A transistor-only current-mode Sigma Delta modulator

Current copier circuits are used in the IC realization of a transistor-only second-order Sigma Delta modulator. By using current copiers, it is possible to implement the modulator without using linear capacitors, as is required for switched-capacitor approaches. Design considerations based on settling time and noise for a current copier circuit are presented, along with the description of the current output D/A and the current quantizer. The modulator has been fabricated in a 0.9- mu m digital CMOS process and achieves 13-b resolution and 12-b linearity over the voiceband frequency range.>