  This paper presents a compact programmable biphasic stimulator for cochlear
implants. By employing double-loop negative feedback, the output impedance of
the current generator is increased, while maximizing the voltage compliance of
the output transistor. To make the stimulator circuit compact, the stimulation
current is set by scaling a reference current using a two stage binary-weighted
transistor DAC (comprising a 3 bit high-voltage transistor DAC and a 4 bit
low-voltage transistor DAC). With this structure the power consumption and the
area of the circuit can be minimized. The proposed circuit has been implemented
in AMS 0.18um high-voltage CMOS IC technology, using an active chip area of
about 0.042mm^2. Measurement results show that proper charge balance of the
anodic and cathodic stimulation phases is achieved and a dc blocking capacitor
can be omitted. The resulting reduction in the required area makes the proposed
system suitable for a large number of channels.
