ACKTS UNIT 1.pdf analog circuits unit 1b

About analog circuits

1. Analog Circuits

2. Syllabus
UNIT I: POWER AMPLIFIERS [T1] [CO1]
•Classification of Power Amplifiers -Class A, B, AB & C power
amplifiers –push pull configuration, complementary symmetry
circuits, Distortion in Amplifiers. Harmonic distortion and Crossover
Distortion in Power Amplifiers–Conversion efficiency and relative
performance.
UNIT II: TUNED AMPLIFIERS [T1] [CO2]
•Introduction to Tuned Amplifiers, Q-Factor. single tuned capacitive
coupled amplifier, tapped single tuned capacitance coupled
amplifier, single tuned inductively coupled amplifier, stagger tuning,
synchronous tuned Amplifier.

3. Syllabus
UNIT III: WAVE SHAPING –Linear and Non-linear: [T2,T3] [CO3]
•RC high pass, low pass circuit response for sinusoidal, step, pulse, square, ramp &
exponential inputs-Differentiator –Integrator. Diode clippers-Transistor clipper-
clipping at two independent levels –Emitter coupled clipper-comparator-–
Applications of voltage comparators.
•Clamping operation –clamping with source, diode resistances-clamping circuits
theorem-practical clamping circuits.
•
UNIT IV: MULTIVIBRATORS: [T2] [CO4]
•Stable states of BistableMultivibrator-A fixed bias transistor Bistable
Multivibrator-A self biased transistor BistableMultivibrator-commutating
capacitor –Unsymmetrictriggering of BistableMultivibrator-triggering through
a unilateral device-symmetrical triggering –Schmitt trigger circuit.
•General operation of monostable-multivibrator, collector coupled mono-stable-
multivibrator-wave forms of collector coupled monostable-multivibrator-
Emitter coupled mono-stablemultivibrator-triggering of mono-
stablemultivibrator. Astable-multivibrator, collector coupled Astablemultivibrator
-Emitter coupled Astable-multivibrator. Designing of Bistable, Mono-stable and
Astable-Multivibrators.

4. Syllabus
UNIT V: TIME BASE GENERATORS: [T2] [CO2]
•General features of time base signals-sweep circuit using a transistor
switch-UJT,UJT characteristics, UJT as a sweep circuit, -General
considerations & principles of Miller & Boot strap time base
generators-the transistor miller time base-the transistor, Boot strap
time base generator-A simple current sweep transistor current time
base generator.
UNIT VI: SAMPLING and LOGIC GATES: [T2] [CO3]
•Basic operating principle unidirectional, Bidirectional sampling gates
using diodes, transistors-reduction of pedestal effect and sampling
oscilloscope.
•LOGIC GATES: Digital operation of a system-OR, AND, NOT, NAND
&NOR gates-DTL Logic–RTL Logic, TTL logic –comparison.

5. Text Books:
•[T1] Integrated electronics-J.Millimanand C.C.Halkias, MC Graw–
Hill-1972
•[T2] Pulse digital and switching wave forms-J. Millmanand H. Taub,
Tata McGraw-Hill, New Delhi,2001.
•[T3] Solid State Pulse circuits -David A. Bell, PHI, 4th Edn., 2002 .
•
References:
•[R1] Pulse and Digital Circuits –A. AnandKumar, PHI, 2005.
•[R2] Wave Generation and Shaping -L. Strauss
•[R3] Electronic Circuit Analysis-K.LalKishore, 2004, BSP

6. UNIT-I
Power Amplifiers
Analog Circuits

7. Contents
•Classification of Power Amplifiers
1.Class A Power Amplifier
2.Class B Power Amplifier
3.Class AB Power Amplifier
4.Class C Power Amplifier
•Push Pull Configuration.
•Complementary Symmetry Circuits
•Distortion In Power Amplifiers
•Harmonic Distortion & Cross Over Distortion In
Power Amplifiers
•Conversion Efficiency and Relative Performance.

8. Power Amplifier
•Power Amplifier basic function is to provide high power to the
load. Hence large voltage and current handling is required.
•Due to large power handling requirement, the transistor used
is a power transistor, which is large in size and having large
power rating.
•Heat sink is required.
•Power Amplifier is the last stage Of any amplifying System.
•It develops and feeds sufficient power to the load
•(Ex of load :Loud speaker, Servomotor etc.)
•It has the capability of handling Large signals. Hence it is also
called “ Large signal amplifier or Power Amplifier”.

9. Applications of Power Amplifiers
•Power amplifiers are used in Public Address systems.
•Radio Receivers
•Driving Servomotors in industrial Control systems
•TV Receivers
•Cathode Ray Tubes etc.

10. Public Address system
•Public address system consists of many voltage
amplifiers connected in cascade .
•Last stage of a Public Address system is a Power
amplifier.
•Input signal is a sound signal of a human Speaker.
•Sufficient voltage gain is obtained by the voltage
amplifiers .
•Last stage is capable of handling large voltage or current
Swings.

11. Public Address system

12. Features of Power Amplifiers
•Input signal or amplitude of the power amplifier is Large Of
the order of few volts.
•Out put of power amplifier has large current and Voltage
swings.
•Graphical Analysis is used in power amplifiers.
•Power amplifiers must have low output impedance.
•Common collector circuit is used in Power amplifiers. CE
circuit with Step down transformer is used for impedance
matching.
•Power Transistors are used in Power amplifiers.
•They are large in Size. They have large power dissipation
rating.

13. Features of Power Amplifiers
•Analysis of signal distortion is very important in Power
amplifiers.
•Power amplifiers supply large power to the loud
speakers. Hence power amplifiers are also known as
Audio Power Amplifiers.

14. Parameter Voltage Amplifier Power Amplifier
Gain Voltagegain Power Gain
Analysis Small signal analysis or h
parameter analysis
Large signal analysis
Transistors used High βtransistors Low βtransistors
Size of transistorsSmall in size Large in size
Heat sink Not Required Required
Coupling Elements Resistors and Capacitors Transformers
Amplification Both AF and RF signals AF signals only
Distortion analysisNot Required Required

15. Load line Analysis

16. •Figure shows a common emitter circuit with base as input and
collector as the output.

17. •These two points can be located to draw a straight line on the
output characteristics.
•A line having the reciprocal of the slope of the Load resistance
which is drawn on the output characteristic is called as” load line” .

18. •The characteristics are plotted for various values of I
B
•The intersection of the output characteristic curve and a load line is the
operating point.
•This point is known as Quiescent Point.
•Q point is respectively.
Note :
1.If an ac signal is super imposed, by the application of ac sinusoidal
voltage at the input , Base current varies sinusoidally.
2.Since the transistor is operated in the active region output is
linearly proportional to input signal.
3.Out put current is βtimes larger than input base current in CE
configuration.
4.Output collector current and output voltage is also varies
sinusoidally about its quiescent value.

19. Graphical representations of collector voltage and Collector
current Swings.

20. Classification of Power Amplifiers
•Position of Q point on the load line Decides the Classification of
Power Amplifiers.
➢Power amplifiers are divided as
1.Class A Power amplifier
2.Class B Power amplifier
3.Class AB Power amplifier and
4.Class C Power amplifier

21. Class A Power amplifier
•The power amplifier is said to be Class A amplifier if the Q
point and input signal are selected such that the output signal
is obtained for a full input cycle.
•Position of the Q-point is at the center of load line.
•In Class A Power amplifier output exists for the whole Input
signal.
•Conduction Angle is .
•Efficiency is Poor.
▪Conversion efficiency(ɳ) is 25% for Direct coupled and 50%
for inductively coupled amplifier.
•Distortion is low for small signals.

22. Waveforms of Class A operation

23. Class B Power amplifier
•InClassBpowerAmplifieroutputsignalisobtainedonlyfor
onehalfcycleforfullinputcycle.
•DuetotheselectionofQpointontheX-axis,thetransistor
remainsintheactiveregion,onlyforpositivehalfcycleofthe
inputsignal.
•Hencethishalfcyclereproducedattheoutput.Butina
negativehalfcycleoftheinputsignal,thetransistorentersinto
acut-offregionandnosignalisproducedattheoutput.
•Thecollectorcurrentflowsonlyfor180
0
(halfcycle)ofthe
inputsignal
•Efficiencyis78.5%
•Thereexistsdistortion.(outputisonlyhalfforhalfoftheinput
signal)

24. Waveforms of Class B Amplifier

25. Class C Amplifiers
•In this Q point and input signal are selected such that output
signal is obtained for less than half cycle for a full input cycle.
•Q point is shifted below x axis.
•Due to selection of Q point below X-axis, the transistor
remains in active region for less than a half cycle.
•Hence only less than half cycle reproduced at the output. For
remaining cycle of the input, the remains in cut-off region
and no signal is produced at the output.
•Conduction Angle is less than 180
0
•Efficiency is high and it is closed to 100%.
•Distortion is more.

26. Waveforms of Class “C” operation

27. Applications
•Not suitable for audio applications.
•Used in tuned Circuits, communication areas and RF
circuits.
•They are also called as tuned amplifiers. These are used
in Mixer circuits and wireless communication systems.

28. Class AB Power Amplifier
•TheQpointandtheinputsignalareselectedsuchthatthe
outputsignalisobtainedformorethanhalfoftheinput
signali.e.morethan180
o
butlessthan360
0,
forafull
inputcycle.
•QpointpositionisaboveX-axisbutbelowthemidpointof
loadline.
•Conductionangleis>180
o
butlessthan360
0.
•ClassABamplifiereliminates“CrossOverDistortion”.
•Qpointmovesawayfromthecentreoftheloadlinebelow
towardsxaxis,theefficiencyincreases.

29. Class AB Amplifier Wave forms

30. Comparison of Power Amplifier
Class A Class B Class C Class AB
Operatingcycle 360
0
180
0
Less than
180
0
180
0
to 360
0
Position of Q pointCentre of load
line
On X axis Below X axis Above X axis but
below the centre
of load line
Efficiency 25%-50% 78.5% closed to 100%.Higher than A but
lesser than B
Nature of output
current waveform
Collector
current flows
from
O to 360
degrees
Collector current
flows from
O to 180 degrees
Less than 180
Degrees
Greater than 180
Degrees

31. Comparison of Power Amplifier
Class A Class B Class C Class AB
Distortion Absent.
No Distortion
Present. Highest Present
Power dissipation in
transistors
Very High Low Very Low Moderate

32. Class A Power Amplifier
The class A amplifier further classified in to two types
1.directly coupled and
2.transformer coupled amplifiers
•In directly coupled amplifier, load is directly connected to collector
terminal.
•In transformer coupled amplifier , load is connected to the collector
terminal through transformer.

33. Series Fed, Directly coupled Class A Amplifier

34. DC operation
•The collector supply voltage V
CCand resistance R
B decides the
d.c. base-bias current I
BQ.
•The expression is obtained applying KVL to the input loop and
with V
BE=0.7V
V
CC=I
BQR
B+V
BE
I
BQ=(V
CC-0.7)/R
B
•The corresponding collector current is then
I
CQ=βI
BQ
•Apply KVL to the output circuit
V
CC=I
C QR
L+V
CE
V
CEQ=V
CC-I
CQR
L
•Hence the Q point can be defined as Q (V
CEQ,I
CQ)

35. D.C. Power Input
•The d.c. power input is provided by supply.
•With no a.c. input signal, the d.c. current drawn is the collector
bias current I
CQ.
•Hence D.C. power input is,
P
DC=V
CCI
CQ
•Even if ac input signal is applied ,The average current drawn
from the d.c. supply remains same. Hence the equation
represents DC power input to Class A series fed amplifier

36. AC operation
•When an input a.c. signal is applied, the base current varies
sinusoidally.
•Output collector current varies around its Quiescent value
while the out put voltage collector to emitter voltage varies
around the quiescent value.
•The varying output voltage and output current delivers ac
power to the load.
•AC analysis includes AC power out put, Conversion efficiency,
maximum efficiency and Power dissipation.

37. A.C. Power output
•V
max = Maximum instantaneous value of the collector (output)
Voltage
•V
min = Minimum instantaneous value of the collector (output)
Voltage
•V
PP = Peak to peak value of output voltage across load
•V
m= Amplitude of AC output Voltage

38. A.C. Power Output
•I
max = Maximum instantaneous value of the collector (output)
Current
•I
min = Minimum instantaneous value of the collector (output)
Current
•I
PP = Peak to peak value of output Current across load
•I
m= Amplitude of AC output Current
I
PP=I
max-I
min

39. A.C. Power Output
•V
RMS= RMS value of output collector voltage
•I
RMS = RMS value of output collector current

40. A.C. Power Output

41. •Alternative expressions of power output using RMS value .
•Alternative expressions of power output using peak value.
•Alternative expressions of power output using peak to peak
value

42. Efficiency or Conversion Efficiency
•This efficiency is also known as Conversion efficiency an amplifier

43. Maximum Efficiency
•Assumption: Maximum Voltage swing and current swing on
output Voltage and current respectively.
•By substituting the above values in equation 12

44. Maximum Efficiency
•Ideal conversion Efficiency of Class A Direct coupled Power
amplifier is 25%.
•Practical value is around 15%

45. Power Dissipation
•Power Dissipation defined as the difference between DC power
and AC power delivered to the load (Ex: Loud Speaker,
Servomotor)
•Maximum dissipation occurs when ac signal is Zero.

46. Advantages and Disadvantages
of Class A Direct coupled Power Amplifier
Advantages
•Simple, Easy to construct, design and implement.
•Number of components are less.
•It is cheap and occupies less space. It is not bulky since there is
no transformer.
Disadvantages
•Efficiency is poor and it is 25%.
•Wastage of Power
•Output impedance is high. Cannot be used for low impedance
loads
•Power dissipation is more. Heat sinks are required.

47. Applications
•Class A Direct coupled Power amplifier is more suited for audio
applications.

48. Transformer coupled Class A power Amplifier
•Impedance matching is necessary for the maximum transfer of
power.
•Output impedance of series fed Direct coupled amplifier is
High.
•Direct coupled amplifier is not suitable to match the loads like
loud speakers which are having low impedance.
•Proper Impedance matching is provided by using Transformer
coupled amplifier.
•Transformer is called out put transformer and the amplifier is
called Transformer coupled Class A amplifier.

49. Transformer coupled Class A power Amplifier
•N1 = Number of turns on primary
•N2 = Number of turns on secondary
•V1 = Voltage applied to primary
•V2 = Voltage on secondary
•Turns Ratio:
•The ratio of number of turns on secodaryto the number of turns on
primary is called turns ratio of the transformer denoted by n
•n= Turns ratio= N
1/N
2
Voltage Transformation:
•The transformer transforms the voltage applied on the primary side
to secondary side is proportional to turns ratio
•(V
2/V
1)= (N
2/N
1)) =n

50. Current transformation:
•The current in the secondary winding is inversely proportional
to the number of turns of the windings.
•(I
2/I
1) = (N
1/N
2) = (1/n)

51. Transformer –Impedance Transformation
•Like currents and voltages impedance seen from either side also
changes.
•Impedance of the load on secondary is
•Primary and secondary winding impedances are assumed to be Zero.
•Load impedance is , Gets reflected on the primary side and
behaves as if connected in the primary side.
•Such impedance transformed from secondary to primary is denoted
as

52. Reflected Load

53. Reflected Load

54. • is the reflected impedance and is related to the square of
turns ratio of transformer.
•For a step down Transformer secondary voltage is less than
primary.
•High voltage side is always high impedance side i.eprimary side.
• for a step down Transformer.
•In the amplifier analysis ,the load is on secondary while the
active device the transistor on primary.
•Hence in all calculations the reflected load impedance must
be considered rather than

55. Transformer coupled Amplifier.

56. Transformer coupled Amplifier.(cont….)
•Figure shows transformer coupled Amplifier.
•Loud speaker connected in the secondary acts as load having impedance
ohms.
•Transformer used is a step down transformer with turns ratio .
DC operation
•It is assumed that the winding resistances are zero ohms.
•There is no Dc voltage drop across the primary winding of the transformer.
•Slope of the Dc load line is reciprocal of dc resistance of collector circuit.
•It is zero in this case.
•Slope of the Dc load line is infinite.

57. Transformer coupled Amplifier.(cont….)
•By Applying KVL to the collector circuit
• (Since the drop across transformer winding is Zero).
•DC Power Input
•AC operation
•For analyzing AC operation we may need to draw AC load line.
•Load on the secondary is load impedance and

58. Transformer coupled Amplifier.(cont….)
•The reflected impedance is
•Load line is drawn with a slope of and
passing through the operating point i.eQuiscentpoint Q is called ac
load line.
•Output current i.ecollector current varies around its Quiescent value
when ac input signal is applied.
•Output voltage varies sinusoid ally around its Quiescent value which
is in this case.

59. Graphical Analysis (Include diagram)

60. Graphical Analysis (AC output Power)
•AC output power
•While calculating ac power developed across the primary winding of
a transformer primary values of voltage, current and reflected power
must be considered.
•(1).For the calculation of AC power-must be considered.
•For the calculation of Load Voltage, Load Current , Load power,
Secondary Voltage, Load resistance must be considered.

61. Graphical Analysis (AC Output Power)
• Magnitude of Peak value of Primary Voltage.
• Magnitude of Peak value of Primary Current.
• RmsValue of Primary Voltage
• RmsValue of Primary Current
•AC power developed on the primary is given by

62. Graphical Analysis (AC Output Power)
•Similarly ac power delivered to the load on secondary can be calculated
using secondary Quantities.

63. Graphical Analysis (AC Output Power)
•.
• Magnitude of Peak value of secondary or load voltage.
• Magnitude of Peak value of secondary or load Current.
• rmsvalue of secondary or load voltage.
• rmsvalue of secondary or load Current.

64. Graphical Analysis (AC Output Power)
•In General transformer is not ideal. Hence power delivered to load
on secondary is slightly less than the power developed by primary.

65. Graphical Analysis (AC Output Power)
•Slope of ac load line is
•Maximum Efficiency

66. Maximum Efficiency

67. Maximum Efficiency
•Assumption: Maximum output voltage swing and current swing are
assumed.
•Q point is exactly at the centre of the load line. For maximum swing
we can write

68. Maximum Efficiency

69. Maximum Efficiency
•Note:
•Maximum possible efficiency =50% (ideal case)
•Practical efficiency = 30 to 35%

70. •.
•Power Dissipation
•It is defined as the difference between ac power output and dc
power input.

71. Significance of Power Dissipation
•It decides the maximum power dissipation rating of a power
transistor to be selected for an amplifier.
•Advantages
•Efficiency of transformer coupled amplifier is higher than the direct
coupled amplifier.
•Impedance transfer for maximum power is possible.
•DC bias current does not flow through the transformer.

72. Disadvantages
•System is Bulky, Occupies space, costly.
•Difficult to design.
•Frequency response is poor.

73. SNO Parameter Series Fed Direct coupled Class
A
Transformer Coupled Class A
1 Load ConnectivityLoad is connected directlyto
the collector circuit
Load is coupledthrough the
transformer
2 Design Simple to design Complex to Design
3 Number of
components
Less number of componentsMore number of components
4 Impedance
matching
o/p impedance is high .cannot
be used for low impedance
Low impedance matchingis
possible due to transformer
5 Power Wastage of power Nowastage of Power
6 Cost Circuit is not bulky. less costCircuit is Bulky-More Cost.
7 Efficiency 25% 50%
8 Frequency
Response
Good POOR

74. Distortion in Power Amplifiers
•Distortion plays a very important role in Power amplifiers
•For faithful amplification amplitude, frequency and phase must be
faithfully reproduced at the output.
•Phase distortion and frequency distortion does not play significant
role in power amplifiers.
•Harmonic Distortion plays vital role in power amplifiers.
•Due to nonlinearity in the dynamic characteristics the wave form of
output voltage differs from that of the input signal which is known as
“Harmonic Distortion”

75. Harmonic Distortion
•Presence of harmonic components in the wave form which are not
present in the input signal.
•The component with frequency same as the input signal is called
fundamental frequency component.
•Additional frequency components present in the output signal are
having frequencies which are integral multiples of the fundamental
frequency.
•These components are called Harmonic Components or Harmonics.
•If the fundamental frequency is f hertz,thenoutput signal contains
frequencies such as 2f,3f ...etc.

76. Harmonic Distortion
•2f component-Second Harmonic
•3f component-Third Harmonic
Note : Fundamental component does not consider as the first
harmonic.
•Second Harmonic amplitude is largest.
•Second Harmonic distortion is more important in the analysis of
amplifiers.
•As the order of Harmonics increases its amplitude decreases.

77. Distortion Due to Harmonic Components

78. Distortion Due to Harmonic Components
•From the above figure it is observed that Distorted wave form can be
obtained by adding the fundamental and the harmonic Components.
•Percentage Harmonic Distortion can be calculated by comparing the
amplitude of each order of the harmonic with the amplitude of
fundamental frequency component.
•If the fundamental frequency component has an amplitude of
and the n thharmonic component has an amplitude of then the
percentage harmonic distortion due to nth harmonic component is
expressed as

79. Distortion Due to Harmonic Components
•.

80. Total Harmonic Distortion
•Total harmonic Distortion is defined as the effective distortion due to
all individual components.
•Mathematically it can be expressed as
•D= Total Harmonic Distortion
D = Total Harmonic Distortion

81. Second Harmonic Distortion (Three point
Method)

82. Second Harmonic Distortion (Three point
Method)
•Dynamic transfer characteristics of the transistor is assumed to be
parabolic (non linear in nature) for the analysis of Second Harmonic
Distortion.
•Input signal causes the base current swing cosine in nature.
•
•Due to this collector current swings around quiescent value.
•Relation between and is nonlinear.

83. •In equation no(4) last term represents second harmonic component and
hence it is concluded that second harmonic is present.

84. Total output collector current waveform
•Total output collector current waveform is shown in the below figure
in which collector current varies around its Quiescent value.

85. Total output collector current
•Total collector current can be expressed in terms of dc bias value , dc
signal Component, fundamental frequency and second harmonic
component as

86. Total output collector current
•Due to the presence of Harmonics DC current increases.
•Harmonics can be calculated by connecting milliammeter at the
output i.ein the collector circuit.
•Milliammeter readings must be observed with the presence of ac
inputsignal& without ac input signal.
•If the milliammetrereading is same in the presence of ac signal and
in the absence of ac signal then it indicates that there are no
harmonics present at the output.
•If there exists increase in the milliammetrereadingin the
presence of ac signal when compared with the absence of ac signal
then it indicates that harmonics are present at the output.

87. Mathematical analysis from the collector wave form
•At point1,ωt =0 by substituting in Equation 5
• by substituting in Equation 5

88. Mathematical analysis from the collector wave
form
•Hence the equations can be written as

89. Mathematical analysis from the collector wave
form (cont…)
•From Eqn13
•Substituting 15 in 14

90. Mathematical analysis from the collector wave
form(cont…)
•Eqn12-Eqn14 results
•From the above Equation
•By adding Eqn12 And eqn14
•But

91. Mathematical analysis from the collector wave
form(cont…)

92. Power output Due to distortion
.

93. •.

94. Analysis of Class B amplifiers
•Q point is located on X axis.
•Collector current flows only half cycle of the input signal.
•To get full cycle across the load pair of transistors is used in class B
operation.
•Two transistors conduct in alternate half cycles of input signal and a
full cycle across the load is obtained.
•The two transistors which are identical in characteristics are termed
as matched transistors.

95. Analysis of Class B amplifiers
•Depending on the type of transistors pnpor npnthe two circuit
configurations of class B are possible.

96. Class B operation
PushpullClass B
•In Class B push Pull two
transistors are of the same type
either pnpor npn.
Complementary symmetry class B
•In Complementary symmetry
class B
two transistors are
complementary to each other
one is pnpand the other is npn

97. Class-B Push pull power Amplifier

98. Few modifications

99. Class-B Push pull power Amplifier
•Circuit Discription
•Both the transistors are of the same type hence this is pushpullClass B
power amplifier.
•The two transformers in the push pull circuit are Input transformer T
R1or
Driver transformer and the output transformer T
R2or load transformer.
•Transformers : Input and out put transformers are Centre Tapped.
•Input Transformer is connected to the base of the transistor T
1.
•Input signal is applied to the base of the input Transformer.
•Input transformer drives the circuit hence it is known as driver Transformer.
•Centre tap on the Primary of the Input transformer is connected to the
supply voltage -V
cc.

100. Class-B Push pull power Amplifier (cont...)
•Out put Transformer
•Turns ratio of the output transformer is specified as 2N
1 : N
2
•Centre tap on the Primary of the out put transformer is connected
to the supply voltage +V
cc.
•Secondary of the out put transformer is connected to load i.eLoud
speaker.
•Transistors
•Transistors T
1 and T
2 are npntype. pnptransistors can also be used.
In that case supply voltage polarity is –V
ccinstead of + V
cc.
•Both the transistors are in CE configuration.

101. Class-B Push pull power Amplifier (cont...)
•Operation :
•During positive half cycles of input signal end A of the centre tapped
driver transformer will be positive ahilepoint B will be negative.
•The voltages in the two halves of the secondary of the driver
transformer will be equal with opposite polarity.
•Input signals applied to the base of the transistors T
1 and T
2 will be
180
0
out of phase.
•When point A is positive transistor T
1 is driven into active region and
T
2 is driven into saturation region. & Vice versa.

102. Class-B Push pull power Amplifier (cont...)

103. Class-B Push pull power Amplifier (cont...)
•T
1 & T
2 conducts for the positive and negative half cycles
respectively producing alternate positive & negative half cycles at
the load.
•Thus whole of the applied input signal appears across the load as
shown in the below wave forms.

104. Input and output waveforms of Class B push pull Power amplifier

105. DC Operation
•Biasing point is adjusted on X axis such that VCEQ= VCC andICEQ= 0.
•Coordinates of Q point are (VCC,0).
DC Power input
•Im is the peak value of output current of each transistor (ic1,ic2).
•DC or average value is .
•Two currents drawn by twotransistors are in the same direction.
•Total dc or average current drawn from the supply is
thealgebraic sum of the individual average current drawn by each transistor.

106. AC Operation
•With the application of ac input signal transistor T
1Conducts during
Positive half cycles and T
2Conducts during Negative half cycles

107. Ac operation-T
1Conduction
•When T
1 Conducts, lower half
of the primary transformer
does not carry any current.
•Only N
1 (Out of 2N1) number of
turns carry the current.

108. Ac operation-T2 Conduction
•When T
2 Conducts, upper half
of the primary transformer
does not carry any current.
•Only N
1 (Out of 2N1) number of
turns carry the current.

109. Load lines of Class b push pull Power amplifier
•Reflected load on the primary can be written as
•
•Step down turns ratio is
•During the calculation of reflected load the ratio
•becomes
•Each transistor shares equal load which is nothing but reflected load.

110. • is the peak value of current.

111. Load lines of Class b push pull Power amplifier

112. Ac power output
•.

113. Efficiency
•.

114. Efficiency
•.

115. maximum Efficiency
•.

116. Maximum Efficiency

117. Maximum Efficiency
•.
•As V
m increases ŋ increases
•Maximum value of
•

118. Power Dissipation
•.

119. Maximum Power Dissipation
•.
•Maxima or minima condition is
•Differentiating the above equation w.r.tV
m

120. Maximum Power Dissipation
•.
• for maximum power dissipation

121. •.

122. •Maximum power per transistor =

123. Advantages of Class B pushpullPower
amplifier
•Efficiency is high compared with Class A transformer Coupled.
•Power dissipation is zero in the absence of AC input signal.
•Even harmonics get cancelled. This reduces harmonic Distortion.
•Dc saturation of the core is avoided. Since current flows in opposite
direction.
•Ripples in the supply voltage is eliminated.
•Due to output transformer impedance matching is possible.

124. Disadvantages of Class B pushpullPower
amplifier
•Two centre tap transformers are necessary.
•System is bulky and costlier.
•Frequency response is poor.

125. Complementary symmetry class B pushpull
power Amplifier.
•In this one transistor is pnpand the other transistor is npn.
•The circuit is transformer less.
•To achieve proper impedance matching complementary transistors
are used in common collector configuration.
•Common collector has lowest output impedance and hence
impedance matching is possible.
•Voltage feed back can be used to reduce the output impedance.

126. Class B Push pull Power Amplifier

127. Positive Half Cycles -Operation
•Circuit is Driven from dual power supply
•Transistor
•During positive half cycles is driven in to active region and starts
conducting.
•If the same signal is applied to the base of it remains off sins it is
complementary of . It remains in the off condition during positive
half cycles of applied input voltage

128. Positive Half cycles operation

129. Negative Half cycles operation
•Transistor
•During negative half cycles is driven in to active region and
starts conducting.
•If the same signal is applied to the base of it remains off since it
is complementary of . It remains in the off condition during
negative half cycles of applied input voltage
•Note : Mathematical analysis is same as Class B push pull power
amplifier.
•Here load resistance value must be used since there is no
output transformer.

130. Negative Half Cycles

131. Advantages
•Circuit does not contain transformer. Hence its weight and cost are
less.
•Impedance matching is possible due to common collector
configuration.
•Better frequency response compared with class B push pull.
Disadvantages
1.Circuit needs two power supplies.
2.Cross over distortion results distorted output.

132. SNO Parameter PushpullClass B Complementary
symmetry class B
1 Transistors
Both are similar
Transistors.
pnpor npn
One Transistor is pnpand
the other one is npn
2 Transformer
Two transformers are
required .one at the
input and the other at
load
No transformer is required
3 Impedance matchingNo Impedance
matching.
Impedance matching is
possible due to CC
configuration
4 Frequency ResponsePoor Improved

133. SNO Parameter PushpullClass B Complementary symmetry
class B
5 Power Supply Single power supplyDual Power Supply
6 Weight & Cost Bulky and Costly Light & Less Cost
7 Efficiency High compared with
ClassA
High compared With
Pushpull