Fully Integratable Class-AB Rear-End with Smart Quiescent Current Control for a General-Purpose Hearing Aid Chip

In this paper, the core of the rear-end of an universally applicable analog hearing aid chip is described. It contains, apart from some subcircuits, a current-level Class-AB controlled preamplifier with three independent gain controls and a novel half-switching bridged final amplifier for various hearing aid receivers from low-power up to medium-power. To minimize supply current, the quiescent current of the amplifier string is optimized at any value of the input current and the controlled gain by a translinear quiescent current processor. Chip area in a 2.5-¿m BiCMOS process amounts to 2.6 mm2. Measured THD remains below 2% in all cases (1kHz). The bandwidth is ≫12kHz. All circuits operate correctly with a battery voltage down to 1.05V. Total current drain (without input signal) varies between 140¿A and 425¿A.