Compact, ultra low power, programmable continuous-time filter banks for feedback cancellation in hearing aid

This paper describes the design of a compact, ultra low power continuous time programmable filter. Compact programmable filters are required for a number of applications. A particular application is in feedback cancellation filters in hearing aids. Here, a feedback cancellation filter that matches the open loop transfer function is used to suppress acoustic oscillations. Because of the complexity of the transfer function the number of poles in the cancellation filter is fairly large. To realize an integrated cancellation filter an ultra-low power transconductance cell with large linear range has been designed. Both measurement and simulation results are presented to confirm our design. The power consumption for a transconductance cell is comparable to the theoretical minimum bound and is measured to be 0.8 μW at a 3 V supply. The linear input range for this cell is 2 V_p-p. In this paper we also compare our design with a programmable switched capacitor implementation and show that there is a significant savings in area as well. The complete filter has been designed, implemented and fabricated in an 2 μ CMOS technology.