CMOS Technology and Current-feedback Op-Amps

The implementation of currentfeedback op-amps (CFB op-amps) relies on the use of a semiconductor process with complementary device structures for class AB operation of the input and output stages. Thus, CMOS technology is an obvious candidate. However, commercial CFB opamps are all realized in bipolar processes. In this paper we identify some of the problems related to the application of CMOS technology to CFB op-amps. Problems caused by the low device transconductance and by the absence of matching between P-channel and N-channel transistors are examined, and circuit solutions providing improved input characteristics are presented. Also, problems related to the achievable output voltage swing are examined and circuits which may be used to achieve a near rail to rail output swing are proposed. It is concluded that mere translations of bipolar circuit designs yield a rather poor performance compared to the bipolar designs, but CMOS has a potential for CFB op-amp design if more ingenious circuit configurations are applied.