power amplifiers

1. Power Amplifiers

2. Classification of Power Amplifiers
 Power amplifiers are classified based on the Q
point
 If the operating point is chosen at the middle of the
load line, it is called Class A amplifier
 If the operating point is chosen at the cut-off point
it is called Class B amplifier
 If the operating point is chosen beyond the cut-off
point it is called Class C amplifier
 It conducts for 3600

3. Concept Preview
• Efficiency is most important in power amplifiers.
• Poor efficiency means that much of the input
power is converted to heat.
• A class A amplifier conducts for the entire signal
cycle and has the lowest efficiency.
• A class B amplifier conducts for only half of the
signal cycle.
• A class C amplifier conducts for less than half of
the signal cycle.
• A class D amplifier switches between cutoff and
saturation.

4. Power
Amplifier
PIN
Efficiency =
Input signal
POUT
POUT
PIN
Output signal
HEAT = PIN - POUTHigh efficiency means less heat.

5. Efficiency
• The dc power supplied to an amplifier is
PIN = VCC xIDC
• Efficiency = POUT/PIN x 100%
• The maximum efficiency for Class A
amplifiers with a dc collector resistance
and a separate load resistance is 25%.
• Class A is usually not acceptable when
watts of power are required.

6. t
IC
t
IC
t
IC
t
IC
ISAT
A B
C D
The major classes of amplifier operation

7. Class and efficiency quiz
If POUT = 100 W and PIN = 200 W, the
efficiency is _________. 50%
The efficiency of an ideal amplifier is
__________. 100%
When efficiency is poor, too much of the input
is converted to ________. heat
An amplifier that conducts for the entire cycle
is operating Class _______. A
An amplifier that conducts for half the cycle
is operating Class _______. B

8. Concept Preview
• Class A amplifiers operate at the center of the load
line and have a large quiescent current flow.
• Class B amplifiers operate at cutoff and have no
quiescent current flow.
• Class B amplifiers are usually operated in push-
pull configurations.
• Class B amplifiers have crossover distortion.
• Class AB reduces crossover distortion.
• Bridge amplifiers provide four times the output
power and eliminate the output coupling capacitor.

9. Class A amplifier
 The Q point is chosen at the middle of load line
 This will give equal swing on either direction
 Both halves of the input comes at the output
 Hence Class A will give (amplitude) distortionless
output
 It can handle only small signals
 Its efficiency is less

10. Ic
Vce
Ib = 60μA
Ib = 50μA
Ib = 20μA
Ib = 30μA
Ib = 40μA
10mA
2mA
4mA
6mA
8mA
24 V0 V
Q
A
B
10mA
6mA
4mA
Class A

11. A large-signal amplifier can also be called a power amplifier.
This class A amplifier has a large quiescent collector current.
C
B
E
VCC = 18 V
RL = 12 ΩRB = 1.2 kΩ
CC
β = 60
ΙΒ =
VCC
RB
18 V
1.2 kΩ
= = 15 mA
IC = β x IB = 60 x 15 mA = 0.9 A

12. 0 2 4 6 8 10 12 14 16 18
0.2
0.4
0.6
0.8
1.0
1.2
1.4
VCE in Volts
IC in A
5 mA
0 mA
25 mA
20 mA
15 mA
10 mA
ISAT =
VCC
RL
18 V
12 Ω
= = 1.5 A
Q
This is a Class A amplifier.
PC = VCE x IC = 7.2 V x 0.9 A = 6.48 W

13. Class B amplifier
 The Q point is chosen at the cut-off point
 This will give swing only on one direction
 Only one half of the input comes at the output
 Hence Class B will give (amplitude) distorted
output
 It can handle large signals
 Its efficiency is high
 It conducts for 1800

14. Ic
Vce
Ib = 60μA
Ib = 50μA
Ib = 20μA
Ib = 30μA
Ib = 40μA
10mA
2mA
4mA
6mA
8mA
24 V0 V
Q
10mA
0mA
Class B

15. 0 2 4 6 8 10 12 14 16 18
0.2
0.4
0.6
0.8
1.0
1.2
1.4
VCE in Volts
IC in A
5 mA
0 mA
25 mA
20 mA
15 mA
10 mA
Q
This is a Class B amplifier.
PC = VCE x IC = 18 V x 0 A = 0 W
Its quiescent power dissipation is zero.

16. 0 2 4 6 8 10 12 14 16
0.2
0.4
0.6
0.8
1.0
1.2
1.4
5 mA
0 mA
25 mA
20 mA
15 mA
10 mA
ClassB
The collector signal
is too distorted for
linear applications.

17. C
B
C
B
E
E
+VCC
The complementary-symmetry Class B
push-pull amplifier has acceptable
linearity for some applications.
NPN
PNP

18. NPN
PNP
Class B

19. C
B
C
B
E
E
+VCC
Since the base-emitter junction potential
is 0.7 V, there is some crossover distortion.
NPN
PNP

20. C
B
C
B
E
E
+VCC
Crossover distortion is eliminated
by applying some forward bias
to the transistors (class AB).
NPN
PNP
1.4 V

21. 0 2 4 6 8 10 12 14 16 18
0.2
0.4
0.6
0.8
1.0
1.2
1.4
VCE in Volts
IC in A
Q
The quiescent power dissipation is moderate for class AB.
The efficiency is much better than class A.

22. Cap. required
+VCC
RL
RL
+VCC
Single-ended amplifier
A bridge-tied load provides four times the output
power for a given supply voltage and load resistance.
+VCC
2
Max.
Max. = VCC
Bridge amplifier
Max. = 2 x VCC
Max.

23. Class A, B, and AB quiz
Class A amplifiers are biased to operate near
the ________ of the load line. center
Class B amplifiers have their Q-points at
____________. cutoff
The conduction angle for class B is
_________. 180o
To reduce distortion, two class B transistors
are arranged in _____________. push-pull
Class AB is a solution for __________
distortion. crossover

24. Concept Review
• Class A amplifiers operate at the center of the load
line and have a large quiescent current flow.
• Class B amplifiers operate at cutoff and have no
quiescent current flow.
• Class B amplifiers are usually operated in push-
pull configurations.
• Class B amplifiers have crossover distortion.
• Class AB reduces crossover distortion.
• Bridge amplifiers provide four times the output
power and eliminate the output coupling capacitor.
Repeat Segment

25. Concept Preview
• Class C amplifiers are biased beyond cutoff for a
small conduction angle and high efficiency.
• Class C amplifiers used tuned tank circuits to
reduce distortion in RF applications.
• Class C amplifiers cannot be used in wideband
applications like audio.
• Class D amplifiers switch between cutoff and
saturation for very high efficiency.
• Class D amplifiers operate at a relatively high
switching frequency and often use PWM.
• Class D can be used in audio applications.

26. 0 2 4 6 8 10 12 14 16 18
0.2
0.4
0.6
0.8
1.0
1.2
1.4
A
B
C
AB
The class of an amplifier
is determined by the bias
which establishes the Q-point.
Class C is established by reverse
biasing the base-emitter junction.

27. Class C amplifier
 The Q point is chosen at the beyond the cut-off
point
 This will give only a partial swing in one direction
 Only a portion of the input comes at the output
 Hence Class C will give (amplitude) severely
distorted output
 It can handle large signals
 It conducts for less than 1800

28. Ic
Vce
Ib = 60μA
Ib = 50μA
Ib = 20μA
Ib = 30μA
Ib = 40μA
10mA
2mA
4mA
6mA
8mA
24 V0 V
Q`
10mA
0mA
Class C

29. Class A
Class B
Class C

30. Conduction Angles
& theoretical max. efficiencies:
 Class A = 360o
50 %*
 Class B = 180o
78.5 %
 Class AB ≅ 200o
(between A & B)
 Class C ≅ 90o
100 %
*Class A amplifiers are seldom driven to
maximum output and typically provide much
less efficiency.

31. C
B
E
VCC
RB
CC
VBB
Class C amplifier
VBB reverse biases the base-emitter junction.
Tank circuit
The transistor is
off for most of
the input cycle
and the conduction
angle is small.

32. VBB
0.7 V
0 A
VBE waveform
IC waveform
VCE waveform
Class C amplifier
waveforms
(with tank circuit)
Low VCE when IC is flowing

33. C
B
E
VCC
RB
CC
Class C amplifier with signal bias
The base-emitter
junction rectifies
the input signal
and charges CC.
Signal bias increases when the input
signal increases in amplitude.

34. IB >> 0
Three transistor operating modes:
IB = 0 IB > 0
Cutoff Linear
(PC > 0)
Saturation
PC = 0 in both of these modes

35. Distortionless amplifier
 Out of the 3 amplifiers, Class C is unsuitable as the
distortion is very heavy
 Class A is the best, as it gives distortionless output
 But Class A cannot handle large signals as
required by the Power Amplifier
 Though Class B gives heavy distortion, it gives out
one half of the signal perfectly
 And Class B can handle large signals

36. Class A Audio Amplifier
 As we have seen out of the 3 classifications, Class
A is the best, as it does not give any distortion
 Among the configurations, we know that CE is the
best as it gives maximum power gain
 A CE amplifier will have high output impedance
 Unfortunately for an audio amplifier, the output
device is the speaker which has a low impedance

37. Impedance Matching
 The speaker impedance is typically about 4 Ω
 Hence there is a mismatch between the high Zo of
the amplifier and the low impedance of the speaker
 This will result in loss of gain
 This can be avoided by connecting a transformer at
the output stage
 The primary winding will match the high Zo of the
amplifier while the secondary will match the low
impedance of the speaker

38. Class A Audio Amplifier
270 K 5.6 K
Re
Rb1
Rb2 Ce
Rc
Vcc
270 K
Re
Rb1
Rb2 Ce

39. Drawback
 The drawback of this circuit is that it cannot
handle large signals
 In a Class A amplifier, the operating point is
chosen around the middle of the load line
 If the signal exceeds the cut-off point, the output
current stops and any signal with a lower
amplitude will not come at the output
 Similarly, if the signal exceeds the saturation
point, the output current cannot increase any
further, even if the input signal increases

40. Ic
Vce
Ib = 60μA
Ib = 50μA
Ib = 20μA
Ib = 30μA
Ib = 40μA
10mA
2mA
4mA
6mA
8mA
24 V0 V
Q
A
B Class A

41. Class B Push-Pull Amplifier
 To avoid this we can use Class B which has a
greater signal handling capacity
 But Class B will give only one half of the signal
 Hence we can use 2 Class B amplifiers
 One for one half and one for the other half
 This type of amplifier is called Push-Pull
Amplifier

42. Class B Push-Pull
Vcc
T1
T2
T3
TR2
TR1

43. Push-Pull Circuit
 TR1 and TR2 are output transistors connected
back to back, with their emitters grounded
 The output transformer TR1 couples the push-pull
output to the speaker
 In the Push-Pull arrangement T1 conducts for one
half of the signal & T2 conducts for the other half
 Both are biased in Class B and each gives one half
of the signal & the combined output is coupled to
the speaker

44. Push-Pull Circuit
 The Driver Transformer TR2 gives 2 out of phase
signals
 During one half, the +ve half forward biases T1
while the –ve half reverse biases T2
 Thus when T1 conducts, T2 is cut-off & vice-
versa
 This way both the transistors conduct alternately
to give the full signal output

45. Class D Amplifier
 During the +ve half cycle Q1 gets Forward Bias and it
conducts
 During the -ve half cycle Q2 gets Forward Bias and it
conducts
 Thus both the transistors conduct alternately
 The amplifier works for 3600
 No distortion
 100% efficiency

46.  During the first half T1
conducts
 Ic flows from the
centre-tapping through
T1 to ground
 This half is coupled to
the speaker through
TR1
Working of Push-Pull Circuit
Vcc
T1
T2
T
3
TR2
TR1

47.  During the second half
T2 conducts
 Ic flows from the
centre-tapping through
T2 to ground
 This half is coupled to
the speaker through
TR1
Working of Push-Pull Circuit
Vcc
T1
T2
T
3
TR2
TR1

48. Drawbacks
 Though this circuit functions well it has a few
drawbacks
 Transformer coupling affects the quality of
output
 Phase shifting circuit is a must
 Both these drawbacks can be avoided if we use
one pair of PNP and NPN transistors at the
output

49. T1
T2
Vcc Complementary Symmetry Amplifier

50. Complementary Symmetry Amplifier
 This circuit uses one NPN transistor & one PNP
transistor at the output stage
 During the +ve half, T1(NPN) base gets forward bias &
it conducts while T2 (PNP) gets reverse biased and does
not conduct
 This gives one half of the signal at the speaker coupled
to the emitter

51. Complementary Symmetry Amplifier
 During the other half, T2 gets forward bias
and conducts while T1 gets reverse biased and
does not conduct
 Thus T1 & T2 conduct alternately giving a
distortionless output
 This circuit does not require a phase shifter

52. Cross – over distortion
 Class B Push-Pull amplifier has one limitation
 As the phase of the signal changes from +ve to –ve
(or vice-versa) one transistor stops conducting
while the other begins conducting
 But the transistor cannot conduct instantaneously
as it requires a minimum Vbe before it starts
conducting
 Thus as the signal crosses over zero, a distortion
occurs
 This is called Cross over distortion

53. Cross – over distortion
Vbe
-Vbe

54. Class AB amplifier
 This circuit overcomes cross-over distortion
 Biasing is done such that even if there is no input
signal, a small current keeps the output transistor
conducting
 This circuit uses 2 diodes whose characteristics
matches with that of the BE junction of the output
transistors
 Biasing resistors R1 & R2 are also identical values

55. T1
T2
Vcc Class AB amplifier
R1
R2
D1
D2

56. Symmetrical components
 Since R1 & D1 are identical to R2 & D2, the diode
junction as well as the output point will be at half
the supply voltage
 Because of symmetry both T1 & T2 will conduct
equally
 Even when there is no input signal, there will be a
current Icq = (I/2 Vcc – 0.6) / R1
 This will keep the output transistors conducting

57. Elimination of cross-over distortion
 Normally, during cross-over there will not be any
output till the non-conducting transistor gets the
minimum Vbe
 This causes distortion
 This has been eliminated here, since the 0.6 V
across the diodes keep the transistors on and gives
a continuous output signal without producing
cross-over distortion

58. Thermal stability
 In addition, the two diodes also provide thermal
stability
 They prevent the output transistors going to
Thermal Run Away
 When the output current is high, heat dissipation is
more
 The increase in temperature produces more charge
carrier in the BE junction of T1 & T2

59.  This increases Ib & hence Ic
 This in turn increases the power dissipation &
hence the heat
 This chain goes on till too much current flows and
destroys the transistors
 This is called Thermal Run Away
 This is arrested by the diodes in the output circuit

60.  When the charge carriers increase in the B-E
junction of T1 & T2, a similar increase takes place
in D1 & D2, due to matching characteristics
 This increase in the diode current, produces more
drop across R1 & R2 and brings down the forward
bias at the base of T1 & T2
 Thus the 2 diodes prevent cross-over distortion as
well as provide thermal stability

61. A switch-mode amplifier uses a
rectangular input signal to drive the
transistor rapidly between cutoff and
saturation. The efficiency is very high.
C
B
E
RB
They are also
called Class D
amplifiers.

62. If the switching frequency is a good deal higher
than the signal frequency, a Class D amplifier is
capable of linear amplification. Pulse-width
modulation and a low-pass filter are often used.
PWM Signal
Input Signal

63. PWM LPF
The low-pass filter rejects
the switching frequency.

64. Class C and D quiz
Class C amplifiers use _______ circuits to
restore sinusoidal signals. tank
The base-emitter junction in a class C
amplifier is ________ biased. reverse
The theoretical maximum efficiency for
class C is ___________. 100%
Class D amplifiers are also known as
__________ amplifiers. switch-mode
Class D amplifiers employ a varying duty-
cycle known as _________. PWM

65. Concept Review
• Class C amplifiers are biased beyond cutoff for a
small conduction angle and high efficiency.
• Class C amplifiers used tuned tank circuits to
reduce distortion in RF applications.
• Class C amplifiers cannot be used in wideband
applications like audio.
• Class D amplifiers switch between cutoff and
saturation for very high efficiency.
• Class D amplifiers operate at a relatively high
switching frequency and often use PWM.
• Class D can be used in audio applications.
Repeat Segment