Electronics Principles & Applications Fifth Edition Chapter 8 Large-Signal Amplifiers ©1999 Glencoe/McGraw-Hill Charles A. Schuler

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

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

Efficiency The dc power supplied to an amplifier is P IN = V CC 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.

t ICIC t ICIC t ICIC t ICIC I SAT AB C D The major classes of amplifier operation

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

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

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

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.

mA 0 mA 25 mA 20 mA  mA 10 mA Class B The collector signal is too distorted for linear applications.

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

NPN PNP Class B

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

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

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.

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

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.

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.

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.

V BB 0.7 V 0 A V BE waveform I C waveform V CE waveform Class C amplifier with tank circuit Low V CE when I C is flowing

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.

I B >> 0 Three transistor operating modes: I B = 0 I B > 0 Cutoff Linear Saturation P C = 0 in both of these modes

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 RBRB They are also called Class D amplifiers.

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

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