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07/03/2018

Compare Class A, Class B and Class C Amplifier

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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