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Current Source With Alpha Compensation

IP.com Disclosure Number: IPCOM000087308D
Original Publication Date: 1977-Jan-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 28K

IBM

Related People

Gersbach, JE: AUTHOR

Abstract

This article describes a novel current source circuit having alpha compensation, in which positive feedback is used to increase the output current as the alpha of the transistor reduces to keep the current constant after alpha attenuation. In this way the tolerance of the current source output is dramatically compensated.

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Current Source With Alpha Compensation

This article describes a novel current source circuit having alpha compensation, in which positive feedback is used to increase the output current as the alpha of the transistor reduces to keep the current constant after alpha attenuation. In this way the tolerance of the current source output is dramatically compensated.

It is desirable to keep the current Ix at a constant value in various logic and miscellaneous circuits where a current is generated and must pass serially through a number of transistors before arriving at an output. Each transistor attenuates the current by a factor of delta, grounded base current gain so that the total attenuation is given by delta/n/ where n is the number of series transistors.

If it is assumed that R2 = R3 = R4, and T1, 3 and 4 are the same size, then the currents in R3 and R4 are equal to the reference current. Ix is delta/3/ x Iref. T3, T8, T7 and D1 form a positive feedback loop around the current reference circuit formed by T1 and T2. The gain of this feedback circuit is the ratio of the emitter area of T7 to D1, times the attenuation from emitter to base of T8. The loop gain is then delta/2/K/beta+1, where K is the area ratio of T7 to D1. The equation for the reference current is:

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The ratio of Iref to I(x) equals delta/3/. It can be shown that little error is introduced by the assumption:

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The compensation is nearly ideal and is in fact within 2% of i...