Universal constant-gm input-stage architectures for low-voltage op amps

In this paper, a novel design technique for low-voltage, constant transconductance (g_m) op amp input stages is presented. The new technique which uses current-mode circuits is based on processing signal currents, rather than handling DC tail currents, to achieve a constant-g_m. Two cases are developed. One is based on processing signal currents (the AC case) while the other is based on processing total instantaneous currents (the TIC case). The adopted design strategy in both cases is universal in that it is independent of the input stage transistor types (FET or bipolar) and their operating regions. It also considerably simplifies the design procedure of low-voltage op amps. To demonstrate the new concepts, universal op amp input stage architectures have been developed and their performances have been verified in both MOS and bipolar design examples. The MOS designs have been verified in both weak and strong inversion. The proposed universal implementations achieve almost constant-g_m, independent of the common mode input voltage range from rail-to-rail