Class B amplifier is a type of power amplifier where the active device (transistor) conducts only for half cycle obey input signal. That means the conduction angle is 180 degree for a class B amplifier. Since the active device is switched off for half the input cycle, the active device dissipates less power hence the efficiency is improved. 78.5% is the theoretical maximum efficiency of Class B power amplifier. the schematic of a single ended class B amplifier and input, output wave form is shown in figure bellow.

(Single ended class B amplifier)

From the above circuit it is clear that the base of the transistor Q1 is not biased and the negative half cycle of the input waveform is missing in the output. Even though it improves the power efficiency, it creates a lot of distortion, only half of information present in the input will be available in the output and that is a bad thing. Single ended Class B amplifiers are not used in present day and they can be found only in some earlier gadgets. Output characterstics of a single ended class B power amplifier is shown in the figure below.

Class B power amplifier output characteristics:

One way to realize a practical Class B amplifier is to use a pair of active devices (transistors) arranged in push-pull mode where transistor conduct one cycle and another amplifier conduct another half cycle. The output from both transistors are then combined together get a scaled replica of the input. Transformer coupling is solution for this problem and such amplifiers are called class B amplifier.

Advantages of Class B amplifies-

  • High efficiency. when compared to the Class. A configuration.
  • Push-pull mechanism avoids even harmonics.
  • No DC components in the output (in deal case)

Disadvantages of Class B amplifier-

  • The major disadvantage is the cross-over distortion.
  • Coupling transformers increases the cost and size. It is difficult to find ideal Transformer.
  • Transformer coupling causes hum in output and also effect the law frequency response.

Transformer coupling is not practically in case of huge loads.

Also Read- Frequency response of OP-AMP

Sachidananda Dhar

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