Download Chapter 8 - Large-signal Amplifiers

8-1
Electronics
Principles & Applications
Eighth Edition
Charles A. Schuler
Chapter 8
Large-Signal Amplifiers
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-2
INTRODUCTION
• Amplifier Class
• Class A
• Class B
• Class AB
• Class C
• Class D
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-3
High efficiency
means
less
HEAT
= PIN
- Pheat.
OUT
Input signal
Power
Amplifier
Output signal
POUT
Efficiency =
PIN
McGraw-Hill
POUT
PIN
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-4
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.
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-5
The major classes of amplifier operation
IC
IC
A
B
t
t
IC
IC
ISAT
C
McGraw-Hill
D
t
t
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-6
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
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-7
A large-signal amplifier can also be called a power amplifier.
This class A amplifier has a large quiescent collector current.
VCC
18 V
IB =
=
= 15 mA
RB
1.2 kW
IC = b x IB = 60 x 15 mA = 0.9 A
RL = 12 W
RB = 1.2 kW
CC
McGraw-Hill
B
C
b = 60
E
VCC = 18 V
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
VCC 18 V
ISAT =
=
= 1.5 A
12 W
RL
IC in A
8-8
This is a Class A amplifier.
Q
25 mA
1.4
1.2
1.0
0.8
0.6
0.4
0.2
20 mA
15 mA
10 mA
5 mA
0 2 4 6 8 10 12 14 16 18
VCE in Volts
0 mA
PC = VCE x IC = 7.2 V x 0.9 A = 6.48 W
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-9
This is a Class B amplifier.
Its quiescent power dissipation is zero.
IC in A
25 mA
1.4
1.2
1.0
0.8
0.6
0.4
0.2
20 mA
15 mA
Q
10 mA
5 mA
0 2 4 6 8 10 12 14 16 18
VCE in Volts
0 mA
PC = VCE x IC = 18 V x 0 A = 0 W
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-10
25 mA
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0 2 4 6
20 mA
15 mA
10 mA
5 mA
8 10 12 14 16
0 mA
The collector signal
is too distorted for
linear applications.
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-11
The complementary-symmetry Class B
push-pull amplifier has acceptable
linearity for some applications.
+VCC
C
B
B
NPN
E
E
PNP
C
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-12
PNP
NPN
Class B
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-13
Since the base-emitter junction potential
is 0.7 V, there is some crossover distortion.
+VCC
C
B
B
NPN
E
E
PNP
C
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Crossover distortion is eliminated
by applying some forward bias
to the transistors (class AB).
+VCC
8-14
C
B
NPN
E
1.4 V
B
E
PNP
C
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-15
The quiescent power dissipation is moderate for class AB.
The efficiency is much better than class A.
IC in A
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Q
0 2 4 6 8 10 12 14 16 18
VCE in Volts
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-16
A bridge-tied load provides four times the output
power for a given supply voltage and load resistance.
Bridge amplifier
Single-ended amplifier
+VCC
+VCC
Cap. required
+VCC
Max. = VCC
Max. = 2 x VCC
RL
2
RL
Max.
Max.
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-17
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
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
A
1.4
1.2
1.0
0.8
0.6
0.4
0.2
8-18
AB
B
C
0 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.
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-19
Conduction Angles
& theoretical max. efficiencies:
•
•
•
•
o
Class A = 360
o
Class B = 180
o
Class AB @ 200
o
Class C
@ 90
50 %*
78.5 %
(between A & B)
100 %
*Class A amplifiers are seldom driven to
maximum output and typically provide
much less efficiency.
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-20
Class C amplifier
VCC
Tank circuit
C
CC
B
RB
VBB
E
The transistor is
off for most of
the input cycle
and the conduction
angle is small.
VBB reverse biases the base-emitter junction.
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-21
0.7 V
VBB
VBE waveform
Class C amplifier
waveforms
(with tank circuit)
0A
IC waveform
VCE waveform
Low VCE when IC is flowing
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Class C amplifier with signal bias
VCC
8-22
C
CC
B
RB
E
The base-emitter
junction rectifies
the input signal
and charges CC.
Signal bias increases when the input
signal increases in amplitude.
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-23
Three transistor operating modes:
IB = 0
IB > 0
Cutoff
IB >> 0
Linear
Saturation
Ideally, PC = 0 in both of these modes
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-24
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.
B
RB
McGraw-Hill
C
E
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Two, 250-W outputs
into 4-W loads, or
one 500-W bridged
output into an 8-W
load
8-25
classdaudio.com
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-26
Power Out
Note that Class D offers high efficiency when the amplifier output is modest or low!
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-27
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
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-28
Triangular wave
Class D amplifier
Comparator
Input signal
Max.
pulse
width
McGraw-Hill
Low-pass filter
Output signal
Min.
pulse
width
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-29
The low-pass filter rejects
the switching frequency.
PWM
McGraw-Hill
LPF
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-30
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 dutycycle known as _________.
PWM
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
8-31
REVIEW
• Amplifier Class
• Class A
• Class B
• Class AB
• Class C
• Class D
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.