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Browse Prior Art Database

Gamma Correction Circuit

IP.com Disclosure Number: IPCOM000052011D
Original Publication Date: 1981-Apr-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 3 page(s) / 63K

Publishing Venue

IBM

Related People

Marple, WP: AUTHOR

Abstract

The gamma characteristic of a CRT display is a nonlinear relationship between the light output of the CRT and the CRT video output voltage, as illustrated in Fig. 1. The gamma characteristic of typical CRT displays is a positive exponential curve. Since it is desirable to achieve a linear relationship between the CRT light output and the video input voltage, conventional gamma correction circuits employ a logarithmic transfer function which, when superimposed upon the exponential transfer function of the CRT itself, results in a linear characteristic. However, this linear characteristic for prior art circuits is difficult to maintain over large temperature ranges.

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Gamma Correction Circuit

The gamma characteristic of a CRT display is a nonlinear relationship between the light output of the CRT and the CRT video output voltage, as illustrated in Fig. 1. The gamma characteristic of typical CRT displays is a positive exponential curve. Since it is desirable to achieve a linear relationship between the CRT light output and the video input voltage, conventional gamma correction circuits employ a logarithmic transfer function which, when superimposed upon the exponential transfer function of the CRT itself, results in a linear characteristic. However, this linear characteristic for prior art circuits is difficult to maintain over large temperature ranges.

The circuit disclosed herein (Fig. 3) provides temperature compensation for temperature excursions in a gamma correction circuit by using a pair of matched junction field-effect transistor devices Q7 and Q8, with the device Q7 in a feedback loop to maintain constant channel resistance over a temperature range.

Fig. 2 is a functional schematic diagram showing the relationship of the gamma correction circuit 1, shown in Fig. 3, to the video amplifier circuit and the CRT itself. The positive input to the video amplifier is grounded, and the negative input is connected through the gamma correction circuit 1 to the video input which engages in voltage swings of from 0 through 4 volts. The video amplifier has a negative feedback through the resistor RF, and the output of the video amplifier is connected to the cathode of the CRT, whose grid is grounded.

For normal operation, the low level video signal (0 to 4 volts) is connected to "A" and the output "K" is connected to the inverting input of the output amplifier which has a 2.8K ohm feedback resistor. "B" is L connected to "D;" "C" is connected to "E;" and "F" is connected to an appropriate supply voltage.

The variable impedance element Q8 is a junction field-effect transistor operated with the gate forward biased. In this mode, the channel resistance varies logarithmically with the drain-to-source voltage. The gate bias for Q8 is supplied from the output of U1A via R14. The output level of U1A is determined by the channel resistance of Q7 which is matched with Q8. Q7 also derives its gate bias from the output of U1A (via R12 which is equal to R14); thus the gate bias for Q7 and Q8 is identical. Since Q7 is in the feedb...