Power amplifiers are classified based on their operating point as Class A, B, or C. Class A amplifiers have their operating point at the middle of the load line, resulting in distortionless output but lower efficiency. Class B amplifiers have their operating point at the cut-off point, handling larger signals with higher efficiency but distorted output. Class C amplifiers operate beyond the cut-off point, severely distorting the output. Class A amplifiers are suitable for audio but cannot handle large signals, so Class B push-pull amplifiers using two amplifiers are often used instead to produce an undistorted output. Complementary symmetry amplifiers avoid using transformers by employing an NPN and PNP transistor at the output

1. Power Amplifiers
Unit – 4.1

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. 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

4. 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

5. 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

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

7. 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

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

9. Class A
Class B
Class C

10. 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

11. 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

12. 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

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

14. 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

15. 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

16. 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

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

18. 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

19. 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

20. 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

21.  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

22.  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

23. 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

24. T1
T2
Vcc Complementary Symmetry Amplifier

25. 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

26. 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

27. 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

28. Cross – over distortion
Vbe
-Vbe

29. 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

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

31. 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

32. 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

33. 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

34.  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

35.  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

36. End of Unit – 4.1