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This paper presents the investigation on the design of automatic gain control (AGC) circuits assumed to be robust against both bias and swing variations in the received unipolar digital signal. We have derived a small signal AGC general model that includes the impact of all relevant AGC components on the AGC transfer function. This model has been used for the calculation of AGC loop parameters of the specified AGC transfer function. The AGC robustness against signal level transients has been improved by introducing the second-order AGC loop. In ac-coupled receivers, the order and corner frequency of high-pass filter (HPF) placed in front of the AGC have been investigated. The HPF has been designed so that the input signal bias variations caused by signal level transients are compressed in acceptable limits, and so that bit-pattern bias variations do not significantly decrease the receiver performance. Signal swing and bias variations become very critical in optical systems using unipolar modulation format. Since the AGC design depends on its application, as an example, we have presented the AGC design concept in dispersion compensating 10.7 Gb/s intensity-modulated direct-detection (IM-DD) nonreturn-to-zero (NRZ) optical receiver using maximum-likelihood sequence estimation (MLSE).