GB1265034A - - Google Patents

Abstract

1,265,034. Facsimile apparatus. XEROX CORP. 2 Jan., 1970 [8 Jan., 1969], No. 237/70. Heading H4F. The signal obtained from a photo-sensitive semi-conductor 10 on scanning an image is fed to a differential operational amplifier 20 provided with feedback circuitry such that signals below a selected value V R are all represented by a single output V R and signals above V R and above or below a value V REF are represented by an output V REF , thereby enabling, e.g., the intelligence on a document to be clearly distinguished from the background. The amplifier 20 is connected, via a D.C. restorer 44, 50, 56 which determines V R , to one input of a long-tailed pair 60, 62, resistors 82, 84 defining a reference potential V REF at the other input. Thus depending on whether the signal at 60 is greater or less than V REF , a capacitor 64 is charged or discharged. Since the gain of amplifier 20 is defined by the ratio between the impedance in its feedback path and the impedance presented at its inverting input 24, i.e. resistance 26 divided by the effective drain-source impedance R DS of a field effect transistor 30, and since R DS varies according to the potential applied to the transistor gate electrode 34 by the capacitor 64, all signals above V R are amplified to appear with an amplitude V REF at an output 42. If the input at transistor 60 is much greater than V REF , transistors 72 and 74 conduct to speed the charging of capacitor 64.