Design Of Digitally Controlled And Variable Gain Amplifier

The variable gain amplifier (VGA) is a common structure of the analog integratedcircuit system, it can provide a range of adjustable gain according to the actual needs.Because of the ability to deal with or output an uncertainty signal, the VGA is widely usedin communication equipment, medical equipment, display imaging devices and so on. TheVGA in this paper is designed for the Micro-nano optical controlled devices, amplify thedigital voltage adjustable at the input port, and get a square wave voltage to theMicro-nano optical controlled devices at the output port.Firstly, this article introduces three common structures of VGA, then analyze itscharacteristics and scope of application, compare the advantages and disadvantagesbetween the various structures, so that the most appropriate circuit structure was choseaccording to the actual situation. Secondly, the operational amplifiers are introduced as themost important circuit in VGA, several common op-amp circuits are analyzed andcompared. Then the appropriate structure is selected according to the main propertyparameters. Thirdly, the circuit framework and functions are defined from up to down,completed the design of VGA from bottom to up, then the simulation results are comparedwith actual demand by analysis and calculation. Finally, the layout design of VGA iscompleted.Closed-loop feedback VGA structure is used in this article,120kinds ofcombinations of signals are got for the switching resistor arrays that can be controlledprogrammed. Meanwhile, a linear variable magnification is obtained at the steps of120. Asecond-level structure with folded cascode and a common source stage is used in op-ampcircuit, the structure with folded casecode and a common drain stage CMFB is used infully differential op-amp circuit. In addition, the circuit is worked in two voltage domainsthat is0~5V and±20V, so the subtraction circuit is used for voltage conversion. Huahong NEC BCD180process is used for the design of VGA, the digitallycontrolled VGA can output a voltage(Vpp) from3.03V to39.95V in a step size of0.15V，the variable gain is-0.746dB~21.69dB. This VGA achieves high accuracy and works atan adjustable frequency from0to100KHz.