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Browse Prior Art Database

Cathode Bias Circuit for Cathode Ray Tube

IP.com Disclosure Number: IPCOM000049834D
Original Publication Date: 1982-Jul-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 2 page(s) / 31K

Publishing Venue

IBM

Related People

Ainscow, F: AUTHOR

Abstract

Conventional bias circuits define the voltages on the grid G1 and cathode of a CRT. These voltages are then adjusted (manually) to compensate for variations in mutual conductance (gm) and cut-off voltage to produce the required brightness. The circuit described here defines the cathode current, regardless of gm and cut-off. By this means, the color balance can be preset as the ratio of 3 resistors. Provided the phosphor sensitivities do not vary widely, this balance will hold over the entire brightness range and will require no adjustment.

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Cathode Bias Circuit for Cathode Ray Tube

Conventional bias circuits define the voltages on the grid G1 and cathode of a CRT. These voltages are then adjusted (manually) to compensate for variations in mutual conductance (gm) and cut-off voltage to produce the required brightness. The circuit described here defines the cathode current, regardless of gm and cut-off. By this means, the color balance can be preset as the ratio of 3 resistors. Provided the phosphor sensitivities do not vary widely, this balance will hold over the entire brightness range and will require no adjustment.

The drawing shows conventional fast video amplifier 1 driving video transitions through capacitor C1 to CRT cathode 2. DC bias of the cathode is fed in via inductor L1. Transistor Q3 and resistor R2 form a current source, which is adjustable by means of brightness control VR1, and is set equal to the required ON-current in the CRT. The choice of R2 value in each of the 3 channels feeding a color CRT fixes color balance over the entire range of brightness.

Transistors Q1 and Q2 form a current switch which diverts the collector current of transistor Q3 into or away from capacitor C2 according to the video signal. The result is that the current in inductor L1 is equal to the defined ON- current multiplied by the video mark/space ratio. This forces the CRT cathode current to be exactly equal to the defined ON-current during the ON time.

Voltage V and resistor R1 are chosen to deliver a cons...