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White Normalizer for Video Signals

IP.com Disclosure Number: IPCOM000086334D
Original Publication Date: 1976-Aug-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 42K

Publishing Venue

IBM

Related People

Siverling, MM: AUTHOR

Abstract

Video signals from optical scanners must often be normalized so that the whitest (e.g., most negative) excursions occur at a predetermined absolute level. The overall gain of amplifier 10, from input 11 to output 12, is unity. The DC signal level, however, shifts so that the most negative excursion is always one Vbe drop below a reference voltage 13.

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White Normalizer for Video Signals

Video signals from optical scanners must often be normalized so that the whitest (e.g., most negative) excursions occur at a predetermined absolute level. The overall gain of amplifier 10, from input 11 to output 12, is unity. The DC signal level, however, shifts so that the most negative excursion is always one Vbe drop below a reference voltage 13.

Transistor Q1 operates as an emitter-follower. If the current through resistor R1 is held constant, the voltage shift between signals 11 and 14 will be constant at Vbe+ (R1 x Ie). Thus, the voltage level of signal 14 can be established by feedback control of transistor Q1's emitter current, Ie. The mechanism for this control is a current mirror including components 22-Q4 and R2-R4.

Signal 14 is isolated from the preceding circuit by emitter-follower Q5, which has an active load device including transistors Q6, Q7 and resistors R5-R7. A second emitter-follower Q8, R8 delivers the output signal 12 through impedance- matching resistor R9.

In normal operation, resistor R10 charges capacitor Cl to increase the base voltage of Darlington follower Q9, Q10, R11. This causes a greater current to be pumped into the Q2, 73 mirror, which increases Ie of transistor 21 and thereby increases the voltage drop between the base of Q1 and signal 14 at the collector of transistor Q2. But, if signal 14 attempts to go more negative than one Vbe drop below reference voltage 13, transistor Q11 turns on. Th...