Electronics Principles & Applications Eighth Edition

1. Electronics Principles & Applications Eighth Edition Charles A. Schuler Chapter 8 Large-Signal Amplifiers

2. INTRODUCTION • Amplifier Class • Class A • Class B • Class AB • Class C • Class D

3. Output signal Input signal High efficiency means less heat. HEAT = PIN - POUT Power Amplifier POUT POUT Efficiency = PIN PIN

4. Efficiency • The dc power supplied to an amplifier is PIN = VCC xIDC • Efficiency = POUT/PIN 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. The major classes of amplifier operation IC IC A B t t IC IC ISAT C D t t

6. Class and efficiency quiz If POUT = 100 W and PIN = 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. VCC 18 V IB= = = 15 mA 1.2 kW RB A large-signal amplifier can also be called a power amplifier. This class A amplifier has a large quiescent collector current. IC = b x IB = 60 x 15 mA = 0.9 A RB = 1.2 kW RL = 12 W VCC = 18 V C b = 60 B CC E

8. VCC 18 V ISAT = = = 1.5 A 12 W RL Q 25 mA 20 mA 15 mA 10 mA 5 mA 0 mA This is a Class A amplifier. 1.4 1.2 1.0 IC in A 0.8 0.6 0.4 0.2 6 16 2 4 10 12 0 14 18 8 VCE in Volts PC = VCE x IC = 7.2 V x 0.9 A = 6.48 W

9. Q 25 mA 20 mA 15 mA 10 mA 5 mA 0 mA This is a Class B amplifier. Its quiescent power dissipation is zero. 1.4 1.2 1.0 IC in A 0.8 0.6 0.4 0.2 6 16 2 4 10 12 0 14 18 8 VCE in Volts PC = VCE x IC = 18 V x 0 A = 0 W

10. 25 mA 1.4 20 mA 1.2 1.0 15 mA 0.8 10 mA 0.6 0.4 5 mA 0.2 0 mA 6 16 2 4 10 12 0 14 8 Class B The collector signal is too distorted for linear applications.

11. C B The complementary-symmetry Class B push-pull amplifier has acceptable linearity for some applications. +VCC NPN E E B PNP C

12. Class B NPN PNP

13. C B Since the base-emitter junction potential is 0.7 V, there is some crossover distortion. +VCC NPN E E B PNP C

14. C B Crossover distortion is eliminated by applying some forward bias to the transistors (class AB). +VCC NPN E 1.4 V E B PNP C

15. Q The quiescent power dissipation is moderate for class AB. The efficiency is much better than class A. 1.4 1.2 1.0 IC in A 0.8 0.6 0.4 0.2 6 16 2 4 10 12 0 14 18 8 VCE in Volts

16. +VCC +VCC Cap. required Max. = 2 x VCC RL +VCC Max. = VCC 2 RL Max. Max. A bridge-tied load provides four times the output power for a given supply voltage and load resistance. Bridge amplifier Single-ended amplifier

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 _________. 180o To reduce distortion, two class B transistors are arranged in _____________. push-pull Class AB is a solution for __________ distortion. crossover

18. A 1.4 1.2 1.0 AB B 0.8 0.6 0.4 0.2 C 6 16 2 4 10 12 0 14 18 8 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. Conduction Angles& theoretical max. efficiencies: • Class A = 360o 50 %* • Class B = 180o 78.5 % • Class AB @ 200o (between A & B) • Class C @ 90o 100 % *Class A amplifiers are seldom driven to maximum output and typically provide much less efficiency.

20. Class C amplifier VCC Tank circuit C The transistor is off for most of the input cycle and the conduction angle is small. B CC E RB VBB VBB reverse biases the base-emitter junction.

21. 0.7 V VBE waveform VBB Class C amplifier waveforms (with tank circuit) IC waveform 0 A VCE waveform Low VCE when IC is flowing

22. Class C amplifier with signal bias VCC C The base-emitter junction rectifies the input signal and charges CC. B CC E RB Signal bias increases when the input signal increases in amplitude.

23. Three transistor operating modes: IB = 0 IB > 0 IB >> 0 Cutoff Saturation Linear Ideally, PC = 0 in both of these modes

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. They are also called Class D amplifiers. C B E RB

25. Two, 250-W outputs into 4- loads, or one 500-W bridged output into an 8- load classdaudio.com

26. Power Out Note that Class D offers high efficiency when the amplifier output is modest or low!

27. PWM Signal Input Signal 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.

28. Triangular wave Low-pass filter Comparator Class D amplifier Input signal Output signal Min. pulse width Max. pulse width

29. The low-pass filter rejects the switching frequency. LPF PWM

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. REVIEW • Amplifier Class • Class A • Class B • Class AB • Class C • Class D