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A low-noise, cryogenically cooled preamplifier has been designed and built for indium antimonide detectors used in IR astronomy. General noise and frequency response equations for the transimpedance amplifier are derived. The effects of finite open-loop gain and bandwidth and of photodetector parameters on circuit performance are shown. A current-injecting background cancellation scheme based on optical feedback and synchronous filtering is described. Total electrical NEP figures as low as 10/SUP -16/ W (at 5 /spl mu/m) and background rejection ratios higher than 3000:1 have been measured using a 0.5 mm diameter detector.