AMPLIFIER HELPS US TO KNOW ABOUT THE LEVEL OF SIGNALS

1. Unit –II
High Power Amplifiers

2. Power Amplifier
• The function of a power amplifier is to raise the power
level of input signal.
• It is required to deliver a large amount of power and has
to handle large current.
• The base of transistor is made thicken to handle large
currents.

3. Difference between Voltage and Power Amplifier
Voltage amplifier increases the magnitude of voltage of the input
signal, while a power amplifier raises the power level of the input
signal.
Applications
The Voltage amplifiers are used in wireless communication &
broadcasting of signals
Power amplifiers are used in the output stages of audio amplifier systems
to drive a loudspeaker load

4. The main factors are:
• Efficiency
• Maximum Power Capability
• Impedance matching to the output device
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5. Amplifier Types
The amplifier conducts through the full 360of the input. The Q-point is
set near the middle of the load line.
The amplifier conducts through 180 of the input. The Q-point is setat
the cutoff point.
This is a compromise between the class A and B amplifiers. The
amplifier conducts somewhere between 180 and 360 . The Q-pointis
located between the mid-point and cutoff.
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Class C
The amplifier conducts less than 180 of the input. The Q-point is located
below the cutoff level

6. DIRECT COUPLED AMPLIFIER

7. Class AAmplifier
The output of a class Aamplifier
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.
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8. Working
• Here (Ic)Q and (Vce)Q represent no signal
collector current and voltage between
collector and emitter respectively
• When signal is applied, the Q-point shifts to
Q1 and Q2
• The output current increases to (Ic)max and
decreases to (Ic)min
• Similarly, the collector-emitter voltage
increases to (Vce)max and decreases to
(Vce)min

9. Advantages
(1) simple construction
(2) Distortion less output voltage
Disadvantage
(1) very low efficiency (25%)
(2) Large power dissipation in the transistors.
(3) Output Impedance is very large.

10. TRANSFORMER COUPLED CLASS A POWER AMPLIFIER
• Instead of connecting the load directly,
the output is connected to the load
through a transformer
• By adjusting the turn’s ratio (N1N2)
the output impedance is matched with
the load impedance.
• RL is the load connected in the
secondary of a transformer.
• RL’ is the reflected load in the primary
of the transformer.
• The number of turns in the primary are
n1 and the secondary are n2.
• Let V1 and V2 be the primary and
secondary voltages and I1 and I2 be the
primary and secondary currents
respectively.
The variation of collector voltage appears
across the primary of the transformer.

14. Class B Amplifier
A class B amplifieroutput
only conducts for 180 or
one-half of the AC input
signal.
The Q-point is at 0V on the
load line, so that the AC
signal can only swing for
one-half cycle.
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ICQ = 0 and VCEQ = VCE(cutoff).

15. The Q-Point Is at Cutoff
The class B amplifier is biased at the cutoff point
so that It is brought out of cutoff and operates in
its linear region
when the input signal drives the transistor into
conduction.

16. Push-Pull Amplifier
2 transistors
2 center tapped transformer

18. Class ABAmplifier
This amplifier is a compromise between the
class A and class B amplifier—the Q-point
is above that of the Class B but below the
classA.
The output conducts between 180 and
360 of the AC input signal.
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19. Class C
The output of the class C
conducts for less than 180 of the
AC cycle. The Q-point is below
cutoff.
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20. Amplifier Efficiency
Efficiency refers to the ratio of output to input power. The lower the amount
of conduction of the amplifier the higher the efficiency.
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21. This is similar to the
small-signal amplifier
except that it will handle
higher voltages. The
transistor used is a high-
power transistor.
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Direct Coupled Class-A Amplifier

22. When an input signal is
applied the output will
vary from its dc bias
operating voltage and
current. A small input
signal causes the output
voltage to swing to a
maximum of Vcc and a
minimum of 0V.The
current can also swing
from 0mA to ICSAT
(VCC/RC)
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Direct Coupled Class-A Amplifier

23. C
R
V2
C(rms)
Po(ac) 
Input Power
The power into the amplifier is from the DC supply. With no input
signal, the DC current drawn is the collector bias current, ICQ.
Pi(dc) VCCICQ
Output Power
or
Po(ac)
%η  100
Pi(ac)
Efficiency
CE(rms) C(rms)
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o(dc)
P  V I
Po(dc)  I2
C(rms)Rc
Direct Coupled Class-A Amplifier

24. Transformer-Coupled Class AAmplifier
This circuit uses a
transformer to couple to
the load. This improves
the efficiency of the Class
A to 50%.
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25. TransformerAction
A transformer improves the efficiency because it is able to transform
the voltage, current, and impedance
Voltage Ratio
V2 
N2
V1 N1
Current Ratio
I 2 
N1
I1 N2
Impedance Ratio
2
L
R
 a
2  2 
 N1 
2
  
N

R
R

L R1
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26. Transformer-Coupled Class AAmplifier
DC Load Line
As in all class A amplifiers the Q-
point is established close to the
midpoint of the DC load line. The
dc resistance is small ideally at 0Ω
and a dc load line is a straight
vertical line.
AC Load Line
The saturation point (ICmax) is at
Vcc/RLand 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 arespread
furtherapart.
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27. Transformer-Coupled Class AAmplifier
8
Po(ac)

(VCEmax VCEmin)(ICmax  ICmin )
(maximum)
Signal Swing and Output ACPower
The voltage swing:
VCE(pp)  VCEmax  VCEmin
The current swing:
Ic( p p)  IC max  IC min
The AC power:
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28. Transformer-Coupled Class AAmplifier
Efficiency
PQ  Pi(dc) Po(ac)

CEmin 
 CEmax  V
 VCEmax  VCEmin 
2
%η  50
V
Maximum efficiency:
Power input from the DC source:
Pi(dc) VCCICQ
Power dissipated as heat across the transistor:
Note: The larger the input and output signal,the
lower the heatdissipation.
Note: The larger VCEmax and smaller VCEmin, the
closer the efficiency approaches the theoretical
maximum of 50%.
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29. Crossover Distortion
If the transistors Q1 and Q2 do
not turn on and off at exactly
the same time, then there is a
gap in the output voltage.
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30. Class AB Push Pull Amplifier

31. Complementary Symmetry Class AB Amplifier

32. Class C Amplifiers
A class C amplifier conducts forless
than 180. In order to produce afull
sine wave output, the class C uses a
tuned circuit (LC tank) to provide
the full AC sinewave.
Class C amplifiers are used
extensively in radio communications
circuits.
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