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Browse Prior Art Database

DC Coupled Video Amplifier

IP.com Disclosure Number: IPCOM000040017D
Original Publication Date: 1987-Sep-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 51K

Publishing Venue

IBM

Related People

Ainscow, F: AUTHOR [+2]

Abstract

The video amplifier circuit disclosed reduces DC drift so that the known DC voltage at the input determines the DC operating point of the whole amplifier, so removing the need for DC restoration circuits, while retaining the required bandwidth and allowing voltage-operated gain control. The input stage is an asymmetric long-tailed pair with gain control which is independent of bias current. The output stage is an NPN-PNP cascode with complementary emitter followers at the output. Video amplifiers for modern color CRT displays require a gain of about 100 in each of the three color channels, combined with a pass band extending from DC to 30 MHz or more. Furthermore, an electrically controlled gain control is needed so that contrast can be adjusted simultaneously in all three channels by a remote contrast control.

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DC Coupled Video Amplifier

The video amplifier circuit disclosed reduces DC drift so that the known DC voltage at the input determines the DC operating point of the whole amplifier, so removing the need for DC restoration circuits, while retaining the required bandwidth and allowing voltage-operated gain control. The input stage is an asymmetric long-tailed pair with gain control which is independent of bias current. The output stage is an NPN-PNP cascode with complementary emitter followers at the output. Video amplifiers for modern color CRT displays require a gain of about 100 in each of the three color channels, combined with a pass band extending from DC to 30 MHz or more. Furthermore, an electrically controlled gain control is needed so that contrast can be adjusted simultaneously in all three channels by a remote contrast control. This is difficult to achieve because established circuit techniques to achieve the bandwidth are subject to drift of their DC operating point with temperature and aging. Commonly, drift is removed by AC coupling followed by complex DC restoration circuits. The circuit diagram shows the video amplifier using a DC coupled design with a differential input. The input-stage transistors T1, T2 form an asymmetric long-tailed pair designed to place zero input voltage (black level) near one end of the linear range of the amplifier. R1 provides a matching termination to the coaxial interface cable, while R2, gain potentiometer R3 and tail resistor R4 set the transconductance gain of this stage. Diode D1 supplies base bias current if the interface cable is removed. The DC collector current drawn by input transistor T1 (even when no video is present) is supplied from the 85 V rail by R5 and diodes D2, D3 regardless of the setting of the gain control so that b...