Analog Baseband Filter Design For RF Integrated Circuits

As the wireless communication system develops, the RF integrated circuits have become a hot research field. Now, more and more RF, analog, digital sections have been required in a single chip. Analog Baseband filter is researched in this paper. After researching the types of active filter, Gm cell, operational amplifier, automatic tuning circuit, Gm-C filter and Active-RC filter are presented.Filter is one of the most important parts in RF system; it is used for selecting channel, blocking high order harmonics, and anti-aliasing. After researching and developing for many years, the theory of filter is mature, such as traditional active-RC filter, switch-capacitor filter with high precision cutoff frequency, and Gm-C filter with high and adjustable frequency.Filter design is divided into two areas: filter topology and sub circuit design. This thesis starts from circuit's theory, analyzing the synthesis methods, stability, noise in filter. In sub circuit design, we emphasize linearity, noise, nonlinear, common mode feedback in Gm cell. And gain, common mode rejection ration in folded cascade amplifier.Two kinds of Baseband filter are presented. One is Gm-C filter used in RF transmitter,the Gm circuit utilized input transistors biased in linear-region to achieve high linearity and low power dissipation. Automatic tuning system is often added to tune the time constant determined by the ratio of C and Gm, which is not accurate due to process, temperature variation. This paper presents a"Gm compensation"method to adjust the cutoff frequency to decrease variation. The other is Active-RC filter used in RF receiver, it utilizes folded cascode operational amplifier. In order to improve stability of active filter, buffer and miller effect capacitor are added. These filters are designed on SMIC(Semiconductor Manufacturing International Corporation)0.18μm Mixed Signal CMOS process. Their simulation results and layouts are presented at the end of the paper. Gm-C filter and Active-RC filter are compared each other.