The document presents an analysis of various power amplifiers, including their classifications (Class A, B, AB, and C) and operational characteristics. It discusses the concepts of power dissipation, efficiency, and thermal stability, along with practical examples and considerations for their design and application. Additionally, it addresses advantages, disadvantages, and references for further study in electronic circuit analysis.

1. 1
RAGHU INSTITUTE OF TECHNOLOGY
AUTONOMOUS
DEPARTMENT OF ELECTRONICS & COMMUNICATION
ENGINEERING
II BTECH II SEMESTER
ELECTRONIC CIRCUIT ANALYSIS
UNIT IV
Prepared By:
Mrs. SUSHMI NAIDU,
Mr. M. PAVAN KUMAR,
Assistant Professors
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

2. 2
Contents:
Classification of Amplifier
Class A Power amplifier and its Analysis
 Example
Harmonic Distortions
Class B push-pull amplifiers and its Analysis
 Example
Complementary Symmetry push-pull Amplifier
Class AB Power Amplifier
Class C Power Amplifier
Thermal Stability and Heat sinks
Distortion in Amplifiers
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

3. 3
 References
• “Integrated Electronics”, J. Milliman and C.C. Halkias, Tata
McGraw Hill, 1972
• “Electronic Circuit Analysis- B.V. Rao, K.R.Rjeswari,
P.C.R.Pantulu- Pearson publication
• “Electronic Devices and Circuits” – Salivahanan, Suresh Kumar,
A.Vallavaraj, TATA McGraw Hill
• www.nptel.ac.in
• www.nesoacademy.org
• www.tutorialspoint.com
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

4. IntroductionIntroduction
Power amplifiers are used to deliver a relatively high amount of
power, usually to a low resistance load.
Typical load values range from 300W (for transmission antennas)
to 8W (for audio speaker).
Although these load values do not cover every possibility, they do
illustrate the fact that power amplifiers usually drive low-
resistance loads.
Typical output power rating of a power amplifier will be 1W or
higher.
Ideal power amplifier will deliver 100% of the power it draws
from the supply to load. In practice, this can never occur.
The reason for this is the fact that the components in the amplifier
will all dissipate some of the power that is being drawn form the
supply.
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

5. Concept of Power AmplifierConcept of Power Amplifier
Provide sufficient power to an output load to
drive other power device.
To deliver a large current to a small load resistance
e.g. audio speaker;
To deliver a large voltage to a large load resistance
e.g. switching power supply;
To provide a low output resistance in order to avoid
loss of gain and to maintain linearity (to minimize
harmonic distortion)
To deliver power to the load efficiently
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

6. 6

7. Power Amplifier Power DissipationPower Amplifier Power Dissipation
P1
= I1
2R1
P2 = I2
2R2
ICQ
RC
RE
R1
R2
VCC
I1
I2
ICC
PC = ICQ
2
RC
PT = ITQ
2
RT
PE = IEQ
2
RE
IEQ
The total amount of power
being dissipated by the
amplifier, Ptot
, is
Ptot
= P1
+ P2
+ PC
+ PT
+ PE
The difference between this
total value and the total
power being drawn from the
supply is the power that
actually goes to the load –
i.e. output power.
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

8. Power Amplifier EfficiencyPower Amplifier Efficiency ηη
A figure of merit for the power amplifier is its efficiency, η .
Efficiency ( η ) of an amplifier is defined as the ratio of ac output
power (power delivered to load) to dc input power .
By formula :
As we will see, certain amplifier configurations have much higher
efficiency ratings than others.
This is primary consideration when deciding which type of power
amplifier to use for a specific application.
%100
)(
)(
%100 ×=×=
dcP
acP
powerinputdc
poweroutputac
i
o
η
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

9. “AUDIO AMPLIFIERS” – Unit-5
“RADIO FREQUENCY AMPLIFIERS” – Unit-6
DC POWER AMPLIFIERS
9
Power Amplifiers ClassificationPower Amplifiers Classification
(Depending on Type of output load)(Depending on Type of output load)

10. Power Amplifiers ClassificationPower Amplifiers Classification
Class-A Power Amplifier: Class-B Power Amplifier:
Class-C Power Amplifier:

11. Power Amplifiers ClassificationPower Amplifiers Classification
Class A - The transistor
conducts during the whole
cycle of sinusoidal input
signal
Class B - The transistor
conducts during one-half
cycle of input signal
Class AB - The transistor
conducts for slightly more
than half a cycle of input
signal
Class C - The transistor
conducts for less than half a
cycle of input signal
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

12. 12

13. Efficiency RatingsEfficiency Ratings
The maximum theoretical efficiency ratings of class-A,
B, and C amplifiers are:
Amplifier Maximum Theoretical
Efficiency, ηmax
Class A 25%
Class B 78.5%
Class C 99%
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

14. Steps for Analysis of Power Amplifiers:
Draw the Circuit Diagram of relevant type of Power
Amplifer
Draw the Graphical operating point representation
Make DC analysis (AC components as 0) & Calculate ‘Pdc’
Make AC analysis (DC components as 0) & Calculate ‘Pac’
Measure the Percentage Efficiency of the amplifier (% n)
Show the Power Dissipation
Advantages and Disadvantages
14

15. Class A AmplifierClass A Amplifier
νoutput waveform → same shape → νinput waveform + π phase
shift.
The collector current is nonzero 100% of the time.
→ inefficient, since even with zero input signal, ICQ is nonzero
(i.e. transistor dissipates power in the rest, or quiescent,
condition)
vin vout
Av
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

16. Class A Amplifier ClassificationClass A Amplifier Classification
Series FED/Direct Coupled Transformer Coupled
RC
+VCC
RE
R1
R2
RL
v in
ICQ
I1
ICC
i C
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

17.  Series FED/Direct Coupled Class A PowerSeries FED/Direct Coupled Class A Power
Amplifier AnalysisAmplifier Analysis
Common-emitter (voltage-divider) configuration (RC-coupled amplifier)
RC
+VCC
RE
R1
R2
RL
vin
ICQ
I1
ICC
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

18.  Typical Characteristic Curves forTypical Characteristic Curves for Class AClass A
OperationOperation
Configuration : No inductor @ transformer are used
(a) Common-emitter amplifier,
(b) dc load line (the Q point is at centre of the load line)
(c) instantaneous power dissipation versus time in the
transistor
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

19.  DC Input PowerDC Input Power
RC
+VCC
RE
R1
R2
RL
vin
ICQ
I1
ICC
The total dc power, Pi
(dc) , that an
amplifier draws from the power supply :
CCCCi
IVdcP =)(
1
III CQCC
+=
CQCC
II ≈ )( 1
IICQ
>>
CQCCi
IVdcP =)(
Note : That this equation is valid for most amplifier power analyses. We can
rewrite for the above equation for the ideal amplifier as
CQCCi IVdcP =)(
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

20.  AC Output PowerAC Output Power
R1//R2
vcevin
vo
ic
RC
//RL
rC
)()()( rmsormsco viacP =
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖
2
)(
Vm
V rmso =
2
Im
)( =rmsoI
2
Vpp
Vm =
2
Ipp
Im =
2
minmax VV
Vpp
−
= 2
ImIm inax
I pp
−
=
2
Im*
2
Im
*
2
)(
VmVm
acPo ==
8
*
2
2
*
2)(
IppVpp
IppVpp
acPo ==
8
)Im(Im*min)max(
)(
inaxVV
acPo
−−
=

21. Efficiency:
21
8
)Im(Im*min)max(
)(
inaxVV
acPo
−−
=
CQCCi IVdcP =)(
100*%
dc
ac
P
P
=η
100*
**8
**2
%
IcqVcc
IcqVcc
=η
Vmax=VCC & Vmin= 0
Imax=2Icq & Imin= 0100*
**8
)Immin)(Immax(
%
IcqVcc
inaxVV −−
=η
%25% =η

22. 22
 Advantages:
1) Circuit is Simple to Design
2) Less no. of Components
 Dis-Advantages:
1) Load Resistor is directly connected to collector. This causes
considerable wastage of Power
2) Power Dissipation is More
3) Efficiency is very poor, due to large power Dissipation

23. Transformer Coupled AmplifierTransformer Coupled Amplifier
• The theoretical maximum efficiency of a basic RC-coupled
class-A amplifier is limited to 25%.
• In practical circuit, the efficiency is less than 25%.
• Used for output power of about 1 W only.
• Transformer coupling can increase the maximum
efficiency to 50%
• Disadvantage of transformer coupling – expensive &
bulky.
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

24. Neglecting transformer resistance and assuming RE is small;
CCCEQ VV ≅
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

25. 25

26.  DC Input PowerDC Input Power
The total dc power, Pi
(dc) , that an
amplifier draws from the power supply :
CCCCi
IVdcP =)(
CQCC
II ≈ )( 1
IICQ
>>
CQCCi
IVdcP =)(
Note : That this equation is valid for most amplifier power analyses. We can
rewrite for the above equation for the ideal amplifier as
CQCCi IVdcP =)(
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖
i C

27.  AC Output PowerAC Output Power
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖
2
minmax VV
Vpp
−
=
)(1)(1)( rmsrmso viacP =
i C
)(2)(2)( rmsrmso viacP =
At Primary side A.C power generated:
RL
V rms
RLI rmsviacP rmsrmso
'
2
1'2
1)( )(1)(1 ===
2
*
2
*
2
)( 1111 mImVmImV
acPo ==
2
Vpp
Vm =
2
Ipp
Im =
2
ImIm inax
I pp
−
=

28. SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖
8
*
2
2
*
2)(
IppVpp
IppVpp
acPo ==
8
)Im(Im*min)max(
)(
inaxVV
acPo
−−
=
 We consider an idea Transformer, means Power delivered on primary side is equal
to the secondary side load, and the values of currents and voltages are similar.
CQCCi IVdcP =)(

29. Efficiency:
29
8
)Im(Im*min)max(
)(
inaxVV
acPo
−−
=
CQCCi IVdcP =)(
100*%
dc
ac
P
P
=η
100*
**8
**4
%
IcqVcc
IcqVcc
=η
Vmax=2VCC & Vmin= 0
Imax=2Icq & Imin= 0
100*
**8
)Immin)(Immax(
%
IcqVcc
inaxVV −−
=η
%50% =η

30. 30
Power Dissipation:
pACpDCpd −=
 In case of Class A amplifiers, Power Dissipation is maximum when no input
signal is applied.

31. 31
 Advantages:
1) Efficiency be somewhat high compared to series fed Class A
2) Impedance matching is required for maximum power transfer is
possible
 Dis-Advantages:
1) Due to Transformer, the circuit becomes bulkier, heavier and
costlier compared to series fed type
2) Frequency response of the circuit is poor
3) Circuit is complicated to design and implementation compared to
series fed type

32. •No cross over distortion
•No switching distortion
•Lower harmonic distortion in the voltage amplifier
•Lower harmonic distortion in the current amplifier
•No signal dependent distortion from the power supply
•Constant and low output impedance
•Simpler design
Class A advantages:Class A advantages:
Class A dis-advantages:Class A dis-advantages:
• Low Efficiency
• Power Dissipation more
• Wastage of Power
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

33. ExampleExample
Calculate the input power [Pi
(dc)], output power
[Po
(ac)], and efficiency [η] of the amplifier circuit for
an input voltage that results in a base current of
10mA peak.
RC
RB
+VCC = 20V
IC
Vi
25=β
Ω20
Ωk1
Vo
( )
%5.6%100
6.9)48.0)(20(
625.0)20(
2
10250
2
250)10(25
20
11000
20
20
4.10)20)(48.0(20
48.05.482)3.19(25
3.19
1
7.020
)(
)(
)(
232
)(
)(
)(
)()(
)(
=×=
===
=Ω
×
==
===
==
==
Ω
==
=Ω−=−=
≅===
=
Ω
−
=
−
=
−
dci
aco
CQCCdci
C
peakC
aco
C
CC
satc
B
P
P
WAVIVP
W
A
R
I
P
peakmApeakmAII
VVV
AmA
V
R
V
I
VAVRIVV
AmAmAII
mA
k
VV
R
VV
I
peakbpeakC
CCcutoffCE
CCCCCEQ
CQ
B
BECC
BQ
η
β
β

34. Class B Power AmplifierClass B Power Amplifier
 Consists of Push-Pull & complementary pair electronic devices
 One conducts for one half cycle of the input signal and the other
conducts for another half of the input signal
 When the input is zero, both devices are off, the bias currents are
zero and the output is zero.
 Ideal voltage gain is unity
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

35.  For input larger than zero, A turn ON and supplies current to the
load.
 For input less than zero, B turn ON and sinks current from the
load
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

36. Class-B transformer coupled push pull
amplifier
36
Driver Transformer
Load Transformer

37.  DC Input PowerDC Input Power
• DC biasing point is on the X-axis
so the Vceq=VCC. And Iceq
becomes 0. hence Q-point (Vcc,0).
mCCdcCC IVIVdcP
π
2
)( ==
π
IdcI
m
=)(1
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖
• Each Transformer output is an Half-rectified
Waveform. Then peak value of the output current is the average d.c current
π
IdcI
m
=)(2
πππ
IIIdcI mmm 2)( =+=

38.  AC Output PowerAC Output Power
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖
)(1)(1)( rmsrmso viacP = )(2)(2)( rmsrmso viacP =
At Primary side A.C power generated:
RL
V rms
RLI rmsviacP rmsrmso
'
2
1'2
1)( )(1)(1 ===
R
VRIVmVm
acP
L
mLm
o '
2'2
222
Im*
2
Im
*
2
)( ====

39. Efficiency:
39
100*%
dc
ac
P
P
=η
• Vm = VCC100**
4
%
V
Vm
CC
π
η =
%5.78% =η
2
Im*
)(
Vm
acPo =
mCC IVdcP
π
2
)( =
100**
4
%
V
Vcc
CC
π
η =

40. 40
Power Dissipation:
pACpDCpd −=
2
*2 ImV m
ImV ccpd −=
π
 In case of Class A amplifiers, Power Dissipation is maximum when no input
signal is applied. But in Case of Class B, when input is 0 then Vm=0. Hence
Power dissipation is very less compared to Class A.

41. 41
 Advantages:
1) Efficiency is much higher than Class A operation.
2) When there is no input signal, the power dissipation is zero.
3) Harmonics get cancelled. This reduces the harmonic distortion.
4) Ripples present in supply voltage also get eliminated.
5) Due to the Transformer impedence Matching is possible.
Dis-Advantages:
1) Frequency response of the circuit is poor
2) Center Tap transformers are necessary.
3) Transformers make the circuit bulky and hence costlier

42. Complimentary Symmetry Class B CircuitComplimentary Symmetry Class B Circuit
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

43. 43
Conducts
Non-Conduct
Non-Conducts
Conducts

44. 44
 Advantages:
1) As the circuit is transformer less, its weight, size and cost are less.
2) Due to the common collector configuration, impedance matching
is possible.
3) Frequency response of the circuit is improves due to transformer
less Class B amplifier circuit
Dis-Advantages:
1) Circuit needs two separate voltage supplies
2) Output is distorted to cross-over distortion

45. DEAD BAND
CROSSOVER DISTORTION
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

46.  Cross-over distortion free circuits
46
• Push-Pull Distortion Less circuit:

47. 47
• Complementary Push-Pull Distortion Less circuit:

48. Class AB Power AmplifierClass AB Power Amplifier
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

49. Class AB Voltage Transfer CurveClass AB Voltage Transfer Curve
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

50. Class C Power AmplifierClass C Power Amplifier
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

51. Load line Operation:
51

52. Collector Currents & Output CurrentCollector Currents & Output Current
CpLCn iii +=
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

53. Comparison of AmplifiersComparison of Amplifiers
Class A B C AB
Operating
Cycle
360˚ 180˚ Less that 180˚ 180˚ to 360˚
Position of Q-
Point
Center of Load
Line
On X-axis Below X-axis In between X-
axis to center
of Load line
Efficiency Poor,
25% to 50%
Better,
78.5%
High In between
78.5% to 50%
Distortion No Present Highest Present
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

54. Amplifier Distortion :
Amplifier Distortion can take on many forms such as
Amplitude, Frequency and Phase Distortion due to Clipping
Distortion in AmplifiersDistortion in Amplifiers
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

55. Amplitude Distortion :
Frequency Distortion :
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

56. Phase Distortion :
SUSHMINAIDU, M.PAVAN ECA Dept of ECE RIT‖ ‖ ‖

57. 57
Thank you

58. 58