An approach to the analysis of the CMOS differential stage with active load and single-ended output

The complementary metal-oxide-semiconductor (CMOS) differential amplifier with active load and single-ended output is one of the most widespread analog building blocks in modern mixed-signal circuits for signal processing applications, because of its good performance in terms of common-mode rejection and voltage gain, combined with an extremely simple circuit structure, which performs directly differential to single-ended conversion. The authors present a straightforward approach to studying the behavior of such a simple circuit and try to explain some of its features in an intuitive way without resorting to tedious calculations. In particular, this study considers the dc operating point and the common- and differential-mode voltage gains, and the results of the proposed analysis are in good agreement with those provided by both accurate analytical solution and simulation of the circuit. Among the other results, this approach emphasizes some interesting, though very often neglected, aspects of the circuit behavior. Index Terms--Active load, complementary metal-oxide-semiconductor (CMOS) analog circuits, common-mode gain, differential amplifiers, differential-mode gain.