A transimpedance amplifier (TIA) has been realized in a 0.6-μm digital CMOS technology for Gigabit Ethernet applications. The amplifier exploits the regulated cascode (RGC) configuration as the input stage, thus achieving as large effective input transconductance as that of Si Bipolar or GaAs MESFET. The RGC input configuration isolates the input parasitic capacitance including photodiode capacitance from the bandwidth determination better than common-gate TIA. Test chips were electrically measured on a FR-4 PC board, demonstrating transimpedance gain of 58 dBΩ and -3-dB bandwidth of 950 MHz for 0.5-pF photodiode capacitance. Even with 1-pF photodiode capacitance, the measured bandwidth exhibits only 90-MHz difference, confirming the mechanism of the RGC configuration. In addition, the noise measurements show average noise current spectral density of 6.3 pA/√(Hz) and sensitivity of -20-dBm for a bit-error rate of 10-12. The chip core dissipates 85 mW from a single 5-V supply.