BentlyNevada P/N 146031-01
引言
当共模输入电压在中间值附近时，P差分对M1、M4与N差分对M2、M3均导通，控制开关M17、M18、M13、M14开启，分别调节它们的栅电压，使其从M15、M16均抽取3/4的电流，
电源电压逐步下降，晶体管的阈值电压并没有减小，但是运放的共模输入范围越来越小，这使设计出符合低压低功耗要求，输入动态幅度达到全摆幅的运放成为一种必须。本文所设计的具有轨至轨(R-R)输入功能的低压低功耗CMOS运算放大电路，在各种共模输入电平下有着几乎恒定的跨导，使频率补偿更容易实现，适合应用于VLSI库单元及其相关技术领域。
理论模型
基本的轨至轨输入结构
在较低的电源电压下，运算放大器的输入级设计是非常重要的。传统的PMOS差动输入级的共模输入电压范围VCM可表示为:
跨导恒定结构
图1所示的轨至轨输入级电路采用互补折叠式结构，使共模输入电压可以在整个从地到电源电压的范围内工作，如果输入级工作在饱和区，电路的跨导由下面的公式确定：
式中mn和mp分别代表NMOS和PMOS的迁移率。从上面的公式可以看出，输入级的跨导会随栅源电压和便置电流的变化而变化。因此，当共模输入电平从VDD到VSS变化时，轨至轨输入差分对的跨导从PMOS差分对的跨导变化到PMOS+NMOS差分对的跨导之和，再变化到NMOS差分对的跨导。中间部分跨导gm几乎是其它部分的一倍，这种跨导的变化会使运放的增益误差发生变化，从而使频率特性变差，因此，需要设计一种电路，使轨至轨输入电路具有恒定的跨导。
目前，可保证R-R输入级的gm恒定不变的设计方法主要有以下几种：1.采用双极(BJT)线性互补差分对形式的输入级。2.由齐纳二极管将P、N差分对的偏置电流连起来实现。3.采用冗余的差分对来实现。4.用电流镜技术，使偏置电流的大小随输入共模电压的变化而变化。
上述第4种方法的电路不仅结构简单，而且对gm的控制也易于实现。因此，本文运用了对输入跨导的控制原理，采用了一种全新的保持R-R输入级gm为常数的电路结构。
电路设计
本文所设计的电路如图2所示，该电路由输入互补差分对、恒定gm电路、共源共栅求和电路组成。M1~M4构成了输入互补差分对。当低共模输入时，P输入差分对M1、M4处于工作状态，N输入差分对M2、M3截止，开关管M17、M18开启，抽取M16上的电流；M13、M14截止。M15的电流全部流入P差分对，则此区间的等效差分跨导为:
INST
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