A peak detector is conventionally utilized to detect an input voltage's peak, thus can be applied to biosensors to detect the peak of bio-signals for obtaining the characteristic of the bio-signals. For this reason, a peak detector with higher sensitivity and accuracy is required. The detection principle of the conventional peak detector is described as follows. An operational amplifier circuit is utilized to receive and compare the input voltage and a charge voltage of a charge capacitor in electric potential. If the input voltage is greater than the charge voltage in electric potential, a transistor is ON and allows a power source to charge the charge capacitor for increasing the electric potential of the charge voltage of the charge capacitor. On the other hand, if the input voltage is lower than the charge voltage in electric potential, the transistor is OFF to stop charging the charge capacitor, so the electric potential of the charge voltage stops increasing and that means that the input voltage reaches a peak. However, the non-ideal operational amplifier may be influenced by an offset voltage, i.e., the offset voltage may cause a peak-to-peak difference between the real input voltage and the voltage comparing with the charge voltage in the operational amplifier and cause occurrence of peak detection error.