Optical receiver with electronically tunable sensitivity

Optoelectronic integrated circuits (OEICs) for optical storage (OS) systems like CD-ROM and DVD include photodiodes, which convert light with a wavelength of 780 nm, 650 nm or 405 nm into a current, and a transimpedance amplifier (TIA), which transforms the photocurrent via a feedback resistor into an output voltage. OEICs achieve higher bandwidths than amplifier chips with external photodiodes due to the avoidance of bondpad capacitances. A small active chip area, a small power consumption, a low noise level, a small offset voltage, a sensitivity being switchable in a wide range and a high bandwidth are desirable in these OEICs. Therefore, BiCMOS instead of CMOS processes are used for DVD OEICs. The project goals are investigation by design, fabrication and experimental verification of OEICs with innovative voltage- or current-controlled transimpedance for OS systems in a 0.6 micron OPTO-ASIC BiCMOS technology. In contrast to some TIAs with variable gain intended for optical receivers where the output voltage is a nonlinear function of the photocurrent, OS applications require a good linearity. The parasitic capacitance of commonly used polysilicon feedback resistors and of MOSFET switches, which limits the bandwidth and the sensitivity, will be avoided by the innovative approach of voltage- (VCR) or current- (CCR) controlled resistors in the TIA. In addition, a new compensation network solves the stability problem of switched TIAs. The advantages summarize to: The equivalent resistance of the VCR (CCR) (sensitivity of the OEIC) can be changed digitally (in case of DVD) or continuously by varying the control voltage (current) over a wide range; VCR/CCR and the new compensation network increase the frequency bandwidth compared to conventional DVD OEICs by factors of 2 to 4. Simultaneously, the maximum transimpedance will be increased by a factor of 2.5 to 10 by avoiding the bandwidth limitation of integrated polysilicon resistors due to their parasitic capacitance towards substrate.