A low-voltage low-power fully-integratable front-end for hearing instruments

The front-end of a universally applicable analog integrated circuit for hearing instruments is presented. This IC comprises the following functions: a microphone preamplifier, a pickup-coil preamplifler, a second-order high-pass filter and a second-order low-pass filter, both with a controllable cutoff frequency, and an input-controlled automatic gain control with an adjustable knee level. By applying a compression/expansion system and operating all the circuits in the current domain as much as possible, all these functions can be implemented in a single IC, without the need for external components. The test chip demonstrates operation down to 1.05 V and a current consumption between 120 and 175 μA. The full-custom chip area in a 2.5-μm BiCMOS process (using only vertical NPN´s and lateral PNP´s) amounts to 2.4 mm²