A high-speed 850-nm optical receiver front-end in 0.18-μm CMOS

A high-speed optical interface circuit for 850-nm optical communication is presented. Photodetector, transimpedance amplifier (TIA); and post-amplifier are integrated in a standard 0.18-mu m 1.8-V CMOS technology. To eliminate the slow substrate carriers, a differential n-well diode topology is used. Device simulations clarify the speed advantage of the proposed diode topology compared to other topologies, but also demonstrate the speed-responsivity tradeoff. Due to the lower responsivity, a very sensitive transimpedance amplifier is needed. At 500 Mb/s, an input power of -8 dBm is sufficient to have a bit error rate of 3.10(-10). Next, the design of a broadband post-amplifier is discussed. The small-signal frequency dependent gain of the traditional and modified Cherry-Hooper stage is analyzed. To achieve broadband operation in the output buffer, so-called "f(T) doublers" are used. For a differential 10 mV(pp) 2(31)-1 pseudo random bit sequence, a bit error rate of 5 . 10(-12) at 3.5 Gb/s has been measured. At lower bit-rates, the bit error rate is even lower: a 1-Gb/s 10-mV(pp) input signal results in a bit error rate of 7 . 10(-14). The TIA consumes 17 mW while the post-amplifier circuit consumes 34 mW.