A 2pAspl radic/Hz 622 Mb/s GaAs MESFET transimpedance amplifier

Telecommunication systems require high sensitivity transimpedance amplifiers with wide dynamic range for loopback testing. Although GaAs MESFET technology has superior noise performance compared to silicon technology for transimpedance amplifiers, this has not been fully exploited. Low voltage gain, which leads to a small feedback resistor, and the use of numerous active devices that have significant channel noise degrades the noise performance of many designs. Silicon bipolar transimpedance amplifiers suffer from poor sensitivity because of the shot noise produced by the base current, and from the thermal noise produced by a low-value feedback resistor. A low-value feedback resistor can result from the parasitic capacitance associated with implanted or thin-film resistors, and the lossy silicon substrate. This parasitic capacitance lowers the bandwidth because it exists in the feedback path with the feedback resistor and/or because it degrades stability. GaAs MESFET technology, on the other hand, does not suffer from this limitation because of the semi-insulating substrate. The challenge for GaAs MESFET technology is to produce adequate voltage gain in spite of the low inherent gain of short-channel MESFETs without degrading noise performance or stability. The circuit described here meets these requirements and achieves an average input-referred noise current of <2pAspl radic/Hz over 500 MHz bandwidth. Self-contained AGC extends dynamic range without external components.<>