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Basics of Power Amplifier


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Amplifiers are devices that process and increase the signal amplitude. By inputting weak signals, the device amplifies them and gives stronger signals at the output. Amplifiers that accept current, voltage and power are classified as current amplifiers, voltage amplifiers, and power amplifiers respectively.

Voltage amplifier

The device converts low voltage signals to a high voltage signals. For example, if we provide a 2V AC signal at the input, then the signal is amplified as a 4V AC signal at the output.

Current amplifier

Works similarly to a voltage amplifier, except a current is amplified.

Power amplifier

It works similarly to both voltage and current amplifier. Here, a low power signal is amplified to a high power signal. The amplifier is also capable of delivering large voltage/current. 

A power amplifier is required because:

  • It can handle a high amount of power (more than 1W).
  • It dissipates less power.

It is classified into 5 types: Class A, Class B, Class AB, Class C and Class D.

Class A Power Amplifier

It amplifies the entire waveform input i.e. it amplifies both positive and negative halves of the waveform. This is because the transistor is at an ON state during full cycle or the transistor conducts during full cycle. It has a conducting angle of 360 degrees.


  • Low signal distortion levels
  • Always conducting
  • No issues regarding turn-on time and charge storage
  • Efficiency of around 25 to 50 per cent
  • Generates a lot of heat

Class B Power Amplifier

Unlike Class A power amplifiers, a Class B power amplifier amplifies only half waveform; either the positive half or the negative half. This is because the amplifier conducts only during half cycle. It has a conducting angle of 180 degrees. 

This amplifier amplifies only half of the input signal.


  • High efficiency of 75 – 78.5 per cent and efficient design compared to Class A amplifier
  • Low heat dissipation
  • Starts conducting from 0.7V, which makes it unsuitable for precise application

Class AB power amplifier

This amplifier uses two transistors that work together. They are active less active than a full cycle but more active than half cycle. It has a conduction angle of around 180 degrees and 360 degrees. The Class AB power amplifier emerges by combining both Class A and Class B power amplifiers.


  • Efficiency of around 50 – 60 per cent
  • Two transistors work together
  • No crossover distortion

Class C Power Amplifier

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


  • High efficiency of around 80 – 90 per cent
  • High signal distortion, making unsuitable for audio amplification
  • Low power dissipation

Class D Power Amplifier

It is a nonlinear switching amplifier having two transistors. These function as a switch, which converts an analogue signal into a pulse width modulated signal.


  • Low Power dissipation
  • Very high Efficiency
  • Low power consumption
  • Provides accurate output

You can also check out the 1.5W Power amplifier circuit here.

Uses of Power Amplifier

These amplifiers are used in Live Sound Events for amplifying musical instruments and Singer voices.

Most modern Sound Systems, Home Theatre and Home Audio System contains audio power amplifier and Preamp amplifier as a single unit and is called as Integrated Amplifier

Video Courtesy: nptelhrd


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