Basics of Power Amplifier

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Amplifiers are the devices that process and increase the signal amplitude, It takes weak signals as input and amplifies them and gives stronger signals at the output. The amplifiers do amplification to current, voltage and power and hence they are classified as per their job as a current amplifier, voltage amplifier and power amplifier.

Voltage amplifier

This device converts low voltage signal to high voltage signal for example if we give the 2V ac signal then it can amplify the signal and can give 4V ac signal.

Current amplifier

This amplifier amplifies the amplitude of input current

Power amplifier

As we know that power is the product of voltage and current so as the name this amplifier amplifies the power this means the product of voltage and current at the output of this amplifier is greater than the input and hence is capable of delivering large voltage /current.

Why do we require such an amplifier?

  • Handling High Amount of power (More than 1 W)
  • Less Power dissipation

Its Classification:

The power amplifier is classified into 5 types Class A, Class B, Class AB, Class C, Class D.

Class A Power Amplifier

This amplifier amplifies the whole waveform input i.e. it amplifies both Positive and negative halves of waveform. This is because the transistor is on during full cycle or you can say the transistor conducts during the full cycle and also has a conducting angle of 360 degrees.


  • Low signal distortion levels
  • Always Conducting
  • No turn-on time and charge storage problem
  • Having efficiency around 25 to 50 %
  • Generates a lot of heat

Class B Power Amplifier

Unlike Class A amplifiers this amplifier amplifies only half part of waveform either positive half or the negetive half of the waveform this is due to the amplifier conducting in only half part during amplification and having a conducting angle of 180 degree. This amplifier amplifies only half of the input signal.


  • Higher efficiency and efficient design in comparison with class A amplifier.
  • Having efficiency around 75- 78.5%
  • It has low heat dissipation
  • It starts conducting from 0.7v which makes it not usable for precise application.

Class AB power amplifier

This amplifier uses two transistors and work together and are active less than a full cycle but more than half cycle and have the conduction angle around 180 degrees and 360 degrees. It emerges from combining the power of class A and class B amplifiers.


  • Have efficiency around 50 – 60 %
  • Have two transistors working together
  • No Crossover distortion

Class C Power Amplifier

This amplifier is the most efficient and has the lowest operating cycle and linearity compared to other Class A, Class B and Class C amplifiers and has a conduction angle around 90 degrees and stays on for less than half the input cycle.


  • High Efficiency having efficiency around 80 – 90 %
  • High Distortion in signal making it useless for audio amplification
  • Lower power dissipation

Class D Power Amplifier

It is a nonlinear switching amplifier having two transistors functioning as a switch and converts the analog signal into pulse width modulation.


  • Have low Power dissipation
  • Very high Efficiency
  • Low power consumption
  • Accurate Output