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8/3/2019 BENE 3143_poweramp_classAamplifier (1)
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BENE 3143: SYSTEM
ELECTRONICS 2
Power Amplifier
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Outline
• Introduction
• The class A amplifier
•
The class B and class AB Push-Pull amplifier • The class C amplifier
Sadhiqin 2011 2
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Introduction
• A power amplifier is designed to deliver a
large amount of power to the load.
• To perform this function, a power amplifier
must itself be capable of dissipating large
amount of power.
Sadhiqin 2011 3
POWER
LOAD
AMPLIFIER
Outline
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• It must so designed that the heat generated
when it is operated at high current and
voltage levels is released into the
surroundings at a rate fast enough to prevent
destructive temperature buildup.
Sadhiqin 2011 4
HEAT RELEASE
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• Consequently, power amplifiers typically
contain bulky component s having large
surface areas to enhance heat transfer to
environment.
• A power amplifier is often the last, or output,
stage of an amplifier system.
• The preceding stages may be designed to
provide amplification, to provide buffering to
a high impedance signal source, or to modify signal characteristics in some predictable way.
Sadhiqin 2011 5
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• Power amplifier arewidely used in audiocomponent such asradio and televisionreceivers, phonographsand tape players, stereoand high fidelity system,recording-studioequipment, and publicaddress system.
Sadhiqin 2011 6
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• The load of these
applications is most often a
loudspeaker (“speaker”)
which requires considerable
power to convert electrical
signal to sound waves.
Sadhiqin 2011 7
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Sound Test Room!
Sadhiqin 2011 8
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• It also used inelectromechanicalcontrol system to drive
electric motors.Example includecomputer disk andtape drives, roboticmanipulators,autopilots, antennarotator
Sadhiqin 2011 9
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• One important aspect in the
design power amplifiers is that itdelivers a specified power to the
load efficiently.
Sadhiqin 2011 10
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• Power amplifiers
are classifiedaccording to the
percent time the
outputtransistors are
conducting, or
‘turn on’.
Sadhiqin 2011 11
Outline
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Class A Amplifier
• Class A operation
• Heat dissipation and Centered Q-Point
• 2 load lines
• Power Gain
• DC Quiescent Power
• Output Power
• Efficiency
• Examples
Sadhiqin 2011 12
Outline
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Class A operation
• A power amplifier is a large signal amplifier
that produces a replica of the input signal on
its output. In the case shown here, the output
is an inverted replica of the input.
Sadhiqin 2011 13
V in V out A
v0 0
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• Output is shown 180o out of phase with the
input (inverted)
Sadhiqin 2011 14
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• The output of a class A amplifier conducts
for the full 360 of the cycle.
Sadhiqin 2011 15
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Class A Amplifier
• Class A operation
• Heat dissipation and Centered Q-Point
• 2 load lines
• Power Gain
• DC Quiescent Power
• Output Power
• Efficiency
• Examples
Sadhiqin 2011 16
Outline
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Notice that a class A amplifierdissipates dc power even with no
signal. The dc power dissipated is
the product of ICQ and V CEQ .
Heat Dissipation and Centered Q-point
• A class A power amplifier is a large signal amplifier that
operates in the linear region. Ideally, a class A amplifier is
designed to operate in the center of the ac load line.
Sadhiqin 2011 170
I CQ
I c(sat)
Q
AC load line
V CEQ V ce(cutoff) V CE
I C
DC load line
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The Q-point is set at the middle
of the load line so that the ACsignal can swing a full cycle.
Sadhiqin 2011 18
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• If the Q-point is not centered on the ac load
line, the output signal will be limited
Sadhiqin 2011 19
a) Amplitude limited by cutoff b) Transistor driven into cutoff
with an increase of the input
amplitude
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• Here is shown when the Q-point is moved
away from center towards saturation.
Sadhiqin 2011 20
a) Amplitude Vce and Ic limited
by saturation
b) Transistor driven into
saturation by a further
increase in input amplitude
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Class A Amplifier
• Class A operation
• Heat dissipation and Centered Q-Point
• 2 load lines
• Power Gain
• DC Quiescent Power
• Output Power
• Efficiency
• Examples
Sadhiqin 2011 21
Outline
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2 Load Lines
Sadhiqin 2011 22
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Class A Amplifier
• Class A operation
• Heat dissipation and Centered Q-Point
• 2 load lines
• Power Gain
• DC Quiescent Power
• Output Power
• Efficiency
• Examples
Sadhiqin 2011 23
Outline
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Power Gain
• A power amplifier delivers power
to the load.
• The power gain of an amplifier is
the ratio of the output power
(power delivered to the load) to
the input power.
Sadhiqin 2011 24
in
L p
P
P A
A p is the power gain
P L is signal power delivered to the load
Pin is signal power delivered to the amplifier
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Class A Amplifier
• Class A operation
• Heat dissipation and Centered Q-Point
•
Power Gain• DC Quiescent Power
• Output Power
• Efficiency• Examples
Sadhiqin 2011 25
Outline
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DC Quiescent Power
• The power dissipation of transistor with no
signal input is the product of its Q-point
current and voltage.
Sadhiqin 2011 26
CEQCQ DQ V I P
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Class A Amplifier
• Class A operation
• Heat dissipation and Centered Q-Point
•
Power Gain• DC Quiescent Power
• Output Power
•Efficiency
• Examples
Sadhiqin 2011 27
Outline
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Output Power
• The maximum power from a class A amplifier
is
Sadhiqin 2011 28
EQC CQout(max) V I P 5.0
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Class A Amplifier
• Class A operation
• Heat dissipation and Centered Q-Point
•
Power Gain• DC Quiescent Power
• Output Power
•Efficiency
• Examples
Sadhiqin 2011 29
Outline
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Efficiency
• The ratio of the output signal power supplied
to a load to the total power from the dc
supply
Sadhiqin 2011 30
%25250
%100
.%η
P
P%
max
i(dc)
o(ac)
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Increase Efficiency with Transformer
Coupling
• By using the transformer coupling, the
efficiency can be increased by 50% but there
are several drawbacks such as:
– Size of transformers
– Cost of transformers
– Potential distortion problems once the
transformer begins to saturate
Sadhiqin 2011 31
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Class A Amplifier
• Class A operation
• Heat dissipation and Centered Q-Point
• Power Gain
• DC Quiescent Power
• Output Power
•Efficiency
• Examples
Sadhiqin 2011 32
Outline
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Example 1
• Determine the voltage gain and the power
gain of the class A power amplifier below:
Sadhiqin 2011 33
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Example 2
• Determine the efficiency of the same power
amplifier
Sadhiqin 2011 34
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Example 3
• Figure below shows aCE power amplifier inwhich the collectorresistor serves also asthe load resistor.Assume
a) Determine the dc Q-
point (ICQ and VCEQ)b) Determine the
voltage gain and thepower gain.
Sadhiqin 2011 35
100 ac DC
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c) The power
dissipated in the
transistor with no
load
d) The total power
from the power
supply with no loade) The signal power in
the load with a
500mV input
Sadhiqin 2011 36
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Example 4
• Determine each Q-point for each amplifier
below:
Sadhiqin 2011 37
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Example 5
• Find the maximum
output signal power
to the load and
efficiency for theamplifier in the figure
below with a 500 Ω
resistor.
Sadhiqin 2011 38
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Example 6
• Calculate the input power, output power, and efficiency of the
amplifier circuit below for and input voltage that results in a
base current of 10mA peak.
Sadhiqin 2011 39
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End of Class A Amplifier
• Class A operation
• Heat dissipation and Centered Q-Point
• Power Gain
• DC Quiescent Power
• Output Power
•Efficiency
• Examples
S dhi i 2011 40
Back to outline