Design Of Broadband Linear CMOS Variable Gain Amplifier

In radio communication system, because of the attenuation caused by the effect of obstacles, air and other factors, signal reception needs to be able to handle the widest range of data conversion. Based on this consideration, Automatic Gain Control (AGC) circuit is generally adopted in a typical RF front-end system. Variable Gain Amplifier (VGA) is the key module of AGC whose stability and sensibility to input signal depend largely on the performance of variable gain amplifier.This subject is in the background of developing of RF chip. Starting from AGC loop system, AGC's basic working principle and VGA'status and role in AGC loop is analyzed. On the basis of concluding the mainstream technology of current CMOS VGA, a wideband linear CMOS VGA is designed and its gain control range is 24dB. According to the requirements of AGC system, VGA's gain is in dB-linear relationship with control voltage. The designed VGA is composed of input matching circuit, control voltage conversion circuit, Signal Summing structure cell and output matching circuit. Input matching circuit adopts band-pass filter to choose the frequency range, where the requirements of input impedance matching and low noise is met. Control voltage conversion circuit changes a large range of control voltage into a smaller range of voltage in order to directly control the main circuit of VGA and the dB-linear relationship between VGA'gain and control voltage comes true. Signal Summing structure cell multiplies input signal and control signal, thus achieving continuous adjustment of gain. Meanwhile, by controlling the drain-source voltage of the input MOS transistor, good linearity is attained by making the input MOS transistor working in linear area when input signal is strong. Output matching circuit adopts Shunt-Peaking structure, in which the zero point introduced by inductance compensates for the decline of gain caused by capacitor, thus expanding bandwidth. Also, in the output matching circuit, source follower is adopted to drive the external load.The design is based on ADS simulation platform developed by Agilent Technologies and adopts a TSMC 0.18μm RF CMOS process. Simulation results show that VGA's 3dB-bandwidth is 570MHz~770MHz. In the condition of the bandwidth from 570MHz to 770MHz and the control voltage from 0.95V to 1.13V, input return loss S11 and output return loss S22 are both less than -10dB. Reverse isolation S12 is less than -50dB, gain dynamic range is from -8.9dB to 15.9dB, and VGA's gain is in approximate dB-linear relationship with the control voltage. The minimum noise figure arrives at 0.34dB, and the maximum noise figure is no more than 10dB. VGA's input third order intercept point (IIP3) varies from -6dBm to 5dBm. Also, the average power consumption is 11.245mW, and the volatility is 0.092mW.