A bio-signal amplifier whose output voltage amplitude is maximized automatically is proposed. The proposed amplifier adjusts not only the voltage gain but also the output bias voltage to maximize the output voltage amplitude. The proposed circuit is suitable for amplifying voltage signals with large spikes such as electrocardiographic signals. The proposed circuit is integrated using a 0.35 μm CMOS process, and its performances are confirmed by measurement. The measurement results show that the proposed circuit can control the output voltage amplitude by adjusting the offset and the gain.