A 12 GHz 30 dB modular BiCMOS limiting amplifier for 10 Gb SONET receiver

In SONET, a limiting amplifier amplifies the voltage output of the transimpedance amplifier to a level high enough for a decision circuit to determine correct 1s and 0s from incoming bit streams. In OC-192, the data rate is 9953.28 Mb/s. The overall receiver must have a bandwidth of 6.5 GHz to avoid intersymbol interference (ISI) but must meet the minimum sensitivity requirement which ranges from -12 dBm for short-haul communication to -24 dBm for long-haul communication at BER of 10-12. For long-haul communication, the minimum photocurrent is approximately 40 μAp-p if an avalanche photodiode with 0.5 A/W responsivity and a gain of 10 are assumed. Then, the rms noise current referred to the input of the receiver should be less than 2.85 μA for BER of 10-12. This includes a noise contribution from the photodiode, transimpedance amplifier, and limiting amplifier. Typically, the photodiode and transimpedance amplifier dominate the receiver noise. Thus, the rms noise voltage contributed by the limiting amplifier should be much less than 2.85 mV if the transimpedance amplifier has 1 kΩ gain. The bandwidth of the overall receiver is determined mostly by the transimpedance amplifier (usually a transimpedance amplifier followed by a first-order low pass filter to limit the noise contribution). The bandwidth of the limiting amplifier must be >10 GHz to cause no ISI