Abstract
In this thesis, we propose an operational transconductance amplifier (OTA) with low distortion by using some techniques: (a) current reusing (b) gain-boosting (c) MOS triode region operation. Therefore, a high linear relationship between input voltage and output current can be obtained. We use the proposed OTA to implement a sixth-order Gm-C filter for wireless communication applications. Moreover, the bandwidth of the Gm-C filter can be tuned by tuning the transconductance of the OTA. By using this property, we can overcome the bandwidth variation of the filter due to the process variation. An on-chip automatic frequency tuning system is also designed to compensate the bandwidth variation automatically. The power consumption and the active area of the automatic-tuning system are reduced by the lots usage of the digital circuits in the automatic-tuning system. Besides, a low noise fourth-order Butterworth filter implemented with the proposed OTA is also designed for low noise application. In order to verify this design, two test chips are fabricated with TSMC 0.18 μm CMOS process. The power supply is 1.8 V. The measurement results of the first chip show that the -3dB frequency is about 8 MHz and the DC gain is 0.056 dB. The total harmonic distortion (THD) is smaller than -40 dB when the input signal is a 1 MHz sinusoidal waveform with 1.2 Vppd amplitude. The input third intercept point (IIP3) is 10 dBV, and the power consumption of the filter and the automatic-tuning circuit are 8.1 mW and 3.53 mW, respectively. The tuning error of the automatic frequency tuning system is less than 5 %. The measurement results of the second chip show that the -3dB frequency is about 11.5 MHz and the DC gain is -0.61 dB. The THD is 0.96 % with 1.1 Vppd input signal at 1 MHz. The IIP3 is 8 dBV, and the power consumption is 13.3 mW. The noise level is 40.7 nV/rt Hz.