A 1.2-V Feedforward Amplifier and A/D Converter for Mixed Analog/Digital LSIs

Takanobu ANBO

IEICE TRANSACTIONS on Electronics   Vol.E79-C    No.12    pp.1666-1678
Publication Date: 1996/12/25
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Issue on Low-Power LSI Technologies)
low-voltage,  low-power,  analog,  pipeline A/D converter,  feedforward amplifier,  

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Very-low-voltage 1.2-V mixed-signal CMOS technology is a device/circuit solution aimed at ultra-low-power portable systems such as digital cellular terminals and PDAs. We have developed an experimental 1.2-V mixed analog and digital LSI circuit/device technology. This technology is based on a new transistor structure that has a 0.3-µm gate length and a low Vth of 0.4 V, and that suppresses the short-channel effect. In this paper, we will mainly discuss low-voltage analog circuit design that uses this technology. We show that low Vth is essential not only to digital circuits, but also to 1.2-V analog amplifier, A/D converter and analog switch designs. To achieve high-conversion rate A/D converters, a pipeline architecture is used for low-voltage operation. To increase the attainable gain-bandwidth of the operational amplifier of the converter, a feedforward phase-compensated three-stage amplifier is proposed. The addition of a feedforward capacitor allows a high frequency signal to pass directly to the second stage, which optimizes use of the second stage bandwidth. Pole-zero canceling is used to achieve a fast settling of the amplifier. Although gain precision is degraded by the positive feedback through the feedforward capacitor, this can be offset by increasing the equivalent second-stage gain with an inner feedforward compensated amplifier. The gain-bandwidth of the proposed double feedforward amplifier is two to three times wider than with the conventional Miller compensation. With these techniques, we used 1.2-V mixed-signal CMOS technology to create a basic logic gate with a 400-ps delay and 0.4-µW/MHz power, and a 9-bit 2-Msample/s pipeline A/D converter with power dissipation of only 4 mW.