One difficulty in designing rail-to-rail input stages for portable ECG amplifier is how to maintain transconductance (gm) as constant as possible throughout the common mode input range of complementary differential input pairs operated in parallel. In the past years, a number of constant-gm techniques have been proposed and those can be categorized by scheme. Among those schemes level shifting is the simplest scheme and has very good constant-gm behavior [6]. In [1], two PMOS source followers, totalling 4 PMOS transistors, are used for level shifting. But in this paper, only one diode connected NMOS transistor is used and similar results to the conventional level shifting method have been achieved. Rail-to-rail input stage with 1.6V supply voltage is proposed and simulated by Cadence SPECTRE with TSMC 0.25-μm CMOS technology.
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