A constant-gm rail-to-rail op amp input stage using dynamic current scaling technique

We introduce an innovative constant-transconductance (g_m) CMOS input stage. Rather than handling the tail currents of the input differential pairs, the proposed circuit scales the output signal currents of the input differential pairs dynamically for a constant g_m while keeping the tail currents of the input transistors unchanged. The operation of the new circuit does not rely on the quadratic characteristic of the input MOS devices, and is independent of the operating regions of the input transistors. The new constant-g_m scheme, which has potential for high-frequency applications, can be employed universally to both short and long channel transistors, and is suitable for new generations of deep submicrometer CMOS technologies. The technique is demonstrated through the design of a rail-to-rail CMOS op amp with supply voltage of 3 V in 0.35 μm CMOS technology. Simulations show that, when the input common-mode voltage swings from rail to rail, the op amp´s input stage g_m varies around ±1.5% and ±2.9%, respectively, for input transistors in the strong and weak inversion regions.