A 3 - V Cmos Optical Preamplifier with Dc Photocurrent Rejection

This paper describes a CMOS optical preamplifier suitable for free-space, infrared wireless communications. The design is differential for improved power supply noise rejection, and an active cancellation scheme is used for eliminating dc photocurrents that are generated by ambient light. Making use of a 0.35-µm CMOS process, the preamplifier provides a transimpedance gain of 20 kΩ over a bandwidth from 1 MHz to 100 MHz and achieves 60 dB attenuation at dc. I. INTRODUCTION The proliferation of infrared (IR) wireless technology in laptops, computer peripherals, and digital cameras has helped drive new developments in the area of optical receiver design. Traditional applications such as fiber-optic networks have emphasized speed, and consequently have used high-speed processes such as GaAs. IR wireless, on the other hand, emphasizes system integration and cost. Consequently, bulk CMOS is the preferred technology. Recent papers on CMOS optical preamplifiers[1-3] have highlighted some of the design challenges of single-chip receivers, specifically that of obtaining sufficient speed and sensitivity given low supply voltages, small transconductances, and noisy power supplies. Because of its wide bandwidth and low noise, the transimpedance amplifier is commonly used in optical receivers[4]. Although inverter-based transimpedance amplifiers have shown good speed and sensitivity[2], they generally have poor power supply rejection because of the capacitive coupling of the supplies to the signal path[5]. A more robust approach is to use a differential structure with shunt feedback[3]. However, this approach has only been realized using bipolar and NMOS devices. This paper describes how the structure can be adapted and optimized for a true CMOS implementation. Apart from the preamplifier, IR wireless communications itself poses numerous challenges. One major concern is the presence of dc photocurrents generated when a photo-diode is exposed to ambient indoor light. Often the dc photocurrent is much larger than the signal current, resulting in reduced signal swing or even saturation. One solution is to ac couple the photodiode to the preamplifier input[6]. However, depending on the desired high-pass frequency, large capacitors and resistors may be required. For mono-lithic implementations, this implies increased chip area and sensitivity to parasitic-coupled noise. Consequently, we propose instead to use an active cancellation scheme for dealing with dc photocurrents.