A low power 10 Gbs AGC optical postamplifier in SiGe

II. AMPLIFIER ARCHITECTURE AND CIRCUIT DESIGN
An AGC amplifier architecture bas a jitter advantage. and it provides increased flexibility by allowing a dynamic tradeoff between gain and bandwidth. A block diagram of the postamplifier is shown in Fig. I .
Fig. 1.
AGC amplifier architecture.
The signal path is fully differential and consists of a low-noise input stage with a constant gain followed by three VGA stages independently controlled by an AGC circuit. Due to a high maximum gain of the amplifier, an offset cancellation loop is provided to compensate for device mismatches induced by process variations.
Abslrorl A 10Gb/s automatic gain control amplifier for use in optical receivers was implemented in a SiGe pmeess with f, = 4SCHz. Active peaking techniques were used to achieve a maximum gain of 48dB with 7.8GHz of bandwidth. The amplifier demonstrates less thuo 0.5dB of peak-to.peak output amplitude variation over a 50dB input amplitude range. It consumes 30mW o f power from a 3.3V supply and remains functional at voltages as low as 2 . 1 V . The amplifier core occupies O.lmm’ and requires no external components. Ider Termr - Optical communication, optical remivcrs, broadband amplifiers, gain control, BiCMOS integrated circuits.
A Low-Power lOGb/s AGC Optical Postamplifier in SiGe
Daniel Kucharski and Kevin T. Kornegay
Cornell Broadband Communications Research Laboratory Cornell University, 330 Phillips Hall, Ithaca, NY, 14853
25
0-7803-8333-8/04/$20.00 0 2004 IEEE
2004 IEEE Radio Frequency Integmee
A schematic diagram of the input stage is shown in Fig. 2. To avoid loading of the TIA, the input is buffered with emitter followers QI driving a differential pair Q2. The input stage was optimized for low noise performance and has a constant gain of 15dB.
level using a feedback loop to adjust gain. LAs are a popular choice for optical receivers due to their relative simplicity. However, 3 variable gain amplifier (VGA) with AGC offers certain advantages. First, it maintains gain stages in their linear region to achieve a linear phase response and low deterministic jitter. Second, the AGC reduces the gain when presented with a large input signal, which avoids excessive noise amplification during signal transitions and reduces random jitter compared to an LA with the same maximum gain. Low jitter is very important for CDR operation, because non-return to zero ( N U )data has no energy at the clock frequency, and the clock must be recovered from data transitions. The remainder of this paper is organized as follows. Amplifier architecture and circuit design are presented in 1 , while fabrication details and experimental section 1 results are presented in sections I11 and 1V respectively.
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1. INTROOUCTION
Data rates in optical communication systems are limited by the speed of available optoelectronic devices such as lasers and photodetectors as well as their electrical interfaces. This is particularly true for short-distance applications where dispersion and attenuation in optical fibers are secondary. Parallel data links can be used to increase data throughput and achieve very high aggregate data rates. However, to make such links economically feasible requires a high level of integration and area efficient design leading to high circuit density. Such circuits also benefit from low power consumption, which alleviates power dislribution and heat dissipation problems and simplifies array implementation. A signal path of an optical receiver begins with a photodetector, which produces a current that is proportional to the incident optical power. The photocurrent is subsequently amplified and converted to voltage by a transimpedance amplifier WA). However, a typical TIA is a single stage circuit with limited gain and an output amplitude on the order of a few tens of millivolts, which can vary over a wide range due lo variations in the transmitter’s output power as well as optical losses. Clock and data recovery (CDR) requires consistent, large signal levels to minimize bit error rate (BER). Consequently, a postamplifer is required with a high gain, a wide dynamic range, and a constant output swing. These requirements can be satisfied with a limiting amplifier (LA) or an automatic gain control (AGC) amplifier. An LA is designed to saturate at the desired signal level, while an AGC amplifier maintains the signal