A video amplifier for use in a document scanning system to provide signals representative of the information content of a scanned document. The amplifier includes an automatic background control portion and an automatic black level adjustment portion to maintain the integrity of the output voltage range independent of the background of the scanned document and throughout the period of time required to scan the entire document. In addition, circuitry is provided for varying the time constant of the automatic background control circuitry to make that circuitry respond quickly to changes of the background toward the "white" direction. Further, the normally white platen back is provided with a gray leading edge to prevent the automatic background control circuitry from being upset by initial runs of white in the event of an imperfectly registered non-white document.