Simple rail-to-rail low-voltage constant-transconductance CMOS input stage in weak inversion

A CMOS input stage for operational amplifiers is presented. To obtain a rail-to-rail input range an nMOS and a pMOS differential pair are driven in parallel. Both differential pairs operate in weak inversion. With the help of only four additional transistors a transconductance g_m is obtained which is independent of the common input voltage. This extension does not increase the minimum required supply voltage.