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27 March 2011 1
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Uploaded By: Khaled Saleh
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Chapter 1 : Power Amplifiers
Class B Amplifier Operation
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Input (DC) Power (drawn from power supply
(Full wave rectified )
dcCCiIVdcP )(
)(2 pIIdc
)(
2)( pIVdcP CCi
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Chapter 1 : Power Amplifiers
Class B Amplifier Operation
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Output (AC) Power
L
L
oR
rmsVacP
)()(
2
L
L
L
L
o R
pV
R
ppVacP
2
)(
8
)()(
22
Efficiency
%100)(
)(%
dcP
acP
i
o
%100
)(
4%100
)()/2(
2/)(%100
)(
)(%
2
CC
L
CC
LL
i
o
V
pV
pIV
RpV
dcP
acP
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Chapter 1 : Power Amplifiers
Class B Amplifier Operation
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Power Dissipated by Output Transistors
acPdcPP oiQ 2
22Q
Q PP
CCCCL VVpV
2636.0)(
L
CCQ
R
VP
2
2
2
2maximum
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Chapter 1 : Power Amplifiers
Class B Amplifier Operation
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EXAMPLE
For a class B amplifier providing a 20-V peak signal to a16- load (speaker) and a power supply of V = 30 V,
determine the input power, output power, and circuitefficiency.
Solution:
A 20-V peak signal across a 16- load provides a peakload current of:
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Chapter 1 : Power Amplifiers
Class B Amplifier Operation
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The dc value of the current drawn from the power supply :
ARpVpIL
LL 25.116/20)()(
ApII Ldc 796.0)25.1(2)(2
The input power delivered by the supply voltage is:
WIVdcPdcCCi
9.23)796.0)(30()(
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Chapter 1 : Power Amplifiers
Class B Amplifier Operation
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The output power delivered to the load is:
WR
pVacP
L
L
o 5.12)16(2
)20(
2
)()(
22
for a resulting efficiency of:
%3.52%1009.23
5.12%100
)(
)(%
dcP
acP
i
o
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Chapter 1 : Power Amplifiers
Class B Amplifier Circuits
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ways to obtain phase-inverted signals from asingle input signal
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Chapter 1 : Power Amplifiers
Class B Amplifier Circuits
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ways to obtain phase-inverted signals from asingle input signal
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Chapter 1 : Power Amplifiers
Class B Amplifier Circuits
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ways to obtain phase-inverted signals from asingle input signal
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Chapter 1 : Power Amplifiers
Class B Amplifier Circuits
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Transformer-Coupled Push-Pull Circuits
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Chapter 1 : Power Amplifiers
Class B Amplifier Circuits
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Complementary-Symmetry Circuits
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Chapter 1 : Power Amplifiers
Class B Amplifier Circuits
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Chapter 1 : Power Amplifiers
Class B Amplifier Circuits
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Complementary-symmetry push-pull circuit usingDarlington transistors.
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Chapter 1 : Power Amplifiers
Class B Amplifier Circuits
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Quasi-complementary push-pull
transformerless power amplifier
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Chapter 1 : Power Amplifiers
Class B Amplifier Circuits
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EXAMPLE (1.7)
Calculate the input power, output power, and
power handled by each output transistor and thecircuit efficiency for an input of 12 V rms.
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Chapter 1 : Power Amplifiers
Class B Amplifier Circuits
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Chapter 1 : Power Amplifiers
Class B Amplifier Circuits
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The peak input voltage is:
Since the resulting voltage across the load is ideally
the same as the input signal (the amplifier has,ideally, a voltage gain of unity),
And the output power developed across the load is:
VVrmsVpV ii 1797.16)12(2)(2)(
VpVL 17)(
WR
pVacP
L
L
O 125.36)4(2
)17(
2
)()(
22
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Chapter 1 : Power Amplifiers
Class B Amplifier Circuits
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ARpVPIL
LL 25.4
417
)()(
ApII Ldc 71.2)25.4(2
)(2
WIVdcP dcCCi 75.67)71.2)(25()(
WPPP
P OiQ
Q 8.1522
2
WdcP
acP
i
o3.53%100
75.67
125.36%100
)(
)(%
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Chapter 1 : Power Amplifiers
Class C Amplifiers
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Is biased to conduct for much less than 180
Used as radio frequencies (RF) Amplifier
Basic class C amplifier operation(noninverting).
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Chapter 1 : Power Amplifiers
Class C Amplifiers
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Basic Operation
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Chapter 1 : Power Amplifiers
Class C Amplifiers
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Basic Operation
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Chapter 1 : Power Amplifiers
Class C Amplifiers
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Basic Operationmaximum collector current
is Ic(sat),
Ideal minimum collectorvoltage is Vce{sat).
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Chapter 1 : Power Amplifiers
Class C Amplifiers
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To avoid complex mathematics, we will use idealpulse approximations for the collector current andthe collector voltage during the ontime of thetransistor.
Power Dissipation
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Chapter 1 : Power Amplifiers
Class C Amplifiers
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The transistor is on for a short time, ton, and offfor the rest of the input cycle T
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