This paper describes a single-ended CMOS chopper amplifier for 1/f noise reduction of n-channel MOS transistors and its application to a low-noise high-gain switched-capacitor (SC) amplifier for sensor interface circuits. Since the chopping is used inside of the operational transconductance amplifier (OTA), this amplifier can be used for high output impedance sensors. To investigate the effect of the proposed chopping technique, a test chip was fabricated using 0.25µm mixed-signal CMOS process. The total input-referred noise is greatly reduced by using a high chopping frequency of 256kHz to 20µV_rms from 295µV_rms without chopping.