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Unformatted text preview: Fall 2005 6.012 Microelectronic Devices and Circuits Prof. J. A. del Alamo Optical Receiver Design Project November 18, 2005 Due: December 6, 2005 on the (no later than 12:55PM) (late project reports not accepted) 1. Overview The explosive growth in data communications has stimulated the development of optical systems for high channel capacity (typically 4-16 channels) and high bandwidth. In a fiber optic system, a transmitter encodes the data in the form of laser pulses that are transmitted over a long optical fiber. At the other end, a receiver detects the attenuated optical signal and amplifies it to digital levels. Figure 1: Block diagram of an optical transmitter and receiver. A block diagram of an optical transmitter and receiver is shown in Figure 1. On the transmitter path, the data is multiplexed, encoded, and error correction bits are added. A laser driver and modulator drive the laser diode, which transmits an optical signal over the fiber. After some loss in the fiber, the optical signal is detected at the receiver end by the photodiode. A transimpedance amplifier converts the small photodiode current into a voltage, which is then amplified to digital levels for subsequent digital signal processing. The transimpedance amplifier is also called a transresistance amplifier in 6.012; for cultural reasons, we will stick with the transimpedance amplifier terminology. 1 MIT course website Integration of all of the functions on either side of Figure 1 onto a single CMOS chip would save costs, but the implementation has eluded system designers in part due to the complexity of realizing high-performance receiver circuits in CMOS. The goal of this design project is to design a fast, high gain, low noise, and low power optical receiver in an inexpensive CMOS process. 2. Design problem statement Figure 2 shows the schematic of the optical receiver. It consists of three CMOS stages: a tran- simpedance amplifier, a saturating or limiting amplifier, and an output driver. We describe these three stages next. Light creates electron-hole pairs that produce a current I light in the reverse-biased photodiode. The diode can be modelled as a current source of value I light which flows in the reverse bias direction of the diode. Although the laser diode produces a large square wave pulse at the other end of the fiber, dispersion and loss make the diode current I light appear sinusoidal. This current is only guaranteed to have a peak value of about 10 A. Depending on the system, loss in the fiber could be lower and the peak diode current could be larger. However, to ensure proper operation for all systems, the worst-case (i.e. minimum) current must be used for the design. The receiver should operate at speeds up to 1 MHz....
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- Fall '05