Design Of CMOS Variable Gain Amplifier

Integrated circuits with small sizes, high reliability and high performance, low costs are largely produced. With the development of microelectronics technology, integrated circuits are widely used in military, communications, medical equipments and so on. CMOS variable gain amplifiers (VGA) with low power, low cost, high linearity, wide bandwidth and high gain control range become more and more important with the reduction of power voltage and channel length.Firstly, CMOS VGA's design and study and its application areas at home and abroad were given currently. Some VGA architectures are elaborated in detail such as VGA based on analog multiplier, VGA based on programmable transconductance, VGA based on programmable load, VGA based on programmable feedback and VGA based on programmable attenuator. A constant gain operational amplifier and programmable attenuator cascode architecture are used to implement the VGA.An operational amplifier is the key of the VGA. A detailed analysis of some kinds of operational amplifiers is presented. The architecture of the operational amplifier is determined from advantages and disadvantages of operational amplifiers above. A self-biasing current sink design increase output swing of the input differential amplifier. A dual cascode amplifier improves the frequency characteristics of the operational amplifier. A source follower reduces the output impedance and increase circuit drive capability. The result of the design satisfied the specification by a series of theoretical calculations and simulations.The digital control method is used in an attenuation network to control circuit reliability and to improve control accuracy. An attenuation network is composed of digital control word generator and R-2R attenuator and a group of MOS switch circuits. The design of the control word generation circuit is referred from cell-based, which uses Verilog HDL and generates by the logic synthesis using DC compiler EDA tools. The digital control circuit can achieve the functions of the circuit by functional verification, transistor-level circuit verification.Finally, the relevant knowledge of layout is given. The layout design of an operational amplifier and a programmable attenuator is finished. The VGA used a low-IF applications, high-gain control range is completed. The design of VGA is based on the Cadence EDA platform. The amplifier is fabricated in 0.35μm SiGe BiCMOS process (SBC35) from the JAZZ semiconductor. This VGA operates at an IF frequency of 4.46MHz, and provides 42dB gain control range in 6dB steps by digital controlled ways. The VGA static power dissipation is 11mW.