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

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 Power Amplifier P IN Efficiency = Input signal P OUT P IN Output signal HEAT = P IN - P OUT High efficiency means less heat.

4. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-4 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.

5. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-5 t ICIC t ICIC t ICIC t ICIC I SAT AB C D The major classes of amplifier operation

6. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-6 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

7. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-7 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

8. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-8 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

9. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-9 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.

10. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-10 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.

11. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-11 C B C B E E +V CC The complementary-symmetry Class B push-pull amplifier has acceptable linearity for some applications. NPN PNP

12. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-12 NPN PNP Class B

13. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-13 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

14. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-14 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

15. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-15 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.

16. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-16 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.

17. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-17 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

18. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-18 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.

19. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-19 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.

20. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-20 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.

21. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-21 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

22. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-22 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.

23. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-23 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

24. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-24 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.

25. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-25 Two, 250-W outputs into 4-  loads, or one 500-W bridged output into an 8-  load classdaudio.com

26. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-26 Power Out Note that Class D offers high efficiency when the amplifier output is modest or low!

27. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-27 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

28. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-28 Low-pass filter Comparator Class D amplifier Triangular wave Input signal Output signal Max. pulse width Min. pulse width

29. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-29 PWM LPF The low-pass filter rejects the switching frequency.

30. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-30 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

31. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill 8-31 REVIEW Amplifier Class Class A Class B Class AB Class C Class D