Differential amplifiers that reject common-mode currents

The transistor differential pair is analyzed and it is shown that the common mode rejection ratio (CMRR) is limited both in magnitude and bandwidth by device parameters. The performance is not significantly improved by series common-mode feedback. It is shown that amplifiers with shunt common-mode feedback reject common-mode input currents and that their common-mode rejection ratio can be made independent of device parameters. Applications of such amplifiers include differential operational amplifiers, rejection of large common-mode voltages, and transducer amplifiers. Basic circuitry is described and the performance of a practical circuit is given.