Abstract : Recently, biosensors have been developed for portable biosensor system. Signals measured in the biosensor system are inevitably accompanied by a variety of internal and external noises. Thus, biosensor systems require very low-level signals detection under noisy environments. Lock-in amplifier (LIA) has been used for the detection of a weak signal concealed in an intense background noise such as thermal noise, flicker noise (1/f) and shot noise in biosensor systems. The LIA is composed of a trans-impedance amplifier (TIA), a phase sensitive detector (PSD) and filters such as low pass filter (LPF) and high pass filter (HPF). In general, output signal of the LIA is that modulated signal by the PSD is represented by the DC voltage after passing through the LPF. However, in order to improve the signal to noise ratio (SNR) and dynamic reserve, a second-order harmonic frequency (2 c  ) which is double of reference frequency is used. For the detection of 2 c  signal, the minimum noise region in frequency domain should be determined. Also, this region should not be affected by the flicker noise. The total noise characteristics of a system almost depend on the first stage of cascade system by Friis formulas. Therefore, total noise figure of the LIA is determined by the noise characteristics of the TIA. In this paper, we analyzed characteristics of the TIA and found the minimum noise region for using 2 c  detection method. We implemented the LIA based on CMOS integrated circuit using 0.18㎛ process and detected the 2 c  signal for improvement of the SNR. As a result of experiment, the minimum noise region is ranged from 20 to 55 ㎑ in the TIA.