Design of a BiCMOS constant-gm rail-to-rail operational amplifier for low-voltage applications

This paper presents a BiCMOS rail-to-rail operational amplifier for low-voltage applications. CMOS differential pairs are used for the input stage to achieve high input impedances. The BJT translinear principle is then employed in the design of a constant-g_m circuit to control the tail currents of the complementary differential pairs properly. Mismatching between K_n and K_p is compensated by a a K_n-K_p matching circuit employing the MOS translinear principle. The resulting variation of the total g_m is within 13% according to the simulation. The output stage is also rail-to-rail with a class AB control. When configured as a unity-gain buffer with a 10 kHz sinusoidal wave applied, the total harmonic distortion simulated from SPICE is less than 0.06% with the input amplitude less than 1.4 V and 0.114% at 1.5 V. This op amp will be fabricated in a CMP/AMS 0.8 μm DPDM BiCMOS process, and the experimental results will be reported later