An amplifier is an electronic device that is designed for the purpose of increasing signal waveform and level of voltage, current, or power and achieving this without changing other parameters of the waveform such as frequency or wave shape. Signals that are amplified are very often not sine waves, though many time they are. The amount of this increase is known as the gain of the amplifier. The actual principle of amplification is that a smaller current or voltage is used to control much larger current or voltage the same as done in active devices, BJT and FET. They can be small signal amplifier or large signal amplifier, which are voltage amplifier and power amplifier respectively. The general symbol of an amplifier is as shown below, and signal direction can be assumed as flowing from left to right of the diagram.


Not all amplifiers are the same, there is a clear distinction made between the way their outputs stages are configured and operate. Also amplifiers are classified according to the characteristics of the collector current waveform with an applied input. The classes are A, B, AB, C and D. Class A Amplifier The class A amplifiers are the simple designed amplifiers and the most commonly used one. They are power amplifier and the best class of amplifiers because of their low distortion level, it is the best in audio system sound. They are formed by the output stage devices which are biased for class A operation. It uses only one transistor amplifier whose amplitude of the input signal are such that the output current flows for the complete cycle (360) of the input signal.


  • The class is more suitable for outdoor music systems, since they contain low level of distortion · They are used in cars, and rarely accepted in a home


  • They have the highest linearity, which is its signal distortion is minimum.
  • It has high fidelity because of the output exact replica of an input signal
  • It has improved high frequency response because the active device is on full time, that is to say no time is required to turn on then device
  • The configuration of class A amplifier is the ideal operating mode, because there is no switch-off distortion to the output waveform even during the negative half of the cycle
  • They are the simplest form of power amplifier, using single switched transistor in the standard CE configuration.


  • The efficiency of class A amplifier is very poor.
  • Due to the large power supply and heat sink, class A amplifier is costly and bulky.
  • Due to the transformer coupling frequency response is not as good


Class B Amplifier

This class require the use of two transistor amplifiers to produce a complete waveform one NPN and another PNP types with both power transistor receiving same input signal together that is equal in magnitude and opposite in phase to each other. The two transistor are biased in such a way that, each transistor conducts during one half cycle of the input waveform. It improves a full power efficiency of class A power amplifier.


  • Each transistor dissipates zero, or almost zero, power in the quiescent condition, so efficiency is improved.
  • Distortion free output is obtained.
  • No DC components in the output(in ideal case)


  • It has poor linearity, that is a crossover distortion
  • Two identical transistors are required.
  • It is difficult to find ideal transformer.
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