Computer Simulation of Nonlinear Distortion of Class D Sound Amplifier on GaN-Transistors

Class A, B, AB and D amplifiers are used in the field of sound reproduction equipment, but each has its own peculiarities. The principle of operation of class D amplifiers is based on pulse width modulation. Transistors act as switches, either in the closed or opened state without intermediate values. The efficiency of real class D amplifiers is approximately 90%, in the most economical models 95%, and they have low dependence of the output power. The class D amplifiers are compact in size. For this paper, GaN-transistors were selected as the base. Because they are, compared to silicon transistors, low resistance in the open state, high breakdown voltage, higher switching speed of the transistors. They also work at high temperatures.

The nonlinear distortions is negative features of Class D amplifiers. They directly affect the sound quality. The distortions include nonlinearity, which is determined by the modulation method and the "dead" period, which is necessary to prevent the occurrence of cross-current at the moment when both half-transistors are open. Modeling the parameters of a Class D amplifier, especially at an early stage of development, is a necessary tool that can significantly reduce the time and money spent on development and production. In this paper, simulations were performed using equivalent circle methods.

Changing the shape of the signal at the output of the cascade is called nonlinear distortion. They affect the output signal formation positively (e.g. in demodulation) and negatively (e.g. in amplifiers). When investigating GaN transistors on Class D amplifiers, the signal shape at the amplifier output will be different from the signal shape at the input nonlinear distortions occur. They are due to the features of the structure of the amplifier.

The purpose of this work is to create a model of the output stage of a class D amplifier on GaN transistors so as to be able to investigate the effect of nonlinear distortions when changing the switching frequency for different signal frequencies.

The GaN transistor EPC2022 transistor based on GaN transducer, which is assembled on the EPC9035 board for researching, it was selected as the basis for the amplifier model.

The simulation was performed in the Multisim 14.1 software environment. The dependence of the occurrence of nonlinear distortions for switching frequencies of 100 kHz, 250 kHz, 500 kHz, 1000 kHz for sound signals of 1 kHz, 2 kHz, 5 kHz and 10 kHz is investigated. The study founds that nonlinear distortions for different conversion frequencies have different frequency dependencies. For the studied conversion frequencies, there is a decrease in the distortion with increasing frequency, which is caused by the decrease of the high-frequency harmonics of the output filter.

The optimal working conditions for obtaining the minimum values of the coefficient of nonlinear distortions are determined. It is investigated that the created model proves the efficiency of using class D amplifiers on GaN transistors for audio paths. Because it allows to obtain a coefficient of nonlinear distortion of up to 1% and high efficiency.