Monolithic GaAs transimpedance amplifier with voltage tunable bandwidth and gain control

A monolithic transimpedance amplifier for optical front-ends with voltage tunable bandwidth between 200 MHz and 2.4 GHz, and 40 dB gain control margin for AGC implementation, is described. The circuit consists of three main sections: preamplifier, controllable gain amplifier, and output buffer. Bandwidth adjust maintains near constant the G-BW product in the whole range, while gain control is independent, and does not modify the bandwidth setting. Both controls have been realized using MESFE1b as variable resistors. The circuit provides a gain of 80 dB¿ at 2.4 GHz bandwidth, input noise below 5 pA/¿Hz for bandwidth settings up to 700 MHz, and output VSWR lower than 1.5:1 over the whole band. The circuit has been designed and fabricated utilizing commercially available 0.5¿m gate length GaAs MESFET process.