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DC Voltage Source
Signal SourceE.g.: CD player,
Human Voice
Small
Signal
Amplifier
Large
Signal
Amplifier
Load
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INTRODUCTION In small-signal amplifiers the main factors are:
Amplification
Linearity
Gain
Since large-signal, or power amplifiers handle relatively large voltagesignals and current levels, the main factors are:
Efficiency
Maximum power capability Impedance matching to the output device
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POWER AMPLIFIER TYPES CLASS A
CLASS B
CLASS AB
CLASS C
CLASS D
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CLASS A POWER AMPLIFIER The output of a class A power amplifier conducts for the full 360 of the cycle.
The Q-point is set at the middle of the load line so that the AC signal can swing a
full cycle.icic
ic sat
t
Vce
ic(cutoff)
ICQ
VCEQ VCE(cutoff)VCE(sat)
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LOAD LINE CONDITION 1
icic
ic sat
The Q-point is set at the middle of the load line so that the AC signal can swing a
full cycle.
Value of ic change from ICQ either increase to ic(sat) or decrease to ic(cutoff)
t
Vce
ic(cutoff)
ICQ
VCEQ VCE(cutoff)VCE(sat)
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LOAD LINE CONDITION 2 When the input signal larger than the Q-point limits, amplifier will operate in cut off
and saturation region.
ic(sat)
ICQ
ic
X- Operate in
Saturation region
Y- Operate in cut-
off region
ic(cutoff)
Vce(sat) Vce(cutoff)VCEQ
VCE
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LOAD LINE CONDITION 3 When the Q-point not in the middle of AC load line.
ic(sat)
Ic
ICQ
ic(cutoff)
Vce(sat) Vce(cutoff)VCEQ
VCE
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LOAD LINE IN POWER AMPLIFICATION Class A power amplifier
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DRAW DC LOAD LINE All capacitor will act as open circuit, AC source will short circuit.
() 0,
()
EC
CC
RR
V
+
()0, ()
Ic
VCE
ICQ
VCEQ
EC
CC
RR
V
+
VCC0
Ic(sat)
VCE(cutoff)
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DRAW DC LOAD LINE
E
BEBCQ
R
VVI
)(
21
2
ECCQCCCEQ
CCB
RRIVV
VRR
RV
+
+=
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DRAW AC LOAD LINEFrom the shown figure, equation 1:
Where
LCL
L
cecLcce
RRr
r
ViriV
//
@
=
==
Ic and Vce can be relates to thechanges in DC value, then, :
)3___(
)2_____(
CEQCEce
CQCc
VVV
IIi
=
=
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DRAW AC LOAD LINE
In saturation region, Ic=ic(sat) and Vce=Vce(sat)0v, thus
(2) (3) 1
L
CEQCECQCr
VVII
+
=
L
CEQ
CQsatcr
VIi +=)(
In cutoff region, VCE = Vce(cutoff) and Ic=ic(cutoff) 0A, thus
LCQCEQoffcutce rIVV += )(
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DRAW AC LOAD LINE
In saturation region, Ic=ic(sat) and Vce=Vce(sat)0v, thus
(2) (3) 1
L
CEQCECQCr
VVII
+
=
L
CEQ
CQsatcr
VIi +=)(
In cutoff region, VCE = Vce(cutoff) and Ic=ic(cutoff) 0A, thus
LCQCEQoffcutce rIVV += )(
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CLASS A POWER AMPLIFIER
Power delivered to load:
Power dissipated in transistor:
CQCEQDQ IVP =
( )2)(rmsV=
Power drawn from supply :
2
)(
)(
po
rms
L
VV =
CQCCdci IVP =)(
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CLASS A POWER AMPLIFIEREfficiency
100P
P
% i(dc)
o(ac)
=
Power Efficiency for CLASS A is between 25% to 50%
Continue with exercise 2.2
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CLASS A POWER AMPLIFIER with RC
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CLASS A POWER AMPLIFIER with RC The AC load line and DC load line overlap each other.
The Q-point is still in the middle of load line
.
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CLASS A POWER AMPLIFIER with RC Power consumption
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CLASS A POWER AMPLIFIER withTRANSFORMER COUPLED
This circuit uses a transformer to couple to the load. This improves the
efficiency of the Class A to 50%.
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CLASS A POWER AMPLIFIER withTRANSFORMER COUPLED
A transformer improves the efficiency because it is able to transform
the voltage, current, and impedance
Voltage RatioVoltage Ratio
22 NV
Current RatioCurrent Ratio
Impedance RatioImpedance Ratio
11 NV
2
1
1
2
N
N
I
I====
2
2
2
1
2
1
L
La
N
N
R
R
R
R====
========
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CLASS A POWER AMPLIFIER withTRANSFORMER COUPLED
DC Load LineDC Load Line
As in all class A amplifiers the Q-point is
established close to the midpoint of the
DC load line.
The saturation point (ICmax) is at Vcc/RLand the cutoff point is at V2 (the
secondary voltage of the transformer).
This increases the maximum output
swing because the minimum and
maximum values of IC and VCE are spreadfurther apart.
Continue with exercise 2.3
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CLASS B POWER AMPLIFIER
Power Amplifier class A :
has a low efficiency due to power dissipated by the transistor itself. This willeffect the power consumption at the load/output.
Improve the situation in Power Amplifier Class B where the transistor work
only in cut-off region and work only for 180 of the input signal and the
transistor dissipated only small amount of power.
With this a lot more power can be transmit to load and eventually increase the
efficiency of power amplification at the circuit.
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CLASS B POWER AMPLIFIER
In class B, the transistor is
biased just off. The AC signal
turns the transistor on.
The transistor only conducts
when it is turned on by one-half
of the AC cycle.
In order to get a full AC cycle out
of a class B amplifier, you need
two transistors:
An npn transistor that provides thenegative half of the AC cycle
Apnp transistor that provides the
positive half.
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CLASS B POWER AMPLIFIER: PUSH-PULL
CIRCUIT
OUTPUT SIGNAL ATPOSITIVE CYCLE
ON
OFF
OUTPUT SIGNAL AT
NEGATIVE CYCLE
ON
OFF
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CLASS B POWER AMPLIFIER: TRANSFORMERCOUPLE PUSH-PULL CIRCUIT
The center-tapped
transformer on the input
produces opposite polarity
signals to the two transistor
inputs.
The center-tappedtransformer on the output
combines the two halves of
the AC waveform together.
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CLASS B POWER AMPLIFIER: TRANSFORMERCOUPLE PUSH-PULL CIRCUIT
During the positive
half-cycle of the ACinput, transistor Q1(npn) is conducting and
Q2 (pnp) is off.
During the negativehalf-cycle of the AC
input, transistor Q2(pnp) is conducting and
Q1 (npn) is off.
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CLASS B POWER AMPLIFIER: CrossoverCrossoverDistortionDistortion
If the transistors Q1 and Q2 do not
turn on and off at exactl the
same time, then there is a gap inthe output voltage.
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CLASS AB POWER AMPLIFIER
Problem in Class B Power Amplifier:
1. Crossover Distortion
2. Thermal Runaway
Crossover Distortion problem:
1. Due to ON and OFF situation in ush- ull circuit.
2. Since the transistor in work in cut-off region only, thus the input voltage mustovercome the VBE voltage which usually 0.7v.
3. To improve this, add class A principle in class B power amplifier by adding
voltage divider at the base of the transistor and usually the R1 and R2 were
design to provide 1.4 v at the base.
4. The Class AB power efficiency for this case from 50% to 75%.
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CLASS AB POWER AMPLIFIER: CROSSOVERDISTORTION
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CLASS AB POWER AMPLIFIERThermal Runaway problem:
1. To maintain Q point in Class AB is difficult due to changes in VBE effected by
temperature.2. When temperature high, VBE decrease, increase and Ic increase.
3. This situation will damage the transistor.
4. Improve by adding diode at the base since diode share the same work
rinci le at base-emitter unction.
Figure 1: Graph of Ic versus VBE with temperature change
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CLASS AB POWER AMPLIFIER: THERMALRUNAWAY
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CLASS C POWER AMPLIFIER
A class C amplifier conducts for less than
180. In order to produce a full sine wave
output, the class C uses a tuned circuit (LC
tank) to provide the full AC sine wave.
radio communications circuits.
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CLASS D POWER AMPLIFIERA class D amplifier amplifies
pulses, and requires a pulsed
input.
There are many circuits that can
convert a sinusoidal waveform to a
pulse, as well as circuits that
convert a pulse to a sine wave.
This circuit has applications in
digital circuitry.