Optimizing drain current, inversion level, and channel length in analog CMOS design

This paper describes a methodology for selecting drain current, inversion level (represented by inversion coefficient), and channel length for optimum performance tradeoffs in analog CMOS design. Here, inversion coefficient replaces width as a design choice to permit a conscious optimization of inversion level while width is implicitly considered. Transconductance, gate-referred thermal-noise voltage, and drain-source saturation voltage are optimized towards weak inversion while transconductance linearity and drain-referred thermal-noise current are optimized in strong inversion. Voltage gain, flicker noise, and dc mismatch are optimized towards weak inversion at long channel length while bandwidth is optimized in strong inversion at short channel length. Optimization expressions are given along with measured transconductance efficiency and Early voltage from weak through strong inversion over a wide range of channel lengths. Transconductance efficiency and Early voltage are used as normalized measures of transconductance and drain-source resistance, independent of drain current. The methodology presented is used to design three 0.5-mu m operational transconductance amplifiers having equal 50-mu A bias currents, but different tradeoffs in gain, bandwidth, noise, and dc mismatch. The amplifiers have measured voltage gains of 16.8, 110, and 326 V/V, -3-dB bandwidths of 350, 5 1, and 5 MHz, input-referred flicker-noise voltage at 100 Hz of 2,000, 450, and 58 nV/Hz(1/2), and input-referred dc mismatch voltages of 10.2, 2.2, and 1.1 mV respectively. The design methodology can be readily extended to deeper submicron CMOS processes.