Noise improvement beyond the “kT/C limit"” in low frequency C-T filters

A technique to improve the noise performance of low frequency continuous-time filters is presented. The technique is based on scaling the filter´s capacitor currents using current mirrors. First a representative current-scaled filter topology is analyzed using a generic current mirror model. It is concluded that the transconductance of the current mirror transistors should be small if a noise improvement is to be achieved. Noise properties of down-scaling bipolar and MOS current mirrors are presented along with comparisons to the generic model. Several current-scaled 100 Hz first-order low-pass filters have been designed and optimized for low noise, using the obtained analysis results. Simulation results indicate that if a class A/B biasing scheme is used for the down-scaling current mirrors, then a noise improvement of at least 8 dB can be obtained, when compared to a nonscaled reference filter. Comparing the simulated noise to the fundamental limit for a passive RC-filter (kT/C), a 5 dB lower noise level is demonstrated