Monolithic SiGe HBT Feedforward Variable Gain Amplifiers for 5 GHz Applications

Abstract

Monolithic SiGe heterojunction bipolar transistor (HBT) variable gain amplifiers (VGAs) with a feedforward configuration have been newly developed for 5 GHz applications. Two types of the feedforward VGAs have been made: one using a coupled-emitter resistor and the other using an HBT-based current source. At 5.2 GHz, both of the VGAs achieve a dynamic gain-control range of 23 dB with a control- voltage range from 0.4 to 2.6 V. The gain-tuning sensitivity is 90 mV/dB. At VCTRL= 2.4 V, the 1 dB compression output power, P1-dB, and dc bias current are 0 dBm and 59 mA in a VGA with an emitter resistor and -1.8 dBm and 71mA in a VGA with a constant current source, respectively. (HBT) technology. These topologies, however, have stability disadvantages, a limitation on bandwidth, and nonlinear-in-dB gain control characteristics. In this work, we have newly developed SiGe HBT feedforward VGAs to improve gain-control range and dB- linear gain control characteristics. The VGAs were designed and fabricated by using ST Microelectronics' SiGe MMIC design library and Si-BiCMOS process. The VGAs were measured for small signal and large signal performances. The measured results of the developed VGAs will be discussed.