Operation of analog MOS circuits in the weak or moderate inversion region

In analog metal-oxide-semiconductor (MOS) circuit design, the ratio of channel width to channel length (W/L) can become very high, often exceeding a value of 1000. This high ratio may result either from wide channel devices, designed to achieve high transconductance, or from short-channel-length devices used in high-frequency circuits. Since drain current is generally proportional to the W/L ratio, the effective gate-to-source voltage to achieve a desired drain current is typically quite low. In many cases, this set of circumstances leads to operation in the weak or moderate inversion region accompanied by electrical behavior that differs significantly from that in the strong inversion region. In designing analog MOS circuits, an understanding of operation below the strong inversion region is necessary to relate design predictions to simulation results. In addition, biasing simple amplifier stages near the weak or moderate inversion region can be used to advantage. Such a bias can lead to maximum voltage gain, low device dissipation, and minimum total harmonic distortion.