Class D Amplifiers Operation and Applications

In this modern world, the main goal of audio amplification in an audio system is to accurately reproduce and amplify the given input signals. And one of the biggest challenges is to have high output power with as least amount of power loss as possible. Class D amplifier technology is making an increasing impact on the live sound world by offering high power with zero power dissipation and less weight than ever before. Nowadays, portable music devices are becoming more popular with the growing demand for external sounds in portable music devices.

Audio amplification is sometimes done with tube amplifier technology but these are bulky in size and not suitable for portable electronic sound systems. For most audio amplification needs, engineers choose to use transistors in linear mode to create a scaled output based on a small input. This is not the best design for audio amplifiers because transistors in linear operation will continuously conduct, generate heat, and consume power. This heat loss is the main reason why the linear-mode is not optimal for battery-operated portable audio applications. There are many classes of audio amplifiers; A, B, AB, C, D, E, and F. These are classified into two different operating modes, linear and switching.

Linear Mode Power Amplifiers – Class A, B, AB and class C are all linear mode amplifiers that have an output that is proportional to their input. Linear mode amplifiers do not saturate, fully turn-on or fully turn-off. Since the transistors are always conducting, heat is generated and continuously consuming power. This is the reason why linear amplifiers have lower efficiency when compared to switching amplifiers. Switching Amplifiers-Class D, E and F are Switching amplifiers. They have higher efficiency, which theoretically should be 100%. This is because there is no energy being loss to heat dissipation.

What is a Class D Amplifier?

Class D amplifier is a switching amplifier and when it is in the “ON” state it will conduct current but have almost zero voltage across the switches, therefore no heat is dissipated due to power consumption. When it is in the “OFF” mode the supply voltage will be going across the MOSFETs, but due to no current flow, the switch is not consuming any power. The amplifier will only consume power during the on/off transitions if leakage currents are not taken into account. Class D amplifier consisting of the following stages:

• PMW modulator
• Switching circuit
• Output lowpass filter

PMW Modulator

We need a circuit building block known as a comparator. A comparator has two inputs, namely Input A and Input B. When Input A is higher in voltage than Input B, the output of the comparator will go to its maximum positive voltage(+Vcc). When Input A is lower in voltage than Input B, the output of the comparator will go to its maximum negative voltage(-Vcc). The below figure shows how the comparator operates in a Class-D amplifier. One input ( let it be Input A terminal) is supplied with the signal to be amplified. The other input (Input B) is supplied with a precisely generated triangle wave. When the signal is instantaneously higher in level than the triangle wave, the output goes positive. When the signal is instantaneously lower in level than the triangle wave, the output goes negative. The result is a chain of pulses where the pulse width is proportional to the instantaneous signal level. This is known as ‘pulse width modulation’, or PWM.

Switching Circuit

Even though the output of the comparator is a digital representation of the input audio signal, it doesn’t have the power to drive the load (speaker). The task of this switching circuit is to provide enough power gain, which is essential for an amplifier. The switching circuit is generally designed by using MOSFETs. It is very crucial to design that the switching circuits produce signals that do not overlap or else you run into the problem of shorting your supply straight to ground or if using a split supply shorting the supplies. This is known as a shoot through, but it can be prevented by introducing non-overlapping gate signals to the MOSFETs. The non-overlapping time is known as Dead time. In designing these signals we must keep the dead time as short as possible to maintain an accurate low-distortion output signal but must be long enough to maintain both MOSFETs from conducting at the same time. The time that the MOSFETs are in linear mode must also be reduced which will help insure that the MOSFETs are working synchronously rather than both conducting at the same time.

For this application, power MOSFETs must be used due to the power gain in the design. The Class D amplifiers are used for their high efficiency, but MOSFETs have a built-in body diode that is parasitic and will allow the current to continue to freewheel during dead time. A Schottky diode can be added in parallel to the drain and source of the MOSFET to reduce the losses through the MOSFET. This reduces its losses because the Schottky diode is faster than the body diode of the MOSFET ensuring that the body diode does not conduct during dead time. To reduce the losses due to high frequency a Schottky diode in parallel with the MOSFET is practical and necessary. This Schottky ensures that the voltage across the MOSFETs before turning off. The overall operation of the MOSFETs and the output stage is analogous to the operation of a synchronous Buck converter. Input and output waveforms of the switching circuit are shown in the figure below.

Output Low Pass Filter

The final stage of a Class D amplifier is the output filter which attenuates and removes the harmonics of the switching signal frequency. This can be done with a common low pass filter arrangement, but the most common is an inductor and capacitor combination. A 2ndorderfilter is desired so that we have a -40dB/Decade roll-off. The range of cutoff frequencies is between 20 kHz to about 50 kHz due to the fact that humans can not hear anything above 20 kHz. The below figure shows the second-order Butterworth filter. The main reason we choose a Butterworth filter is that it requires the least amount of components and has a flat response with a sharp cut off frequency.

Applications of Class D amplifier

It is more suitable for portable devices because it does not contain any extra heat sink arrangement. So easy to carry. High power class D amplifier has become standard in many consumer electronic applications such as

• Television sets and home-theatre systems.
• High volume consumer electronics
• Headphone amplifiers
• Mobile technology
• Automotive

Thus, this is all about class D amplifiers operation and applications. We hope that you have got a better understanding of this concept. Furthermore, any queries regarding this concept or to implement any electrical and electronics projects, please give your feedback by commenting in the comment section below. Here is a question for you, What are the applications of the Class D amplifier?