Schuler Electronics Instructor CH08 amplifiers part 3.ppt
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This document discusses different classes of power amplifiers: - Class A amplifiers operate near the center of the load line and have a conduction angle of 360 degrees but maximum efficiency of only 25%. - Class B amplifiers have their operating point at cutoff, a conduction angle of 180 degrees, and a maximum theoretical efficiency of 78.5%. - Class AB amplifiers reduce crossover distortion by forward biasing transistors, providing an intermediate conduction angle between Class A and B. - Class C amplifiers use tank circuits and have a small conduction angle below 90 degrees but maximum theoretical efficiency of 100%. - Class D or switch-mode amplifiers use pulse-width modulation for very high efficiency power amplification.
Schuler Electronics Instructor CH08 amplifiers part 3.ppt
- 1. Electronics Principles & Applications Sixth Edition Chapter 8 Large-Signal Amplifiers ©2003 Glencoe/McGraw-Hill Charles A. Schuler
- 2. • Amplifier Class • Class A • Class B • Class AB • Class C • Class D INTRODUCTION
- 3. Power Amplifier PIN Efficiency = Input signal POUT POUT PIN Output signal HEAT = PIN - POUT High efficiency means less heat.
- 4. Efficiency • The dc power supplied to an amplifier is PIN = VCC x IDC • 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. t IC t IC t IC t IC ISAT A B C D The major classes of amplifier operation
- 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. A large-signal amplifier can also be called a power amplifier. This class A amplifier has a large quiescent collector current. C B E VCC = 18 V RL = 12 W RB = 1.2 kW CC b = 60 IB = VCC RB 18 V 1.2 kW = = 15 mA IC = b x IB = 60 x 15 mA = 0.9 A
- 8. 0 2 4 6 8 10 12 14 16 18 0.2 0.4 0.6 0.8 1.0 1.2 1.4 VCE in Volts IC in A 5 mA 0 mA 25 mA 20 mA 15 mA 10 mA ISAT = VCC RL 18 V 12 W = = 1.5 A Q This is a Class A amplifier. PC = VCE x IC = 7.2 V x 0.9 A = 6.48 W
- 9. 0 2 4 6 8 10 12 14 16 18 0.2 0.4 0.6 0.8 1.0 1.2 1.4 VCE in Volts IC in A 5 mA 0 mA 25 mA 20 mA 15 mA 10 mA Q This is a Class B amplifier. PC = VCE x IC = 18 V x 0 A = 0 W Its quiescent power dissipation is zero.
- 10. 0 2 4 6 8 10 12 14 16 0.2 0.4 0.6 0.8 1.0 1.2 1.4 5 mA 0 mA 25 mA 20 mA 15 mA 10 mA The collector signal is too distorted for linear applications.
- 11. C B C B E E +VCC The complementary-symmetry Class B push-pull amplifier has acceptable linearity for some applications. NPN PNP
- 12. NPN PNP Class B
- 13. C B C B E E +VCC Since the base-emitter junction potential is 0.7 V, there is some crossover distortion. NPN PNP
- 14. C B C B E E +VCC Crossover distortion is eliminated by applying some forward bias to the transistors (class AB). NPN PNP 1.4 V
- 15. 0 2 4 6 8 10 12 14 16 18 0.2 0.4 0.6 0.8 1.0 1.2 1.4 VCE in Volts IC in A Q The quiescent power dissipation is moderate for class AB. The efficiency is much better than class A.
- 16. Cap. required +VCC RL RL +VCC Single-ended amplifier A bridge-tied load provides four times the output power for a given supply voltage and load resistance. +VCC 2 Max. Max. = VCC Bridge amplifier Max. = 2 x VCC Max.
- 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. 0 2 4 6 8 10 12 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. 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. C B E VCC RB CC VBB Class C Amplifier Tank circuit The transistor is off for most of the input cycle and the conduction angle is small. VBB reverse biases the base-emitter junction.
- 21. VBB 0.7 V 0 A VBE waveform IC waveform VCE waveform Class C amplifier waveforms (with tank circuit) Low VCE when IC is flowing
- 22. C B E VCC RB CC Class C Amplifier with Signal Bias The base-emitter junction rectifies the input signal and charges CC. Signal bias increases when the input signal increases in amplitude.
- 23. IB >> 0 Three transistor operating modes: IB = 0 IB > 0 Cutoff Linear Saturation PC = 0 in both of these modes
- 24. A switch-mode amplifier uses a rectangular input signal to drive the transistor rapidly between cutoff and saturation. The efficiency is very high. C B E RB They are also called Class D amplifiers.
- 25. 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
- 26. PWM LPF The low-pass filter rejects the switching frequency.
- 27. 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
- 28. REVIEW • Amplifier Class • Class A • Class B • Class AB • Class C • Class D