Principles & Applications Large-Signal Amplifiers

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

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

Dear Student: This presentation is arranged in segments. Each segment is preceded by a Concept Preview slide and is followed by a Concept Review slide. When you reach a Concept Review slide, you can return to the beginning of that segment by clicking on the Repeat Segment button. This will allow you to view that segment again, if you want to.

Concept Preview Efficiency is most important in power amplifiers.
Poor efficiency means that much of the input power is converted to heat. A class A amplifier conducts for the entire signal cycle and has the lowest efficiency. A class B amplifier conducts for only half of the signal cycle. A class C amplifier conducts for less than half of the signal cycle. A class D amplifier switches between cutoff and saturation.

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

Concept Review Efficiency is most important in power amplifiers.
Poor efficiency means that much of the input power is converted to heat. A class A amplifier conducts for the entire signal cycle and has the lowest efficiency. A class B amplifier conducts for only half of the signal cycle. A class C amplifier conducts for less than half of the signal cycle. A class D amplifier switches between cutoff and saturation. Repeat Segment

Concept Preview Class A amplifiers operate at the center of the load line and have a large quiescent current flow. Class B amplifiers operate at cutoff and have no quiescent current flow. Class B amplifiers are usually operated in push-pull configurations. Class B amplifiers have crossover distortion. Class AB reduces crossover distortion. Bridge amplifiers provide four times the output power and eliminate the output coupling capacitor.

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

Concept Review Class A amplifiers operate at the center of the load line and have a large quiescent current flow. Class B amplifiers operate at cutoff and have no quiescent current flow. Class B amplifiers are usually operated in push-pull configurations. Class B amplifiers have crossover distortion. Class AB reduces crossover distortion. Bridge amplifiers provide four times the output power and eliminate the output coupling capacitor. Repeat Segment

Concept Preview Class C amplifiers are biased beyond cutoff for a small conduction angle and high efficiency. Class C amplifiers used tuned tank circuits to reduce distortion in RF applications. Class C amplifiers cannot be used in wideband applications like audio. Class D amplifiers switch between cutoff and saturation for very high efficiency. Class D amplifiers operate at a relatively high switching frequency and often use PWM. Class D can be used in audio applications.

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 %* Class B = 180o % Class AB @ 200o (between A & B) Class C @ 90o % *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 (PC > 0) 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

Concept Review Class C amplifiers are biased beyond cutoff for a small conduction angle and high efficiency. Class C amplifiers used tuned tank circuits to reduce distortion in RF applications. Class C amplifiers cannot be used in wideband applications like audio. Class D amplifiers switch between cutoff and saturation for very high efficiency. Class D amplifiers operate at a relatively high switching frequency and often use PWM. Class D can be used in audio applications. Repeat Segment

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

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