Simple CMOS low-voltage op amps with constant-gm rail-to-rail input stage

Two simple 3-V CMOS low-voltage operational amplifiers with constant-g_m rail-to-rail complementary input stage are presented. The constant g_m ensures constant open-loop gain and unity gain frequency, etc. within the whole common mode input range. The first op amp is designed so that the transition regions of the two pair drivers overlap directly, keeping the g_m of the input stage constant within 9% over the whole region. DC level shifts are used in the second op amp so that the transition region of the n-pair is shifted rightward to overlap that of the p-pair, keeping the g_m constant within 8%. Both methods are effective and simple to implement