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This paper presents a fast-settling compact feedforward automatic-gain-control (AGC) circuit suitable for use in wireless local-area network receivers with orthogonal frequency-division multiplexing (OFDM). The use of these signals introduces stringent settling-time constraints which limit the use of traditional closed-loop feedback amplifiers. Furthermore, the amplitude detection of OFDM signals cannot be performed by a typical peak detector (PD) due to their high peak-to-average power ratio; as a consequence, a novel fast-settling PD is employed to solve this task. The AGC has been implemented in a low-cost 0.35-μm CMOS technology. Supplied at 1.8 V, it operates with a power consumption of 2.4 mW at frequencies as high as 100 MHz, while its gain ranges from 0 to 22 dB in 2-dB steps through a 5-b word. The settling time of the circuit is below 2.4 μs (three symbols).
Date of Publication: Aug. 2010