Active-Feedback-Freqcompen - IEEE JOURNAL OF SOLID-STATE...

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IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 38, NO. 3, MARCH 2003 511 Active-Feedback Frequency-Compensation Technique for Low-Power Multistage Amplifiers Hoi Lee , Student Member, IEEE, and Philip K. T. Mok , Senior Member, IEEE Abstract— An active-feedback frequency-compensation (AFFC) technique for low-power operational amplifiers is presented in this paper. With an active-feedback mechanism, a high-speed block separates the low-frequency high-gain path and high-frequency signal path such that high gain and wide bandwidth can be achieved simultaneously in the AFFC amplifier. The gain stage in the active-feedback network also reduces the size of the compen- sation capacitors such that the overall chip area of the amplifier becomes smaller and the slew rate is improved. Furthermore, the presence of a left-half-plane zero in the proposed AFFC topology improves the stability and settling behavior of the amplifier. Three-stage amplifiers based on AFFC and nested-Miller com- pensation (NMC) techniques have been implemented by a com- mercial 0.8- m CMOS process. When driving a 120-pF capacitive load, the AFFC amplifier achieves over 100-dB dc gain, 4.5-MHz gain-bandwidth product (GBW) , 65 phase margin, and 1.5-V/ s average slew rate, while only dissipating 400- W power at a 2-V supply. Compared to a three-stage NMC amplifier, the proposed AFFC amplifier provides improvement in both the GBW and slew rate by 11 times and reduces the chip area by 2.3 times without sig- nificant increase in the power consumption. Index Terms— Active feedback, active-capacitive-feedback net- work, amplifiers, frequency compensation, multistage amplifiers. I. INTRODUCTION W ITH THE advanced technology scaling in CMOS pro- cesses, supply voltages of digital circuits are required to be reduced to 0.9 V by the year 2008, according to the Semicon- ductor Industry Association’s roadmap [1]. The continuous de- crease in the supply voltage, however, poses challenges and dif- ficulties to the design of analog circuits in mixed-signal systems as the threshold voltage of the transistors does not scale down proportionally to the supply voltage. The operational amplifier, which acts as a fundamental block in most analog systems, is required to achieve high gain and large bandwidth simultane- ously in low-voltage condition. To achieve high gain, a conven- tional cascode amplifier, which increases the gain by stacking up transistors, is not suitable in low-voltage design as the cas- code structure results in small voltage swings. Instead, a mul- tistage amplifier is widely used to boost the gain by increasing the number of gain stages horizontally. However, all multistage amplifiers suffer from the closed-loop stability problem due to the presence of multiple poles. A frequency-compensation tech- Manuscript received July 25, 2002; revised October 22, 2002. This work was supported by the Research Grant Council of Hong Kong SAR Government, China, under Project HKUST 6022/01E. The authors are with the Department of Electrical and Electronic Engi-
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This note was uploaded on 09/19/2010 for the course EE 6378 taught by Professor Hle during the Spring '10 term at University of Texas at Dallas, Richardson.

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Active-Feedback-Freqcompen - IEEE JOURNAL OF SOLID-STATE...

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