# An Energy-Efficient Integration of a Digital Modulator and a Class-D Amplifier
## 2 State-of-the-Art  
In the past few years, several class-D amplifier designs and modulation schemes have been presented, and some of them are briefly discussed in the following section.  
The class-D amplifier [3] is based on a digital pulse-width modulator. It receives digital input data and uses a digital loop filter to reduce distortion and mitigate the need of an analog-to-digital converter. The system is designed for low output power up to 1.2 W, and to keep away from the necessities of additional filtering, a sigma-delta modulator is used. The proposed system is designed using complementary metal-oxide-semiconductor (CMOS) technology and has been tested to drive a load of an 8 \(\Omega\) speaker with a signal-to-noise ratio (SNR) of 99.9 dB with 91% efficiency.  
A class-D amplifier based on the delta-sigma modulator with uniform pulse-width modulation is presented in [4]. The major design blocks are an interpolation filter, digital delta-sigma modulator, uniform pulse-width modulator, driver circuitry, and power stage. A 1-bit, 6 th-order noise shaping modulator with a uniform pulse-width modulator quantizer is used along with the pole moving technique to balance the modulator and make the noise transfer function more determined. The system has been simulated with a 1 kHz sine wave, and a 120 dB SNR has been achieved.  
In [5], a high-power pulse-width modulation model based on the class-D amplifier is discussed. To eliminate the effect of 3 rd and 5 th harmonics, the proposed triple-pulse model is designed by adding two narrow pulses with an equivalent comparative to the original single pulse. Initially, the model is verified by simulations, and then a platform is designed with underwater acoustic transducers submerged in a water tank to test the functionality of the system. From the experiments, it was found that the proposed amplifier consumes low power, which reduces 41% of the 3 rd and 5 th harmonic energy [5].  
A class-D amplifier using pulse-width modulation is designed in [6]. The system consists of modulation, and power and filter stages. In this research, two modulation techniques: (a) Pulse-width modulation and (b) pulse density modulation, are used. The system has been verified by PSpice simulations to test the total harmonic distortion using both modulation techniques with a 1 kHz test tone and switching frequency in the range of 300-800 kHz. From the simulations, it was found that the total harmonic distortion of the pulse density modulation is less at higher frequencies.  
A low-voltage class-D amplifier design is presented in [7]. The objective of the research is to propose a class-D amplifier for audio applications with minimum distortion. The analysis is performed using a pulse density modulation (PDM) technique for different test tone frequency ranges from 0.7 to 5 kHz with three amplitudes, 0.5, 0.8, and 1.0 V, using the carrier frequency range of 500-2000 kHz. From the results, it was found that the total harmonic distortion is a function of both amplitude and frequency of the test tone. For the highest switching frequency, i.e., 2000 kHz, and at the smallest amplitude of the input signal, the lowest value of total harmonic distortion was achieved at less than 1%.