Title: Implementing compensation capacitor in logic CMOS processes
Authors: Lin, TC
Wu, JC
Department of Electronics Engineering and Institute of Electronics
Keywords: anti-parallel connected PMOS capacitors;accumulation MOSFET (AMOS);Miller compensation;two-stage amplifier
Issue Date: 1-Aug-2002
Abstract: MOSFETs can be used as capacitors, but its capacitance can vary by 5 to 7 times as its terminal voltage varies. To reduce the voltage dependence of the capacitance, this paper proposed two types of devices: one is called accumulation MOSFET (AMOS) and the other is formed by two conventional PMOS connected in anti-parallel. These two devices are readily available in the standard digital CMOS processes. The proposed capacitors were implemented in three different CMOS processes. The measured results show that the capacitances of both devices have less voltage dependence than a single PMOS. The voltage dependence of the AMOS capacitance can be as small as 17%. The minimum capacitance per unit area of the AMOS is 1.8 times that of the double-poly capacitor in an analog/mixed-mode CMOS process. To verify the usefulness of these two types of capacitors, they are used as compensation capacitors in a conventional two-stage amplifier. The measured results show that the amplifier compensated by the AMOS capacitor has little variation (6%) of the unity-gain frequency over the input common-mode range. Due to its smaller die area and cheaper digital process, AMOS can be used as compensation capacitor without resorting to more expensive analog process.
URI: http://hdl.handle.net/11536/28590
ISSN: 0916-8524
Volume: E85C
Issue: 8
Begin Page: 1642
End Page: 1650
Appears in Collections:Articles