A 2 Gbit/s 0.18 μm CMOS Front-End Amplifier for Integrated Differential Photodiodes Markus Grözing, Michael Jutzi, Winfried Nanz, and Manfred Berroth Institute of Electrical and Optical Communication Engineering University of Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany Abstract — A front-end amplifier for use with an integrated differential spatially modulated CMOS photodiode using unmodified 0.18 µm technology is presented. The circuit comprises a low-noise transimpedance amplifier (TIA) connected with a high-gain limiting amplifier (LIA) and a 50 Ω driver. A combination of thick and thin oxide transistors is used to allow for 2.2 V photodiode bias for enhanced opto-electrical sensitivity and bandwidth. The two-stage TIA is designed for minimum integrated input referred noise current by careful dimensioning of the TIA input stage. The TIA-LIA overall transimpedance is 58 MΩ (78 dBΩ) and it supports data rates up to 2 Gbit/s. The integrated input referred noise current is 1 µA. The driver provides 125 mV single-ended voltage swing. The power consumption from the 3.3 V supply is 34 mW for the TIA- LIA-combination plus 17 mW for the 50 Ω driver. The optical sensitivity of the amplifier integrated with a CMOS photodiode at an incident wavelength of 850 nm for a bit error rate smaller than 10 -11 is -7 dBm at 2 Gbit/s and -10 dBm at 1.25 Gbit/s with a 2 31 -1 PRBS input signal. Index Terms — transimpedance amplifier, CMOS OEIC, CMOS photoreceiver, integrated photoreceiver. I. INTRODUCTION Serial data transmission with rates of several Gbit/s becomes popular to connect memory and processors in server applications [1] and in router backplanes. But electrical transmission lines on standard circuit board substrates like FR4 suffer from losses, reflections, dispersion and coupling between lines. Optical interconnects with their large bandwidth, low loss and immunity to electromagnetic interference can provide an interesting alternative [2]. Optical detectors can already be integrated with standard CMOS technologies [3]. But up to now, the bandwidth of CMOS photodiodes illuminated by red light from low-cost vertical-cavity surface-emitting lasers (VCSELs) is limited. This is due to the large 1/e- penetration depth of 850 nm light in silicon. Carriers generated deep in the substrate generate a slow diffusion tail in the opto-electrical impulse response of the photodiode. The differential spatially modulated photodiode is one approach for high-speed optical receivers that can be integrated within unmodified CMOS technologies [4],[5]. But as this approach halves the photoresponsitivity, a high-gain low-noise amplifier is exceptionally important for the use of such a photodiode. A short description of the differential photodiode is given in section II of this paper. A detailed description of the used photodiode and its modeling is given in [6]. This paper will concentrate on the implementation issues of the amplifier chain in section III. Section IV presents the measurement results and section V summarizes the achieved results. II. DIFFERENTIAL CMOS PHOTODIODE The differential photodiode comprises two cathodes. As shown in Fig. 1, cathode K i is connected to illuminated n- well fingers and K s is connected to n-well fingers that are shaded by floating metal. The active area is 100 μm x 100 μm and the spatial period of the illuminated cathode n-well fingers is 9 μm. The equivalent circuit of the photodiode used for circuit design and simulation is shown in Fig. 2. The cathode to substrate capacitances C Di and C Ds are about 200 fF and the capacitance between the two cathodes C Dis is about 250 fF. The differential photocurrent shaded d illuminate Diff , PD I I I - = (1) is used as the output signal of the photodiode. Its opto- electrical bandwidth is in excess of 1 GHz at 2 V reverse bias and at a wavelength of 850 nm, because it only comprises drift carriers from the illuminated space charge region (SCR) and carriers diffusing from the p-substrate region close to the illuminated fingers. Thus bit-rates up to 2 Gbit/s can be supported. But its photoresponsitivity is only about 20 mA/W, as most carriers are generated deep in the substrate. The common mode photocurrent ( ) shaded d illuminate 2 1 CM , PD I I I + ⋅ = (2) mainly comprises slow diffusing carriers deep from the substrate. Its photoresponsitivity is about 40 mA/W. The common mode photoresponsitivity bandwidth is only a few ten MHz. SiRF 2006 361 0-7803-9472-0/06/$20.00©2006 IEEE