# An Energy-Efficient Integration of a Digital Modulator and a Class-D Amplifier
## 1 Introduction  
With the increasing demand for mobility, more devices incorporating digital wireless communication systems are being operated by electric accumulators (batteries), e.g., human surveillance to crops monitoring, and aerial drones to autonomous underwater vehicles (AUVs). Preserving an accumulator's energy is of paramount importance in difficult-to-access areas, which is also the case in underwater communication systems. If the system incorporates a communication transceiver, the most energy-consuming element is the power amplifier that follows the modulator. Designers have conceived different technologies for power amplification, which are denominated into classes and denoted by capital letters: Class A, B, AB, and D. Class-D amplifiers are the most efficient in terms of power, with values on the order of 90% in practical designs [1], and are preferred in applications where autonomy is essential.  
A traditional class-D amplifier consists of an analog modulator of a digital carrier followed by a switch circuit that drives the current from a DC power supply. This current passes to the load through a filter that retrieves the baseband signal. Due to their complexity, class-D amplifiers are not appropriate for small signals, but in power applications, they are the best choice because of their low losses. Obviously, the power delivered to the load is sensitive to the voltage of the DC source, and the switch has to be able to drive high current. In many of their applications, class-D amplifiers are used at high fidelity (hi-fi, 20 Hz-20 kHz) frequencies and the retrieving filter is a low-pass filter (LPF). If the amplifier is intended to be at the output stage of an ultrasonic communication system, a band-pass filter (BPF) is incorporated.  
Usually, a PW-modulator is available at the input stage of a class-D amplifier. The frequency of the digital carrier of the PW-modulator must be greater than the highest frequency of the analog input signal, typically ten times. However, again, if the amplifier is part of a digital communication system, the analog input signal to the amplifier will be the output signal of a digital modulator that could be a phase-shift keying (PSK) or a frequency-shift keying (FSK) modulator. That is, the digital signal at the input of the communication transmitter will modulate an analog carrier, modulate again a digital carrier at a much higher frequency, and finally be restored to the digital modulated signal.  
In our proposed design, integration of a PW-modulator of a class-D amplifier and BPF provides (a) signal amplification, (b) filtering, and (c) generation of a PSK-modulated output. The digital signal applied at the input of the class-D amplifier directly drives the power switch. This novelty in the design prevents the need of modulation before amplification, which makes the proposed design energy efficient as well as inexpensive. Finally, a BPF is used instead of a LPF at the output of the class-D amplifier to obtain an amplified PSK-modulated signal. This complex modulation is performed on a real-time basis using the ability of the class-D amplifier to provide high-efficiency in order to prolong battery life [2].  
The remaining article is structured as follows. Section 2 briefly explains the state-of-the-art. Section 3 provides the theoretical background. Section 4 discusses the block diagram of the integrated system. Section 5 presents the experimental setup and results. Finally, Section 6 has the concluding remarks.