A floating gate common mode feedback circuit for low noise amplifiers

Most low noise amplifier designs focus on eliminating sources of noise that are intrinsic to the amplifier (thermal noise, I If noise). As integrated circuit design moves increasingly towards mixed signal implementations, the design of low-noise analog amplifiers must be re-evaluated to consider the switching noise generated by on-chip digital circuitry. We designed three fully differential versions of a previously reported single-ended low-noise amplifier for biomedical applications. Each design uses a different common mode feedback (CMFB) circuit. The first uses a standard continuous-time CMFB circuit, the second uses a switched capacitor CMFB circuit, and the third uses a novel floating gate CMFB circuit. A test chip has been fabricated in a 1.5 μm CMOS process. The fully differential amplifiers outperform the single-ended amplifier in the presence of switching noise. The amplifier with the floating gate CMFB circuit has the lowest total harmonic distortion over the critical range and exhibits the smallest fluctuation in the common mode output level.