Class A
Amplifier
- If the collector
current flows all the time during full cycle of input signal, the power
amplifier is called as class A amplifier.
- The operating point Q lies at the centre of the load line.
- The power
amplifier is biased such that no part of the signal is cut off.
- As the output
waveform is same as the input waveform there is least distortion.
- The
transformer coupled class A power amplifier has maximum collector efficiency 50%.
- It means that the maximum 50% DC supply power is converted into AC power.
- The
efficiency of class A amplifier is less than 50% due to power losses in the
output transformer.
- The maximum power dissipation in the transistor occurs
under zero signal condition.
- Low power output and low collector efficiency.
Class B
Amplifier
- If the collector
current flows all the time during positive half cycle of input signal, the
power amplifier is called as class B amplifier.
- The operating point Q lies at
the collector cut off voltage.
- The power amplifier is biased such that negative
part of the signal is cut off.
- The input circuit is forward biased during
positive half cycle of input signal whereas the it is reverse biased during
negative half cycle.
- It is also called as half wave rectification.
- As the
negative half cycle is cut off, severe distortion occurs. Higher power output
and high collector efficiency.
- Range of collector efficiency 50 to 60%.
- The
class B amplifier is used in push pull arrangement in which one amplifier
operates in positive half cycle and other amplifier operates in negative half
cycle.
- The maximum collector efficiency of class B power amplifier is 78.5%.
Class C
Amplifier
- If the collector
current flows for less than half cycle of the input signal, the power amplifier
is called as class C amplifier.
- The negative bias is given to the base of
transistor therefore the collector current does not flow when the positive half
cycle of the signal starts.
- The class C amplifiers are never used for power
amplification.
- As they amplify narrow band of frequencies near the resonant
frequency, they are used as tuned
amplifiers.
- The maximum collector efficiency of class C power amplifier is
nearly 100%.
- As the power losses are very small in the high Q resonant circuit,
narrow pulse will compensate all the losses in the class C power amplifier.
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