Research And Design Of Switched-Capacitor Variable Gain Amplifier

The signals received by wireless communication system, image sensors and many other devices are influenced by a variety of external factors, the strength of the signal is not fixed. Order to be able to properly receive and process the signal obtained, the circuit of the variable gain amplifier (VGA) must be increased to ensure that the signal remains at a relatively fixed value. In this paper, the design of the VGA is used in the front of the ADC, to increase its dynamic range, and process the signal of14-bit100MHz.This paper describes several popular structures of the VGA circuit and its principle. Finally chose the fixed gain amplifier and variable attenuator network of VGA, the system can be more stable and higher linearity. A new network of Switched-Capacitor for variable gain amplifier is proposed and implemented in an analog interface system. Using Switched-Capacitor network as a feedback circuit can lead to lower power consumption, smaller chip area and reduced KT/C noise.VGA circuit is divided into two stages, is composed of the switched capacitor array and op amp circuit. The first stage is a coarse adjustment unit. The gain in this stage varies from0to18dB with6dB step. The second stage circuit is an accurate adjustment unit. The gain in this stage varies from0to6dB with0.094dB step.Op-amp circuit uses a fully differential telescopic two-stage operational amplifier. The first stage is used to amplify signals. The second stage using cascade-transistor has a high output swing. The circuit is fabricated in SMIC0.18um process with the power voltage of3.3V. The clock frequency is100MHz. The gain of the op amp is106dB, the phase margin is65, the unity gain bandwidth is560MHz, the setting-time within0.003%accuracy is9.37ns.After the simulation of the circuit, the gain of the SC-VGA varies from OdB to24dB with0.094dB step. Having a large dynamic range, high-precision and high linearity, the SC-VGA meets the design requirements of the application. The SC-VGA can be used in analog circuits such as the front of the ADC, to improve the system’s dynamic range and stability.