Highly efficient xDSL line drivers in 0.35-μm CMOS using a self-oscillating power amplifier

The design of two highly efficient line drivers in a digital 0.35-μm, 3.3-V technology are presented. The self-oscillating power amplifier (SOPA) architecture has been developed in order to obtain a high efficiency for systems with a high crest factor like discrete multitone modulated xDSL modems. The SOPA architecture is an unclocked switching-type line driver. By using self-oscillation and noise-shaping, a high signal linearity can be obtained for low over-switching ratios. By coupling two SOPA line drivers with a signal transformer, the two limit cycle oscillations are pulled toward synchronization. This gives an important mean switching frequency suppression toward the line. The need for an extra filter dealing with the mean switching frequency is in that way heavily relaxed. A zeroth-order SOPA and a third-order SOPA are prototyped. The zeroth-order line driver meets ADSL-Lite specifications with a missing tone power ratio (MTPR) of 41 dB for an 800-kHz bandwidth. The maximum efficiency is 41%. The third-order version meets ADSL and VDSL specifications with an MTPR of 56 dB and an 8.6-MHz bandwidth. An efficiency of 47% was measured for an ADSL signal with a crest factor >5.