Principles & Applications Large-Signal Amplifiers Electronics Principles & Applications Sixth Edition Charles A. Schuler Chapter 8 Large-Signal Amplifiers ©2003 Glencoe/McGraw-Hill

INTRODUCTION Amplifier Class Class A Class B Class AB Class C Class D

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

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.

A B C D The major classes of amplifier operation IC IC t t IC IC ISAT

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

RB = 1.2 kW RL = 12 W VCC = 18 V C b = 60 B CC E VCC 18 V IB = = A large-signal amplifier can also be called a power amplifier. This class A amplifier has a large quiescent collector current. IB = VCC RB 18 V 1.2 kW = = 15 mA 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

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

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

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 Class B The collector signal is too distorted for linear applications.

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

Class B NPN PNP

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

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

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

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

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

The class of an amplifier is determined by the bias 1.4 1.2 1.0 AB B 0.8 0.6 0.4 0.2 C 2 4 6 8 10 12 14 16 18 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.

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.

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.

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

Signal bias increases when the input signal increases in amplitude. 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.

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

C B E RB A switch-mode amplifier uses a rectangular input signal to drive the transistor rapidly between cutoff and saturation. The efficiency is very high. They are also called Class D amplifiers. C B E RB

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

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

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

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