AEC_21EC34_Module 3_Power Amplifiers.pdf

AEC NOTES

1. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 1
Subject Name: Analog Electronic Circuits
Subject Code: 21EC34

2. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 2
Module 3
Power Amplifiers

3. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 3
Module 3: Chapter 2

Output
Stages
and
Power
Amplifiers

4. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 4
Suggested Reading
Text 1: “Microelectronic Circuits, Theory and Applications,”
Adel S Sedra, Kenneth C Smith, 6
th
Edition, Oxford,
2015.ISBN:978-0-19-808913-1
Text 1:
Sections: 13.1, 13.2, 13.3(13.3.1, 13.3.2, 13.3.3, 13.4, 13.7)

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6. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 6
•Amplifiers are the circuits which increases the strength of
the input signal without changing the shape of the input
signal.

•An amplifier receives a signal from some pickup transducer
or other input source and provides a larger version of the
signal to some output device or to another amplifier stage.

•An input transducer signal is generally small and needs to
be amplified sufficiently to operate an output device.
Introduction:
Definitions and Amplifier types

7. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 7
There are different types of amplifiers:
1.Based on frequency
•Audio amplifiers
•Video amplifiers
•Tuned amplifiers
2.Based on input signal
•Voltage amplifiers
•Current amplifiers
•Power amplifiers
 If the input voltage is amplified Voltage amplifier
 If the input current is amplified Current amplifier
 If the current & voltage, both are amplified Power amplifier
A power amplifier is simply an amplifier with a high-power output stage. Since BJTs can handle much
larger currents than MOSFETs, they are preferred in the design of output stages

8. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 8
Module 3: Chapter 2
•Introduction
•Classification of output stages✓
•Class A output stage
•Series fed Class ‘A’ Amplifier
•Transformer coupled class ‘A’ Amplifier
•Class B output stage
•Transfer Characteristics
•Power Dissipation
•Power Conversion efficiency
•Class AB output stage
•Class C tuned Amplifier
Output
Stages
and
Power
Amplifiers

9. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 9
Model QP 1 Classification of Output stages / Power Amplifiers
Q: Explain with a neat sketches, how power amplifiers are classified.
Output stages are classified according to the collector current waveform that results when an
input signal is applied. Power amplifiers are classified based on the location of Q- point on the
dc load line.

•Class ‘A’ power amplifiers
•Class ‘B’ power amplifiers
•Class ‘AB’ power amplifiers
•Class ‘C’ power amplifiers
•Class ‘D’ power amplifiers

10. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 10
• Class A amplifiers are the usual means of implementing small signal amplifiers. They are not
very efficient.

11. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 11
• To get both the cycles of input as output, two class ‘B’ amplifiers are combined.
• Such a combination is called push – pull configuration.

12. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 12
Push – Pull configuration.

13. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 13
Class AB Amplifier :
• In class ‘B’ amplifier, the Q – point is at V
CE = V
CC and I
C = 0mA

• To bring the transistor back into active region, the input voltage V
i should exceed cut-in voltage
V
γ .

• During the period when V
i < V
γ , the output current will be zero and the distortion occurs at every
zero – crossing of the input signal. The distortion is called as cross – over distortion.

14. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 14
• To overcome the cross over distortion, the operating point is located slightly above cut – off.

• The amplifiers whose Q – point is present in between the centre and cut – off region are called
Class ‘AB’ amplifiers.

• The conduction angle is 180º < ϴ < 360º

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16. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 16
Class C:

• In class ‘C’ amplifier, the transistor is biased below cut – off.

• The amplifier whose Q – point is present below the cut – off region is called class ‘C’ amplifier.

• The output current flows for less than one half – cycle of the input cycle. Hence, the conduction
angle is less than 180°

17. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 17
Class D:

• This operating class is a form of amplifier operation using pulse (digital) signals.

• Class D amplifiers are much more efficient than Class AB power amplifiers.

• They are smaller and lighter in weight than an equivalent Class AB amplifier.

• All power devices in a Class D amplifier are operated in on/off mode.

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20. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 20
Comparison of various power amplifiers :

21. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 21
Module 3: Chapter 2
•Introduction
•Classification of output stages✓
•Class A output stage
•Series fed Class ‘A’ Amplifier ✓
•Transformer coupled class ‘A’ Amplifier
•Class B output stage
•Transfer Characteristics
•Power Dissipation
•Power Conversion efficiency
•Class AB output stage
•Class C tuned Amplifier
Output
Stages
and
Power
Amplifiers

22. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 22
 Class ‘A’ Power amplifiers

Depending upon how the load is connected at the amplifier output, class ‘A’ amplifiers are
classified into:

1.Series fed class ‘A’ amplifier
2.Transformer coupled class ‘A’ amplifier

23. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 23
•The circuit is called series fed class ‘A’
amplifier because the load Rc is connected in
series with the collector.

•This is similar to the small-signal amplifier
except that it will handle higher voltages.
The transistor used is a high power
transistor
1.SERIES-FED CLASS A AMPLIFIER (RC Coupled Amplifier)

24. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 24

25. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 25
AC Analysis of series-fed class A power amplifier
AC Operation

26. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 26
OUTPUT POWER
Input power:
Power Considerations

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28. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 28

29. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 29
Power Conversion Efficiency of Series fed class A Amplifier
Q: Prove that the power conversion efficiency of series fed class A amplifier is 25%

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31. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 31
 Advantages of Series fed class ‘A’ amplifier

• The circuit is simple to design and to implement

• The load is connected directly in the collector circuit hence the output transformer is
not necessary. This makes the circuit cheaper.

• Less number of components required as load is directly coupled.
 Disadvantages of Series fed class ‘A’ amplifier

• It has very poor efficiency i.e. 25%

• There is an impedance mismatch between the power amplifier and the load.

32. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 32
Module 3: Chapter 2
•Introduction
•Classification of output stages✓
•Class A output stage
•Series fed Class ‘A’ Amplifier ✓
•Transformer coupled class ‘A’ Amplifier ✓
•Class B output stage
•Transfer Characteristics
•Power Dissipation
•Power Conversion efficiency
•Class AB output stage
•Class C tuned Amplifier
Output
Stages
and
Power
Amplifiers

33. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 33
2.TRANSFORMER -COUPLED CLASS A AMPLIFIER
Q: Explain the operation of transformer coupled class A amplifier and prove that
maximum power efficiency is 50%.
•To improve efficiency, the transformer-coupled class A power
amplifier is preferred.

•In these amplifiers, a transformer is used to couple AC power
to the load.

•By adjusting the turn ratio of the primary windings to the
secondary windings, one can match the source and load
impedance for maximum power transfer.

•This makes transformed-coupled power amplifiers more
efficient as compared to RC-coupled power amplifiers, as
maximum power transfer can take place.

34. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 34
VOLTAGE TRANSFORMATION CURRENT TRANSFORMATION

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IMPEDANCE TRANSFORMATION

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37. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 37
Fig (a): Transformer-coupled class A amplifier
•This is also sometimes referred to as single ended
power amplifier. The term “single ended”
(denoting only one transistor) is used to
distinguish it from the push-pull amplifier using
two transistors. (fig. (a))

•The resistor R
1 and R
2 are used to bias the
transistor for class A operation.

38. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 38
•The DC and AC load lines for the
amplifier is shown in below fig (b).
These load lines are drawn for ideal
conditions.

•The DC load line is vertical at V
CC , while
the slope of the AC load line is -(1/R
ac),
where R
ac is the AC resistance of the
primary windings.


•The intersection of DC and the AC load
lines gives operating point of the
amplifier. The output signal will swing
from 0 to 2V
cc

39. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 39
Power Conversion Efficiency of Transformer coupled class A amplifier
Q: Prove that the power conversion efficiency of Transformer coupled class ‘A’ amplifier is 50%
Model QP 1

40. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 40
Note: The efficiency of the transformer coupled class A amplifier is twice that of the RC-coupled class A power amplifier.

41. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 41
Comparison of Series fed and Transformer coupled Amplifiers
Sl. No Series fed ‘Class A’ Amplifier Transformer coupled ‘Class A’ Amplifier
1
Load is directly connected in collector so
transformer not required
Output transformer is used to connect the load
2 Simple to design and implement Complicated to design
3
The output impedance is high hence cannot be
used for low impedances
Low impedance matching is possible due to transformer
4 Less number of components are required More number of components are required
5 The circuit is not heavier, bulkier, and costlier
The transformer makes circuit is heavier, bulkier, and
costlier
6 The maximum efficiency is 25% The maximum efficiency is 50%
7 The frequency response is better The frequency response is poor

42. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 42
Problem 3.2 :
Calculate the effective resistance seen looking into the primary of a 15:1 transformer connected to an 8Ω load.

43. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 43

44. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 44
Problem 3.3 :
What transformer turns ratio is required to match a 16 Ω speaker load so that the effective load resistance seen at the
primary is 10 k Ω ?

45. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 45

46. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 46
Problem 3.4 :
A transformer coupling is used in the final stage of multistage amplifier. If the output impedance of transistor is 1kΩ and the
speaker has a resistance of 10 Ω, find the turn ratio of the transformer so that power is transferred to the load ?

47. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 47

48. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 48
Problem 3.5 :
Determine the necessary transformer turn ratio for transferring maximum power to a 16Ω load from a source that has an
output impedance of 10kΩ. Also calculate the voltage across the external load if the terminal voltage of the source is 10V
r.m.s?

49. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 49

50. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 50
Problem 3.6 :
A class A power amplifier has a transformer as the load. If the transformer has a turn ratio of 10 and the secondary load is
100 Ω , find the maximum a.c. power output. Given that zero signal collector current is 100 mA.

51. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 51

52. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 52
Problem 3.7 :
A transformer-coupled class A power amplifier supplies power to an 80Ω load connected across the secondary of a step-
down transformer having a turn ratio 5:1. Determine the maximum power output for a zero signal collector of 120 mA.
Model QP 2

53. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 53

54. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 54
CLASS B TRANSFORMER -COUPLED POWER AMPLIFIER

55. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 55
CLASS B TRANSFORMER -COUPLED POWER AMPLIFIER
•In the class B amplifier, the operating point is located in the cut-off region, hence the
transistor conducts only for 180˚ or half of the input signal.

• To obtain the output for full cycle of the signal, two transistors are used.

•Since one part of the circuit pushes the signal high during one half-cycle and the other part
pulls the signal low during the other half-cycle, the circuit is referred to as a push-pull
circuit.

•The class B amplifier is used in RF transmitter circuits, in which the output signal of the
amplifier is fed to the transmitter antenna.

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57. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 57

58. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 58
Working of Class B push-pull power amplifier
• The input transformer is a 1:1:1 transformer which provides two equal voltages which are 180°
out of phase with each other.

• The voltage divider network consisting of R
1 and R
2 is used to keep Q
1 and Q
2 in the verge of
conduction.

• The output transformer is used to provide impedance matching between amplifier output and the
load.

• During positive half cycle of the input V
S, the voltage at C is positive while voltage at D is
negative going, i.e.,
the base-emitter junction of Q
1 gets forward biased
whereas
the base-emitter junction of Q
2 goes further below cut-off.

• Thus i
c1 increases above zero (because I
CQ = 0) and ic2 is zero.

59. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 59

60. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 60
Fig: Current flowing through each transistor in the class B amplifier

61. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 61
Conversion Efficiency of class ‘B’ power amplifier
Q: Prove that maximum conversion efficiency of idealized class B push pull amplifier is 78.50%
Model QP 2

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63. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 63

64. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 64
Module 3: Chapter 2
•Introduction
•Classification of output stages✓
•Class A output stage
•Series fed Class ‘A’ Amplifier ✓
•Transformer coupled class ‘A’ Amplifier ✓
•Class B output stage ✓
•Transfer Characteristics
•Power Dissipation
•Power Conversion efficiency
•Class AB output stage
•Class C tuned Amplifier
Output
Stages
and
Power
Amplifiers

65. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 65
CLASS B OUTPUT STAGE
(Complementary-Symmetry Circuits)
•Figure shows a class B output stage. It consists of a
complimentary pair of transistors (an npn and a pnp)
connected in such a way that both cannot conduct
simultaneously.

• Using complementary transistors (npn and pnp), it is
possible to obtain a full cycle output across a load using
half cycles of operation from each transistors.
When the input voltage v
I is zero, both transistors are cut
off and the output voltage v
0 is zero.

66. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 66
•The npn transistor will be biased into
conduction by the positive half cycle of the
signal.

•As v
I goes positive and exceeds about 0.5V, Q
N
conducts and operates as an emitter follower.
In this case v
0 follows v
I (i.e., v
0 = v
I ) and Q
N
supplies the load current.

•Meanwhile, the emitter-base junction of Q
P will
be reverse-biased. Thus Q
P will be cut-off.
During positive half cycle :

67. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 67
During negative half cycle :
•The pnp transistor is biased into conduction
when the input goes negative.

•If the input v
I goes negative by more than
0.5V, Q
P turns on and acts as an emitter
follower. In this case v
0 follows v
I (i.e., v
0 = v
I )

•In this case Q
P supplies the load current and
Q
N will be cut-off.
The circuit operates in a PUSH-PULL fashion: Q
N pushes (sources) current into the load when v
I is
positive, and Q
P pulls (sinks) current from the load when v
I is negative.

68. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 68
Module 3: Chapter 2
•Introduction
•Classification of output stages✓
•Class A output stage
•Series fed Class ‘A’ Amplifier ✓
•Transformer coupled class ‘A’ Amplifier ✓
•Class B output stage ✓
•Transfer Characteristics✓
•Power Dissipation
•Power Conversion efficiency
•Class AB output stage
•Class C tuned Amplifier
Output
Stages
and
Power
Amplifiers

69. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 69
Transfer Characteristics :
•A sketch of the transfer characteristics of the class
B stage is shown below.

•There exists a range of v
I centered around zero
where both transistors are cut off and v
0 is zero.

•The region is called dead band and it results in
the crossover distortion.

•Dead band: Dead band is an interval of a band
where no action occurs, the system is 'dead' or the
output is zero. It is also called as neutral zone.

70. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 70
The dead band in the class B transfer characteristic results in crossover distortion

71. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 71
Cross-over distortion
Model QP 2

72. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 72
Cross-over distortion
•For the transistor to be in the active region, the
input voltage V
i should exceed cut-in voltage (V
BE
or V
γ) which is generally 0.7 V for silicon and 0.2 V
for germanium transistors.

•If the transistors Q
1 and Q
2 do not turn on and off
at exactly the same time, then there is a gap in the
output voltage.

•During the period when V
i < V
γ , the output current
will be zero and the distortion occurs at every zero
– crossing of the input signal. The distortion is
called as cross – over distortion.
Model QP 2

73. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 73
Reducing Cross-over distortion
•The crossover distortion of a class B output stage can be reduced substantially by employing a high-gain
op-amp and overall negative feedback.

•The ±0.7 V dead band is reduced to ±0.7 /A
0 volt, where A
0 is the DC gain of the op amp.

Model QP 2

74. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 74
Module 3: Chapter 2
•Introduction
•Classification of output stages✓
•Class A output stage
•Series fed Class ‘A’ Amplifier ✓
•Transformer coupled class ‘A’ Amplifier ✓
•Class B output stage ✓
•Transfer Characteristics ✓
•Power Dissipation ✓
•Power Conversion efficiency ✓
•Class AB output stage ✓
•Class C tuned Amplifier
Output
Stages
and
Power
Amplifiers

75. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 75
CLASS AB OUTPUT STAGE :
•The amplifiers whose Q – point is present
in between the centre and cut – off region
are called Class ‘AB’ amplifiers.

•Crossover distortion can be virtually
eliminated by biasing the complementary
output transistors at a small nonzero
current. The result is the class AB output
stage as shown in the fig.

76. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 76
•A bias voltage V
BB is applied between the bases of Q
N and Q
P.

•For v
I = 0, v
0 = 0, and a voltage V
BB /2 is appears across the base-
emitter junction of each of Q
N and Q
P.

•Assuming matched devices



The value of V
BB is selected to yield the required quiescent
current I
Q

77. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 77
Circuit Operation
•When v
I goes positive by a certain amount, the voltage at the base of Q
N increases by same
amount and the output becomes positive at an almost equal value.

……………… .. (1)

•The positive v
0 causes a current i
L to flow through R
L, and thus i
N must increase; that is


……………… .. (2)


•Since the voltage between the two bases remain constant ay V
BB, the increase in v
BEN will result
in an equal decrease in v
EBP and hence in i
P .

78. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 78
•The relationship between i
N and i
P is given by

……………… .. (3)


……………… .. (4)

……………… .. (5)

•Thus as i
N increases, i
P decreases by the same ratio while the product remains constant.

•Substituting equation (2) in equation (5), we get



•For positive output voltages, the load current is supplied by Q
N, which acts as output emitter follower.
•For negative input voltages, the load current will be supplied by Q
P, which acts as output emitter follower.

79. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 79
Conclusion

•The class AB stage operates in much the same
manner as the class B circuit, with one important
exception.

•For small v
I, both transistors conduct, and as v
I is
increased or decreased, one of the two transistors
take over the operation.

•Since the transition is a smooth one, crossover
distortion will be almost totally eliminated.

•Figure shows the transfer characteristic of the class
AB stage.
Transfer characteristic of the
class AB stage

80. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 80
Module 3: Chapter 2
•Introduction
•Classification of output stages✓
•Class A output stage
•Series fed Class ‘A’ Amplifier ✓
•Transformer coupled class ‘A’ Amplifier ✓
•Class B output stage ✓
•Transfer Characteristics ✓
•Power Dissipation ✓
•Power Conversion efficiency ✓
•Class AB output stage ✓
•Class C tuned Amplifier✓
Output
Stages
and
Power
Amplifiers

81. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 81
CLASS C OUTPUT STAGE :
• In class ‘C’ amplifier, the transistor is biased below cut – off.

• The amplifier whose Q – point is present below the cut – off region is called class ‘C’ amplifier.

• The output current flows for less than one half – cycle of the input cycle. Hence, the conduction
angle is less than 180°
Why Class C Power Amplifiers?

•The class A power amplifier is biased in the active region to produce a linear output signal with
minimal distortion.

•Due to this biasing arrangements, the transistor remains ON even for no input signal. This
results in the poor efficiency of the class A power amplifiers.

•In order to improve efficiency, the class C power amplifier is used.

82. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 82
• In the class C amplifier, the transistor is biased such that it remains OFF for no signal conditions
and operates in the saturation region when an input signal is present.

• When the transistor is off, the current through it is very small and hence the transistor dissipates
negligible power.

• Similarly, when the transistor operates in saturation, the voltage across it is very small, and
again the power dissipation is small.

• Therefore, in the class C amplifier, as the transistor dissipates less power, its efficiency is higher
than that of the class A amplifier.

• However, the class C amplifier is highly nonlinear and produces distorted output.

• This drawback of the class C amplifier is overcome by connecting a low-pass filter at the output.

• The low-pass filter blocks all high-frequency harmonics and passes only signal frequency to the
load.

83. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 83
The schematic diagram of a class C amplifier is shown below.
Fig (a) : Class C output stage

84. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 84
The input and the waveforms at the collector terminal are shown below.
Fig (b) : The input and the waveforms at the collector terminal

85. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 85
Working of Class C Amplifier:
•When the input signal is positive and above the cut-in
voltage of the transistor, the transistor operates in the
saturation region.

• During this period, the output voltage is equal to the
saturation voltage of the transistor and remains constant as
long as the input signal is above the cut-in voltage.

• When the input voltage is less than the cut-in voltage, the
transistor remains off, while the induced emf in the inductor
provides the collector voltage as shown in fib (b).

• This output voltage is fed to the low-pass filter as shown in
fig (a). The low-pass filter suppresses the high-frequency
harmonics present at the collector and produces output
similar to the input signal.

86. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 86
Advantages of Class C power amplifier.
•High efficiency.
•Excellent in RF applications.
•Lowest physical size for a given power output.

Disadvantages of Class C power amplifier.
•Lowest linearity.
•Not suitable in audio applications.
•Creates a lot of RF interference.
•It is difficult to obtain ideal inductors and coupling transformers.

Applications of Class C power amplifier.
•RF oscillators.
•RF amplifier.
•FM transmitters.
•Booster amplifiers.
•High frequency repeaters.
•Tuned amplifiers etc.

87. Guruprasad KN, Assistant Professor, Dept. of E&C, ATME CE, Mysuru 87
Thank You