Low-power single-loop and dual-loop AGCs for bionic ears

Bionic ears or cochlear implants for the deaf require low-power wide-dynamic-range automatic-gain-control circuits (AGCs) to interface between microphone preamplifiers and analog processing circuits or A/D converters. Hearing-impaired patients need strategies which switch intelligently between time constants for speech and time constants for interfering transients. We describe two AGC designs which use a programmable continuous current-mode feedback network to control a variable gain amplifier. The first design implements a log-linear controller to demonstrate level-invariant closed-loop response. The second design is a dual-loop controller which implements a simplified version of a well-known algorithm for speech in noisy environments. The dual-loop strategy implements a continuous-discrete hybrid controller with implicit state control using two filter-and-hold peak detectors and a charge-pump hold-timer. Both AGCs exhibit 78 dB of input dynamic range, have digitally programmable time constants, operate from a 2.8 V supply, and consume less than 36 mu W in a 1.5 mu m BiCMOS process. For typical compression settings, the minimum instantaneous input dynamic range is greater than 58 dB.