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Increasing Dynamic Range of Video Log Amplifier

IP.com Disclosure Number: IPCOM000079464D
Original Publication Date: 1973-Jul-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 43K

Publishing Venue

IBM

Related People

Dreska, RE: AUTHOR

Abstract

This is a method of overcoming body resistance effect that limits the dynamic range of a video-log amplifier when a diode is used as the log element.

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Increasing Dynamic Range of Video Log Amplifier

This is a method of overcoming body resistance effect that limits the dynamic range of a video-log amplifier when a diode is used as the log element.

In a conventional video-log amplifier system (Fig. 1), the body resistance R(B1) of log diode 1 introduces a linear term to the log characteristics of the diode. This results in an error in the amplifier log characteristic when voltages are large.

To compensate for this error, two matched pair log diodes 1, 2 (Fig. 2) are used to provide equal currents I(1), I(2) to each leg. Resistance R(B) is added and made equal to body resistance R(B1); and R(x2) is added and made equal to resistance R(x1). Under this condition, output voltage, V(o) = I(2)R(B) + alpha log(e) I(1) + I(1)R(B1) where alpha represents the gain constant. But, since I(1) = I(2) and R(B) = R(B1), V(o) = alpha log(e) I(1) + I(1) (R(B1) - R(B)) = alpha log(e) I(1) with the result that output voltage V(o) is a true log function.

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