Broadband Variable Gain Amplifier Research And Design

Variable gain amplifier(VGA) is the core of the automatic gain control(AGC) system, which is widely used in various electronic systems. As the mixed-signal is approaching gigahertz, VGA design is becoming more and more challenging. Currently, however, few VGAs with bandwidth higher than gigahertz have been reported no matter in academic area or in industry. Comparison of the performance of various types of VGAs is revealed, then two specific wideband VGAs are designed combined with bandwidth extension techniques.In this thesis, a wideband VGA used in IR-UWB is designed and simulated on SMIC 0.13μm CMOS technology. The low-end and high-end-3dB bandwidth of this VGA are 4.2GHz and 4.8GHz, respectively. And its gain can be tuned from OdB to 40 dB. A modified Cherry-Hooper amplifier is adopted as the core amplifier. Source coupled capacitors are added to hold back the common mode signal and avoid the necessity of adding DC offset cancellation feedback circuits. At the same time, neutralization capacitors are embedded to fulfill the requirement of bandwidth. Temperature compensation circuit is adopted to decrease the gain variation. Finally, a capacitor array is added at the output to make the center frequency tunable.In addition, another wideband VGA is designed and simulated based on TSMC 0.18μm SiGe BiCMOS technology. This VGA can be adopted in the applications of measurement equipments, with a bandwidth of DC-2GHz. Its gain can be tuned from-1 dB to 39 dB by a step of 2dB. After detailed analysis of different kinds of bandwidth extension techniques and gain control methods, an architecture which-can balance the noise figure and input dynamic range is used. In terms of the core fixed gain amplifier, the advanced current feedback technique is adopted to realize better flatness in band. Due to the closed loop characteristic, the gain which is determined by the resistance is very accurate. And at the same time, the passive attenuator makes the gain be controlled accurately. Non-ideal effects, such as mismatch and process uncertainty, have also gained a great concern. DC offset is calibrated through a digital DC offset cancellation circuit. Moreover, a novel pole compensation method is proposed for the current feedback amplifier. And capacitor arrays which are controlled by digital signals are added to make the results tunable, hence improve the feasibility and probability of success.