Question

explain advantages and disadvantages of class A, B, AB, and D amplifiers, and know the approximate efficiencies of each. Plus, be able to explain how class D amplifiers function

Answer 1

Advantages of class A amplifier :

• The design of the circuit is simple
• The output signal is exact replica of the input signal so it provides high fidelity.
• The transistor conducts for entire cycle so no cross over distortion.
• It provides high frequency response.

DisAdvantages of class A amplifier :

• Poor efficiency.
• circuit looks like bulky because of usage of transformer.
• cost is high.

Advantages of class B amplifier :

• Provides high efficiency compared to the Class A configuration.
• The use of pushpull system in class B eliminates the even order harmonics in the output signal.
• DC components will not present in the output because the dc components of two collector currents will flow in opposite directions and cancell each other.

Disadvantages of Class B amplifier:

• cross-over distortion is present in this amplifier.
• Coupling transformers increases the cost and size.
• Transformer coupling causes hum in the output and also affects the low frequency response.

Advantages of class AB amplifier :

• The class AB amplifier provides a linear behavior.
• The design of this amplifier is very simple
• Cross over distortion is removed.
• Useful to design Hi-Fi systems and provides good sound quality.

DisAdvantages of class AB amplifier :

• Power dissipation of this amplifier is more.
• Cost will be more because of usage of additional circutary like Heat sinks.
• This amplifier has low power efficiency and the average efficiency is less than the 50%.

Class D amplifier :

The most basic type of class D amplifier is a pulse-width modulatior (PWM) with a triangle-wave oscillator. The below image shows a simplified block diagram of a PWM-based,Class D amplifier. It consists of a pulse-width modulator, two output MOSFETs, and an external lowpass filter to recover the amplified audio signal.

As shown in the image, the p-channel and n-channel MOSFETs operate as current-steering switches by alternately connecting the output node to VDD and ground. Because the output transistors switch the output to either VDD or ground, the resulting output of a Class D amplifier is a high-frequency square wave.

The output square wave is pulse-width modulated by the input audio signal. The resulting duty cycle of the square wave is proportional to the level of the input signal. When no input signal is present, the duty cycle of the output waveform is equal to 50%.

Then the output of switching controller is passed through low pass filter to recover the amplified audio signal from this PWM waveform.

Advantages of class D audio amplifier:

• High Eficiency that could be greater than 90% and approximately equal to 100%.
• Low power dissipation.

Disadvantages:

• Distortion is more compared to class AB , the distortion is caused because of the switching of the output devices.

Efficiencies of POWER AMPLIFIERS :

• Class A - 50%
• Class B - 78.5%
• Class AB - 50-78.5%
• Class C - 90%
• Class D - 100%