Abstract
The variable gain amplifier (VGA) is one of the essential blocks in wireless transceiver systems, which provides relatively constant output power for fluctuated input signals and hence improves the dynamic range of the transceiver system. The wideband VGA with GHz bandwidth is required in modern wireless transceiver systems to support the Gbps data rate for the next generation of the wireless mobile network. However, the design of VGA with a broad bandwidth is facing several challenges. One of them is the bandwidth fluctuation, while the VGA switches between different gain settings. The trade-off between gain and bandwidth limits gain range and bandwidth extension. One way to satisfy the wideband requirement is to utilize a combination of variable attenuators and post-amplifiers while the dB-linearity performance is not achieved. In this dissertation, a current injection approach is proposed to solve the bandwidth variation and limited gain range problems for classical Cherry-Hooper structure, which is popular in wideband designs. This approach provides a guideline for wideband amplifier design with constant bandwidth. Analyzed, simulated, and measurement results show that the proposed approach is effective in achieving constant bandwidth and large gain range simultaneously. The achieved performance is very competitive comparing with the other state-of-the-art designs. Gain and bandwidth trade-off is one of the most critical considerations in amplifier designs and becomes more severe in VGA design as the gain is not a fixed parameter. The proposed solution to this problem is to separate the design purpose of wideband and large gain range into two blocks. On the other hand, in the proposed design, the gain range can be tuned, and the gain can be varied in both analog and digital methods. Variable attenuators applied to VGA designs is another method to achieve wideband while the circuits are not able to be dB-linear controlled directly by external signals. A correction network is proposed to generate an internal control signal automatically and realize accurate dB-linearity for VGAs. This method expands applications of wideband attenuators and resolves issues of attenuators application in dB-linear VGAs. Measurement results show the proposed VGA design with good performance and dB-linearity accuracy.