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In this paper, we propose a robust and scalable constant- rail-to-rail CMOS input stage for VLSI cell libraries. The proposed circuit does not rely on the characteristics and particular operation (strong, moderate, and weak inversion) regions of the input transistors and is insensitive to mismatches between p- and n-channel devices. Only standard CMOS transistors are used in the circuit without any special devices, such as floating-gate or depletion-mode transistors. Very small variations (less than ) have been achieved without sacrificing the large-signal behavior. The proposed circuit is proven effective for both long-channel and deep sub-micron CMOS technologies and is suitable for VLSI cell libraries, audio/video, embedded mixed-signal system-on-chip (SoC), and other applications. A prototype amplifier with rail-to-rail input common-mode range has been designed and fabricated in a standard 0.35-m CMOS technology. Experimental results confirm the effectiveness and robustness of proposed techniques.