A Gain-Tunable Output Buffer for Audio-DAC with Common-Mode Output Independent of Gain Variation

A novel gain-tunable output buffer for audio-DAC is proposed in this paper. With this proposed architecture, the common-mode output voltage can be independent of gain variation. In practical applications, supply voltage might change. With this proposed architecture, the common-mode output voltage could be set separately, to guarantee that it stays in the mid-scale position of the supply. Therefore, the ability of reaching the maximum output swing for the buffer features the proposed architecture. In addition, the threshold current reference is utilized to generate bias currents for the other building blocks. Therefore, it is always located in the source position of the signal chain. Its output noise would be amplified by all the following circuit blocks. To guarantee high-quality performance of the audio-DAC, the output noise of the threshold current reference should be suppressed. In this letter, noise performance of the threshold current reference is analyzed theoretically and improved significantly. Simulation result shows that the integrated noise current of the threshold current reference can be reduced from 0.61 to 0.15 nA when the integral frequency range sweeps from DC to 50 kHz, i.e., a reduction of 75.4% has been achieved.