This document discusses Class B amplifiers. It explains that Class B amplifiers use two transistors to conduct for alternating half cycles of the input signal, improving efficiency over Class A amplifiers. The theoretical maximum efficiency of a Class B amplifier is 78.5%. Circuit diagrams of common-collector and push-pull Class B amplifier configurations are presented, along with their input/output waveforms and operating principles. Distortion caused by crossover regions when the input signal is low is also discussed.

1. CLASS B
AMPLIFIER
Gauravsinh Parmar
(170410117023)
Neel Patel
(170410117033)

2. CLASS B AMPLIFIER
 Class B amplifier is a type of power amplifier
where the active device (transistor) conducts
only for one half cycle of the input signal. That
means the conduction angle is 180° for a Class B
amplifier. Since the active device is switched off
for half the input cycle, the active device
dissipates less power and hence the efficiency is
improved.
 Theoretical maximum efficiency of Class B
power amplifier is 78.5%. The schematic of a
single ended Class B amplifier and input , output
waveforms are shown

3. CIRCUIT DIAGRAM

4. CLASS B
 The class B amplifier uses two complimentary
transistor (NPN-PNP) for each half of the o/p
waveform.
 One transistor conducts for one-half of the signal
waveform while the other conducts for the other or
opposite half of the signal waveform. This means
that each transistor spends half of its time in the
active region and half time in cut-off region thereby
amplifying only 50%of the I/p signal.

5. COMMON-COLLECTOR CLASS B AMPLIFIER.

6. CLASS B PUSH-PULL AC OPERATION.

7. TRANSFER CHARACTERISTICS

8. CROSSOVER DISTORTION

9. TRANSFORMER COUPLED PUSH-PULL AMPLIFIERS. Q1
CONDUCTS DURING THE POSITIVE HALF-CYCLE; Q2
CONDUCTS DURING THE NEGATIVE HALF-CYCLE. THE TWO
HALVES ARE COMBINED BY THE OUTPUT TRANSFORMER.

10. BIASING THE PUSH-PULL AMPLIFIER TO ELIMINATE
CROSSOVER DISTORTION.

11. COLLECTOR WAVEFORM

12. TRANSFER CHARACTERISTIC

13. CLASS B OUTPUT STAGE
 Q1 and Q2 form two
unbiased emitter followers
 Q1 only conducts when the
input is positive
 Q2 only conducts when the
input is negative
 Conduction angle is,
therefore, 180°
 When the input is zero,
neither conducts
 i.e. the quiescent power
dissipation is zero

14. CLASS B CURRENT WAVEFORMS
Iout
IC1
IC2
time
time
time

15. CLASS B EFFICIENCY
Average power drawn from the
positive supply:
IC1
Phase, q
A/RL
0 p 2p
A sin(q)
   
LL
C
R
A
R
A
II
p
qq
p
qq
p
pp
  0
2
0
C11 dsin
2
1
d
2
1
  1CSve IVP 

16. EFFICIENCY / POWER DISSIPATION
Peak efficiency of the class B output stage is
78.5 %, much higher than class A.
Unlike class A, power dissipation varies with
output amplitude.
Remember, there are two output devices so
the power dissipation is shared between
them.

17. CLASS B
Class B amplifiers are used in low cost
designs or designs where sound quality is not
that important.
Class B amplifiers are significantly more
efficient than class A amps.
They suffer from bad distortion when the signal
level is low (the distortion in this region of
operation is called "crossover distortion").

18. ADVANTAGES
 The Collector efficiency quite high due to class B
operation.
 Distortion free o/p is obtained, they give more ac
o/p power. They are light in weight.
 o/p transformer use to push pull amplifier circuit
are light, smaller and less expensive.

19. DISADVANTAGES
Two identical transformer required.
It required two equal and opposite voltage at
the I/p.
If the parameter of the two transistor differ,
there’ll be unequal amplification of the two
halves of the signal which introduce more
distortion.