Class A amplifiers have the highest linearity because the transistor is always conducting. They are the least efficient at 30% due to continuous power loss. Class B amplifiers only conduct for half of the signal cycle, improving efficiency to 50% but introducing crossover distortion. Class AB balances efficiency and distortion by conducting more than half but less than the full cycle. Class C amplifiers have the greatest efficiency of 80% but introduce heavy distortion as they conduct for less than half of the input cycle. They are used for radio frequency amplification rather than audio.

1. POWER AMPLIFIERS

2. • INTRODUCTION TO POWER AMPLIFIERS
Amplifier system
• Power amplifier is meant to raise the power level of the i/p signal. In order
to get large power at the output, it is necessary that the i/p signal voltage is
large, so an electronic system a voltage amplifier always precedes the
power amplifiers are called large signal amplifiers
• Power amplifier does not amplify power. Power amplifier draws power
from DC supply connected to the output circuit and converts it into useful
AC signal power
• Power amplifier also defined as a device that converts DC power and
whose action controlled by the input signal
TRANSDUCER
SMALL
SIGNAL
AMPLIFIER
LARGE SIGNAL
AMPLIFIER
TRANSDUCER

3. • Transistor power amplifiers handle large signals .amplifiers are generally
classified according to their mode of operation
• The classification is based on the amount of transistor bias and amplitude
of the input signal
CLASS A AMPLIFIER
• Transistor is biased that the output current flows for the entire cycle of the
input signal
• The operating point is so selected that the transistor operates only over the
linear region of its load line (center of load line).
• Class A amplifier has the highest linearity over the other amplifier classes
(because its operate in the linear portion of the characteristics curve)
• The transistor never turns “OFF” which is one of the main disadvantage .
Output stage is always ON(conducting) all the time, so there is a
continuous loss of power in the amplifier
• Due to continuous loss of power amplifier create large amount of heating
• Problems: Low efficiency (30%) and over heating

4. The single output device conducts through a full 360 degree of the output
waveform

5. Series fed class A amplifiers
•It is a fixed bias circuit.
•This circuit is rarely used for power amplification due to its poor
collector efficiency

6. When an ac signal is applied to the amplifier, the output current and voltage will vary
about the operating point Q . In order to achieve the maximum symmetrical swing of
current and voltage (to achieve maximum output power ), the Q point should be located
at the centre of the dc load line.

7. Thus, the maximum collector efficiency of a class A series-fed
amplifier is 30% . In actual practice, the collector efficiency is
far less than this value .

8. Transformer coupled class A amplifier
This is similar to the normal amplifier circuit but connected with a transformer in the
collector load.

9. •Here R1 and R2 provide potential divider arrangement. The resistor
Re provides stabilization, Ce is the bypass capacitor and Re to prevent
a.c. voltage. The transformer used here is a step-down transformer.
•The high impedance primary of the transformer is connected to the
high impedance collector circuit. The low impedance secondary is
connected to the load (generally loud speaker).

14. Class A push pull amplifier.

15. • Q1 and Q2 are two identical transistor and their emitter
terminals are connected together.
• R1 and R2 are meant for biasing the transistors.
• Collector terminals of the two transistor are connected to the
respective ends of the primary of the output transformer T2.
• Power supply is connected between the center tap of the T2
primary and the emitter junction of the Q1 and Q2
• Base terminal of each transistor is connected to the respective
ends of the secondary of the input coupling transformer T1.
• Input signal is applied to the primary of T1 and output load
RL is connected across the secondary of T2.

16. • From the figure phase splited signals being applied to the
base of each transistors. When Q1 is driven positive using
the first half of its input signal, the collector current of Q1
increases. At the same time Q2 is driven negative using the
first half of its input signal and so the collector current of
Q2 decreases.
• From the figure understand that the collector currents of Q1
and Q2 ie; I1 and I2 flows in the same direction trough the
corresponding halves of the T2 primary. As a result an
amplified version of the original input signal is induced in
the T2 secondary. It is clear that the current through the T2
secondary is the difference between the two collector
currents. Harmonics will be much less in the output due to
cancellation and this is results in low distortion.

17. Class B amplifier
• The output current flows only during positive half cycle of the
input signal
• At zero signal , the collector current is zero and no biasing
system is required in class B amplifiers
• The operating point is selected at cut-off region
• Compare to class amplifiers average current is less, power
dissipation is less .so efficiency is increased (50%)
• Class B Amplifier only conducts through one half or 180
degree of the output waveform

19. Class B push pull amplifier.

20. • The circuit arrangement of the Class B push pull
amplifier is similar to the Class A push pull amplifier
except for the absence of the biasing resistors.
• T1 is the input coupling capacitor and the input signal
is applied to its primary.
• circuit that uses a balanced center-tapped input
transformer, which splits the incoming waveform signal
into two equal halves and which are 180o out of phase
with each other. Another center-tapped transformer on
the output is used to recombined the two signals
providing the increased power to the load.
• The transistors used for this type of transformer push-
pull amplifier circuit are both NPN transistors with
their emitter terminals connected together.

21. • Since the transistors are not biased they remains OFF when
there is no input signal and no current flows through the load.
Each transistor starts conduction only when the amplitude of
the input signal goes above the base-emitter voltage (Vbe) of
the transistor which is about 0.7 V. This improves the
efficiency but creates a problem called cross-over distortion
• COMPLEMENTARY SYMMETRY CLASS B AMPLIFIER
Check note book

23. Class AB power amplifier.
• Class AB is a combination of class A and class B
• Class AB amplifiers is currently one of the most common used types
of audio power amplifier design
• In Class AB configuration, the active elements (transistors) are
slightly biased so that the conduction angle is slightly more than
180° but much less than 360°.
• The transistors conduct for more than a half cycle but much less
than the full cycle. That means there will be no instant where both
transistors are OFF simultaneously and thus cross-over distortion is
eliminated.
• Class AB power amplifiers are slightly inefficient than the Class B
configurations but far better in terms of distortion when compared to
Class A configurations.
• Typical efficiency of a well designed class AB power amplifier is
around 70%.

24. Q point selected above the cut-off region

26. • The active elements used in this circuit (transistor Q1 and
Q2) are complementary symmetric and it means the
transistor are similar in all aspects except one is NPN and
the other is PNP.
• The use of this complementary pair eliminates the bulky
transformer for phase splitting the input signal for driving
the individual transistors.
• The NPN transistor alone will conduct the positive half
cycle and PNP transistor alone will conduct the negative
half cycle.
• NPN transistor will start conducting when its base voltage is
above the base emitter voltage (Vbe~0.7V) and a PNP
transistor will start conducting when its base voltage is
below the base emitter voltage (Vbe~ -0.7V).
• A forward biased diode will drop approximately 0.7V
across it and the biasing diodes used here will keep the
transistor slightly forward biased even if there is no input
signal.

28. Class C amplifiers
• Greatest efficiency ,poorest linearity
• Class C amplifiers are heavily biased so that the
output current is zero for more than one half cycle
of an input AC signal
• The conducting angle is less than 180 degree and
is generally around the 90 degree area
• Biasing gives efficiency around 80% to the
amplifier , it introduce a very heavy distortion of
the output signal
• Class C amplifiers are not suitable for use as
audio amplifiers

29. Commonly used in high frequency sine wave oscillators and certain types of radio
frequency amplifiers

30. Class C amplifier circuit diagram

31. • Biasing resistor Rb pulls the base of Q1 downwards and q
point set below the cot-off point in the DC loadline
• As the result the transistor will start conducting only after
input signal amplitude rise above base emitter voltage (.7v)
• The major portion of the input signal is absent in the output
signal.
• Transistor here is to produce a series of current pulses
according to the input and make it flow through the resonant
circuit.
• Inductor L and capacitor C forms a tank circuit which aids
in the extraction of the required signal from the pulsed
output of the transistor. (pulsed output converted to
complete sine wave of a particular frequency by using LC
circuit)
• Class C power amplifiers are never used for power
amplification but as tuned amplifier i.e. to amplify a
narrow band of frequencies near the resonant frequency