Volts I C in A 5 mA 0 mA 25 mA 20 mA  mA 10 mA Q This is a Class B amplifier. P C = V CE x I C = 18 V x 0 A = 0 W Its quiescent power dissipation is zero.

14. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-14 0 24 6 8 1012 14 16 0.2 0.4 0.6 0.8 1.0 1.2 1.4 5 mA 0 mA 25 mA 20 mA  mA 10 mA Class B The collector signal is too distorted for linear applications.

15. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-15 C B C B E E +V CC The complementary-symmetry Class B push-pull amplifier has acceptable linearity for some applications. NPN PNP

16. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-16 NPN PNP Class B

17. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-17 C B C B E E +V CC Since the base-emitter junction potential is 0.7 V, there is some crossover distortion. NPN PNP

18. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-18 C B C B E E +V CC Crossover distortion is eliminated by applying some forward bias to the transistors (class AB). NPN PNP 1.4 V

19. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-19 0 24 6 8 1012 14 16 18 0.2 0.4 0.6 0.8 1.0 1.2 1.4 V CE in Volts I C in A Q The quiescent power dissipation is moderate for class AB. The efficiency is much better than class A.

20. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-20 Cap. required +V CC RLRL RLRL Single-ended amplifier A bridge-tied load provides four times the output power for a given supply voltage and load resistance. +V CC 2 Max. Max. = V CC Bridge amplifier Max. = 2 x V CC Max.

21. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-21 Class A, B, and AB quiz Class A amplifiers are biased to operate near the ________ of the load line. center Class B amplifiers have their Q-points at ____________. cutoff The conduction angle for class B is _________. 180 o To reduce distortion, two class B transistors are arranged in _____________. push-pull Class AB is a solution for __________ distortion. crossover

22. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-22 Concept Review Class A amplifiers operate at the center of the load line and have a large quiescent current flow. Class B amplifiers operate at cutoff and have no quiescent current flow. Class B amplifiers are usually operated in push- pull configurations. Class B amplifiers have crossover distortion. Class AB reduces crossover distortion. Bridge amplifiers provide four times the output power and eliminate the output coupling capacitor. Repeat Segment

23. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-23 Concept Preview Class C amplifiers are biased beyond cutoff for a small conduction angle and high efficiency. Class C amplifiers used tuned tank circuits to reduce distortion in RF applications. Class C amplifiers cannot be used in wideband applications like audio. Class D amplifiers switch between cutoff and saturation for very high efficiency. Class D amplifiers operate at a relatively high switching frequency and often use PWM. Class D can be used in audio applications.

24. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-24 0 24 6 8 1012 14 16 18 0.2 0.4 0.6 0.8 1.0 1.2 1.4 A B C AB The class of an amplifier is determined by the bias which establishes the Q-point. Class C is established by reverse biasing the base-emitter junction.

25. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-25 Conduction Angles & theoretical max. efficiencies: Class A = 360 o 50 %* Class B = 180 o 78.5 % Class AB  200 o (between A & B) Class C  90 o 100 % *Class A amplifiers are seldom driven to maximum output and typically provide much less efficiency.

26. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-26 C B E V CC RBRB C V BB Class C amplifier V BB reverse biases the base-emitter junction. Tank circuit The transistor is off for most of the input cycle and the conduction angle is small.

27. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-27 V BB 0.7 V 0 A V BE waveform I C waveform V CE waveform Class C amplifier waveforms (with tank circuit) Low V CE when I C is flowing

28. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-28 C B E V CC RBRB C Class C amplifier with signal bias The base-emitter junction rectifies the input signal and charges C C. Signal bias increases when the input signal increases in amplitude.

29. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-29 I B >> 0 Three transistor operating modes: I B = 0 I B > 0 Cutoff Linear Saturation Ideally, P C = 0 in both of these modes

30. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-30 A switch-mode amplifier uses a rectangular input signal to drive power transistors rapidly between cutoff and saturation. The efficiency is high, allowing large power outputs from relatively small boards and heat sinks. C B E RBRB They are also called Class D amplifiers.

31. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-31 Two, 250-W outputs into 4-  loads, or one 500-W bridged output into an 8-  load classdaudio.com

32. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-32 Power Out Note that Class D offers high efficiency when the amplifier output is modest or low!

33. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-33 If the switching frequency is a good deal higher than the signal frequency, a Class D amplifier is capable of linear amplification. Pulse-width modulation and a low-pass filter are often used. PWM Signal Input Signal

34. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-34 Low-pass filter Comparator Class D amplifier Triangular wave Input signal Output signal Max. pulse width Min. pulse width

35. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-35 PWM LPF The low-pass filter rejects the switching frequency.

36. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-36 Class C and D quiz Class C amplifiers use _______ circuits to restore sinusoidal signals. tank The base-emitter junction in a class C amplifier is ________ biased. reverse The theoretical maximum efficiency for class C is ___________. 100% Class D amplifiers are also known as __________ amplifiers. switch-mode Class D amplifiers employ a varying duty- cycle known as _________. PWM

37. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-37 Concept Review Class C amplifiers are biased beyond cutoff for a small conduction angle and high efficiency. Class C amplifiers used tuned tank circuits to reduce distortion in RF applications. Class C amplifiers cannot be used in wideband applications like audio. Class D amplifiers switch between cutoff and saturation for very high efficiency. Class D amplifiers operate at a relatively high switching frequency and often use PWM. Class D can be used in audio applications. Repeat Segment

38. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-38 REVIEW Amplifier Class Class A Class B Class AB Class C Class D