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Linearity Improvement for Bipolar Transistors Used as Electrically Variable Resistors

IP.com Disclosure Number: IPCOM000092618D
Original Publication Date: 1966-Dec-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 31K

Publishing Venue

IBM

Related People

Davidson, FS: AUTHOR [+2]

Abstract

The circuit employing two bipolar transistors is used as an electrically variable resistor and overcomes the limited signal handling capabilities of a single transistor. Drawing A shows an idealized family of I(C) versus V(CE) characteristics near the origin for a single transistor. The transistor equivalent resistance is the reciprocal of the slope of any given I(B) curve. The resistance can be varied by changing the value of I(B). Since the change in slope of any given I(B) curve is not symmetrically disposed about the origin, the signal handling capabilities of a single transistor are limited.

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Linearity Improvement for Bipolar Transistors Used as Electrically Variable Resistors

The circuit employing two bipolar transistors is used as an electrically variable resistor and overcomes the limited signal handling capabilities of a single transistor. Drawing A shows an idealized family of I(C) versus V(CE) characteristics near the origin for a single transistor. The transistor equivalent resistance is the reciprocal of the slope of any given I(B) curve. The resistance can be varied by changing the value of I(B). Since the change in slope of any given I(B) curve is not symmetrically disposed about the origin, the signal handling capabilities of a single transistor are limited.

A circuit which reduces this asymmetry is shown in drawing B. The two transistors are of the same type to realize the lowest distortion. T(2) is in parallel with T(1) but inverted. Hence the characteristics of T(2) correspond to rotating the frame of reference for I(C) and V(CE)in drawing A by 180 degrees. A composite transistor characteristic is obtained by adding the slopes of the T(1) family to the slopes of the inverted T(2) family.

A superposition of one I(B) curve from each family and the composite resultant is shown in drawing C. Other values of I(B) produce similar results with different slopes. If the slopes of the T(1) family exactly supplement the slopes of the T(2) family, linear resultants are obtained. In practice, the slopes of common transistors are sufficiently suppleme...