This chapter discusses power amplifiers. It defines power amplifiers as amplifiers used to deliver relatively high power, usually to low resistance loads. Power amplifiers are classified based on the conduction angle of the transistors, with Class A amplifying over the entire cycle but being inefficient, and Class B being more efficient but amplifying over only half the cycle. The chapter covers BJT and MOSFET power amplifiers, describing their characteristics and ratings. It also discusses concepts such as power dissipation, efficiency, load lines, and distortion in power amplifiers. An example calculation of efficiency for a Class A amplifier is provided.

1. CHAPTER 4
1
POWER AMPLIFIER

2. Outcome of Chapter 4
 Ability to perform simple DESIGN and EVALUATE
A, B and AB classes of BJT and FET amplifiers, in
terms of their frequency response, equivalent circuit,
termal management (power dissipation) and gain.
2

3. Outline
3
 Introduction
 Concept of Power Amplifier
 Power Amplifier Classification
 BJTs/ MOSFETs Power Amplifier
 Class A Power Amplifier

4. Introduction
 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.
4

5. Concept of Power Amplifier
5
 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

6. 6
Power 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.

7. 7
Power Amplifier Efficiency 
 A figure of merit for the power amplifier is its efficiency,  .
 Efficiency (  ) 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 



dc
P
ac
P
power
input
dc
power
output
ac
i
o


8. Power Amplifiers Classification
8
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

9. 9
Efficiency Ratings
 The maximum theoretical efficiency ratings of
class-A, B, and C amplifiers are:
Amplifier Maximum Theoretical
Efficiency, max
Class A 25%
Class B 78.5%
Class C 99%

10. BJT Power Amplifier
 Comparison of the characteristics and
maximum ratings of a small-signal and
power BJT
Parameter
Small-signal BJT
(2N2222A)
Power BJT
(2N3055)
Power BJT
(2N6078)
VCE (max) (V) 40 60 250
IC (max) (A) 0.8 15 7
PD (max) (W) 1.2 115 45
 35 – 100 5 – 20 12 – 70
fT (MHz) 300 0.8 1
10

11. Typical dc beta characteristics (hFE versus
Ic) for 2N3055
11

12. BJTs Power Amplifier
12
 Current gain is smaller in power amplifier BJT.
 The gain depends on IC and temperature may be related to the
following:
 maximum current that connecting wires can handle
 at which current gain falls below a stated value
 current which leads to maximum power dissipation.
 maximum voltage limitation associated with avalanche
breakdown in reverse-biased collector-base junction.
 second breakdown in BJT operating at high voltage and
current.

13. 13
VCE(sus) =115 volt at which these curve
merge and the minimum voltage
necessary to sustain the transistor in
breakdown.
The breakdown voltage, VCE0
~130 volt when the base terminal
is open circuited, IB=0

14.  Instantaneous power dissipation
 The average power over one cycle
 The maximum rated power,
C
CE
B
BE
C
CE
Q i
v
i
v
i
v
p 




T
C
CE
Q dt
i
v
T
P
0
1
14
C
CE
T I
V
P 

15. MOSFETs Power Amplifier
 Characteristic of two power MOSFETs
Parameter
Power
MOSFET
2N6757
Power
MOSFET
2N6792
VDS (max) (V) 150 400
ID (max) (A) (at T = 25C) 8 2
PD (max) (W) 75 20
15

16. Performance Characteristic of MOSFETs Power
Amplifier
16
 Faster switching times
 no second breakdown.
 Stable gain and response over wide temperature range.

17. 17
Class AAmplifier
 output waveform  same shape  input waveform + 
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

18. 18
Basic Operation
Common-emitter (voltage-divider) configuration (RC-coupled
amplifier)
RC
+VCC
RE
R1
R2
RL
vin
ICQ
I1
ICC

19. Typical Characteristic Curves for Class A
Operation
19
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

20. 20
DC 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 :
CC
CC
i
I
V
dc
P 
)
(
1
I
I
I CQ
CC


CQ
CC
I
I  )
( 1
I
ICQ

CQ
CC
i
I
V
dc
P 
)
(
Note that this equation is valid for most amplifier power analyses. We can rewrite
for the above equation for the ideal amplifier as
CQ
CEQ
i
I
V
dc
P 2
)
( 

21. 21
AC Output Power
R1//R2
vce
vin
vo
ic
RC
//RL
rC
AC output (or load) power, Po(ac)
Above equations can be used to
calculate the maximum possible
value of ac load power. HOW??
L
rms
o
rms
o
rms
c
o
R
v
v
i
ac
P
2
)
(
)
(
)
(
)
( 

Disadvantage of using class-A amplifiers is the fact that their
efficiency ratings are so low, max  25% .
Why?? A majority of the power that is drawn from the supply by a
class-A amplifier is used up by the amplifier itself.

22. 22
IC
(mA)
VCE
VCE(off) = VCC
IC(sat) = VCC/(RC+RE)
DC Load Line
IC
VCE
IC(sat) = ICQ + (VCEQ/rC)
VCE(off) = VCEQ + ICQrC
ac load line
IC
VCE
Q - point
ac load line
dc load line
L
PP
CQ
CEQ
CQ
CEQ
o
R
V
I
V
I
V
ac
P
8
2
1
2
2
)
(
2















%
25
%
100
2
2
1
%
100
)
(
)
(





CQ
CEQ
CQ
CEQ
dc
i
ac
o
I
V
I
V
P
P


23. 23
Limitation

24. 24
Example
Calculate the input power [Pi(dc)], output power
[Po(ac)], and efficiency [] of the amplifier circuit for
an input voltage that results in a base current of
10mA peak.
RC
RB
+VCC
= 20V
IC
Vi
25



20

k
1
Vo
 )
%
5
.
6
%
100
6
.
9
)
48
.
0
)(
20
(
625
.
0
)
20
(
2
10
250
2
250
)
10
(
25
20
1
1000
20
20
4
.
10
)
20
)(
48
.
0
(
20
48
.
0
5
.
482
)
3
.
19
(
25
3
.
19
1
7
.
0
20
)
(
)
(
)
(
2
3
2
)
(
)
(
)
(
)
(
)
(
)
(






































dc
i
ac
o
CQ
CC
dc
i
C
peak
C
ac
o
C
CC
sat
c
B
P
P
W
A
V
I
V
P
W
A
R
I
P
peak
mA
peak
mA
I
I
V
V
V
A
mA
V
R
V
I
V
A
V
R
I
V
V
A
mA
mA
I
I
mA
k
V
V
R
V
V
I
peak
b
peak
C
CC
cutoff
CE
C
C
CC
CEQ
CQ
B
BE
CC
BQ




25. Example
25
 The common source circuit parameters are
VDD=10V, RD=5kΩ and the transistor
parameters are Kn=1mA/V2, VTN=1V and
=0.
 Assume the output voltage swing is limited
to the range between the transition point and
vDS=9V to minimize nonlinear distortion.
 Calculate the actual efficiency of a class A
output stage.

26. Exercise
26
The Q-point of common source circuit is
VDSQ=4V
a) Find IDQ
b) Determine the max peak to peak
amplitude of a symmetrical sinusoidal
output voltage if the min value of
instantaneous drain current must be no
less than 0.1IDQ and the min value of
instantaneous drain source voltage
must be no less than vDS=1.5V.
c) Calculate the power conversion
efficiency where the signal power is
the power delivered to RL. Ans:
60mA,
5V,
31.25mW, 5.2%

27. Design of Class A C-E Amplifier
 To find R1, R2, RE, RC use the DC analysis and
design formula;
 To find Zi, Zo, Av, Ai use AC analysis (without
loading effect)
 To find Zi, Zo, Avs, Ai use AC analysis (with loading
effect if have Ri and RL
27
L
C
CQ
C
E
CC
CQ
CC
EQ
I
I
I
I
R
R
V
V
V
V
10
;
R
have
If
sheet.
data
in
given
10
;
2
;
10
L
2








28. Example & Exercise
 Will be given in our class.
28

29. Class B Power Amplifier
 Consists of 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
29

30.  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
30

31. Complimentary Push-Pull Circuit
31

32. DEAD BAND
CROSSOVER DISTORTION
32

33. The Ideal Class B
t
V
v p
o 
sin

t
R
V
i
L
p
Cn 
sin

i Cn
i Cp
t
V
v P
o 
sin

sin t
R
V
i
L
p
Cn 

33

34. • Maximum possible value of Vp is VCC
• The instantaneous power in Qn is;
t
V
V
v p
CC
CEn 
sin


Cn
CEn
Qn i
v
p 
 ) 








 t
R
V
t
V
V
p
L
p
p
CC
Qn 
 sin
sin
34

35.  The average power in Qn is
 Differentiating for maximum PQn with respect to Vp equal to
zero gives us
 Maximum average power dissipation;
L
p
L
p
CC
Qn
R
V
R
V
V
P
4
2



 )
L
CC
Qn
R
V
P 2
2
max




CC
P
L
p
L
CC
p
Qn
V
V
then
R
V
R
V
dV
dP
2
0
4
2




35

36.  The average power delivered to the load is
 Power source supplies half sinewave of current,
the average value is;
 The total power supplied by the two sources is
L
p
S
R
V
I


L
p
L
R
V
P
2
2
1











 

L
p
CC
S
CC
s
S
R
V
V
I
V
P
P











L
p
CC
S
CC
S
R
V
V
I
V
P

2
2
36

37.  The efficiency is
 )
 
%
5
.
78
785
.
0
4
when
efficiency
maximum
4
2
2
2
1












CC
P
CC
p
R
V
CC
R
V
S
L
V
V
V
V
V
P
P
L
P
L
P
37

38. Class AB Power Amplifier
Small quiescent bias
on each output
transistor to
eliminate crossover
distortion
38

39. Class C Power Amplifier
39

40. Class AB Voltage Transfer Curve
40

41. Collector Currents & Output Current
Cp
L
Cn i
i
i 

41

42. Example
The parameters are VDD=10V,
RL=20Ω. The transistor are
matched and K=0.2A/V2,
VT=1V, IDQ=0.05 when vo=5V.
Determine the required
biasing in a MOSFET class
AB output stage.
42

43. Inductively Coupled Amplifier
43

44.  The maximum possible average signal power delivered to the load
 The possible average signal power supply by VCC
 The maximum possible power conversion efficiency
L
CC
L
CQ
L
R
V
R
I
P
2
2
2
1
2
1
(max) 

L
CC
CQ
CC
S
R
V
I
V
P
2


%
50
5
.
0
2
1
(max)
(max) 2
2
2
1





L
CC
L
CC
R
V
R
V
S
L
P
P

44

45. Transformer Coupled Amplifier
45
• 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.

46. 46
Neglecting transformer resistance and assuming RE is small;
CC
CEQ V
V 

47. 47
a
v
v
ai
i C
L
1
2
and 

For ideal transformer;










2
1
ratio
turn
N
N
a
C
L
L
ai
a
v
i
v
R
/
1
2


2
1 1
a
i
v
R
C
L 

i C
C
L
i
v
R 1
' L
L R
a
R 2
'

48. 48
Turn ratio is designed for
maximum symmetrical
swing, hence;
The maximum average power delivered to load equals
maximum average power delivered to the primary of the
transformer
L
CQ
CC
CQ
CC
L R
a
I
V
I
V
R 2
2
2
' 


 ) CQ
CC
L I
V
P
2
1
max 
(VCC and ICQ are maximum
possible amplitudes of signal)

49. 49
The average power supplied by the VCC source is;
CQ
CC
S I
V
P 
The maximum possible efficiency is;
 ) %
50
5
.
0
max



S
L
P
P
