Abstract: Total Harmonic Distortion (THD) is an important index to measure the nonlinear degree of channel. The automatic measurement device applied in arbitrary output waveform identification and total harmonic distortion of the audio small signal amplification system has become an effective means to detect the signal amplification performance. In order to accurately calculate the total harmonic distortion of the amplified waveform, FFT technique is adopted. In order to improve the system efficiency, high frequency sampling and half extraction techniques are adopted. Dynamic control relays, resistance-capacitance components and multi-stage amplification are used to reasonably adjust the position of the static operating point of the amplification system and the size of the input waveform to realize the output of different types of distorted waveforms. Based on microcontroller stm32f103c6, the combination switch completes the collection, identification, calculation and display of arbitrary waveform in one-button, greatly improving the working efficiency of small signal amplification systems. Matlab algorithm simulation, Multisim circuit simulation and practical test show that, the automatic calculation device of arbitrary waveform THD for small signal amplification systems consisting of multi-stage transistor amplification consistency adjustment circuit and microcontroller FFT operations can accurately and real-time identify waveforms and calculate THD. The one-button switching design is convenient to operate, saves cost, and has practical value.

Key words: multistage amplifier circuit, gain control, total harmonic distortion(THD), fast fourier transform (FFT), system design, circuit simulation