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Wide Range Gain Controlled Amplifier

IP.com Disclosure Number: IPCOM000077839D
Original Publication Date: 1972-Sep-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Hess, WJ: AUTHOR

Abstract

Circuit 10 uses photoemitter sensors 11, 12 to control the gain of an inverting operational amplifier 13. The controlled resistor sensors 11a, 12a are photoconductive layers with a wide range of resistance, determined by the light falling on their surfaces. The particular devices 11, 12 employ miniature incandescent lamps 11b, 12b as the controlled light-source emitters.

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Wide Range Gain Controlled Amplifier

Circuit 10 uses photoemitter sensors 11, 12 to control the gain of an inverting operational amplifier 13. The controlled resistor sensors 11a, 12a are photoconductive layers with a wide range of resistance, determined by the light falling on their surfaces. The particular devices 11, 12 employ miniature incandescent lamps 11b, 12b as the controlled light-source emitters.

The two photoemitter sensors 11 are provided with inverse current control. This is provided by voltage to current converters 14, 15 and the control signal Vc applied to their common input 16. Resistors 11a and 12a represent, respectively, the feedback resistance Rf and the input gain determining resistance R1 of inverting operational amplifier 13. The current polarity of converter 15 is inverted with respect to the input control voltage, resulting in a 90 DB dynamic range of gain versus control voltage.

Each individual photoemitter sensor 11a, 12a has a nonlinear log of resistance versus control current characteristic. However, operational amplifier 13's gain G is determined by the ratio of the resistances of the two resistors 11a, 12a, that is G = Rf/R1, for large open-loop gains (A). Since Rf and R1 exhibit similar transfer functions, their nonlinearities (difference from a log resistance versus current characteristic) tend to cancel over approximately a 90 DB gain range. In addition to this, their variations due to temperature and aging also tend to cancel....