Classes of Power Amplifiers !

Classes of Power Amplifiers !
Classes of Power Amplifiers !
Classes of Power Amplifiers !

The purpose of an audio amplifier is to increase the small audio signal voltage coming from a detector circuit. The output of the audio amplifier goes to the power amplifier, which increases the signal power in order to drive a speaker. The audio amplifier and the power amplifier work together to produce an audio output voltage that is converted into sound by the speaker. Here, we will introduce the main classes of power amplifiers.

Class A Amplifiers:

When a common-emitter common-collector or common-base amplifier is biased so that it operates in the linear region (within the load line) for the full 360° of the input cycle, it is a class A amplifier. In this mode of operation, the amplifier does not go into either cut-off or saturation; therefore, the output voltage waveform has the same shape as the input waveform. A class A amplifier can be inverting or non-inverting.

here are some important considerations when using class A amplifiers:

  • The Q-point must be centered so that a maximum output signal is obtained.
  • The maximum efficiency of a class A amplifier is 25%

Classes of Power Amplifiers !

Class B push-pull Amplifiers:

When an amplifier is biased so that it operates within the linear region for 180° of the input cycle and is in cut-off for 180°, it is called a class B amplifier. The main advantage of a class B amplifier over a class A is that the class B is more efficient. You can get more output power for a given amount of input power. A disadvantage of class B is that it is more difficult to implement the circuit in order to get a linear reproduction of the input waveform. The term push-pull refers to a common type of class B amplifier circuit in which the input wave is approximately reproduced at the output by means of using two transistors, one of which conducts for one half-cycle and the other conducts for the other half-cycle. When current flows through the devices, it will usually be dissipated in the form of heat.

Some important remarks concerning class B amplifiers:

  • Their Q-point is at cut-off for class B operation.
  • The maximum efficiency of a class B amplifier is 78.5%

    Classes of Power Amplifiers !

Class C Amplifiers:

This type of amplifiers is biased so that conduction occurs for much less than 180°. That means that they operate in the linear region for only a small part of the input cycle and it is then biased below cut-off. Class C amplifiers are more efficient than either class A or push-pull class B, which means that more output power can be obtained from class C operation. Because the output waveform is severely distorted, class C amplifiers are normally limited to applications such as tuned amplifiers at radio frequencies (RF) to produce a sinusoidal amplitude.

Remarks concerning class C amplifiers:

  • Under conditions of low power dissipation and high output power, their efficiency can approach 100%

Classes of Power Amplifiers !

Class AB Amplifiers:

Class A amplifiers are very inefficient. On the other hand, class B amplifiers cannot produce a clean audio signal because their output transistors are not biased all the time. A class AB amplifier is considered to be a compromise between the types of amplifier A and B. A class AB amplifier is a class B amplifier which has a small amount of bias current flowing through the output transistors at all times, which eliminates virtually all the distortion that can arise with class B amplifiers. The bias current flows because the output transistors always conduct current, even without an audio signal, as opposed to a B class transistor which has no current flowing through the output when there is no audio signal present. A class AB amplifier is more efficient than class A, less than class B, but without the distortion introduced by class B amplifiers. So if you remember the class B amplifier, there are two transistors, each conducting for half of a cycle. For class AB amplifiers, each device is conducting for more than half a cycle but less than the whole cycle.

  • The efficiency of class AB amplifiers is about 50%

Class D Amplifiers:

Class D ampliefiers use output transistors which are either completely turned on or completely turned off (operating in switch mode), significantly reducing the power losses in the output devices. When the transistors are conducting, there is virtually no voltage across the transistors and when there is a significant voltage across the transistor (switched off), no current flows. This is similar to the operation of a switch-mode power supply, which is very efficient. The audio signal is used to modulate a PWM carrier signal which drives the output devices, with the last stage being a low-pass filter to remove the high frequency PWM carrier frequency.

  • Efficiencies of 90-95% are possible with this kind of amplifier

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