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Automatic Beta Compensation

IP.com Disclosure Number: IPCOM000107421D
Original Publication Date: 1992-Feb-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 2 page(s) / 57K

Publishing Venue

IBM

Related People

Montegari, F: AUTHOR

Abstract

When process tolerances result in low beta transistors, low beta within current mirror devices will reduce the current sink capacity of the mirror circuit. Base currents of devices that load a current mirror can also become a significant portion of the total current and can affect voltage levels in associated circuits.

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This is the abbreviated version, containing approximately 65% of the total text.

Automatic Beta Compensation

       When process tolerances result in low beta transistors,
low beta within current mirror devices will reduce the current sink
capacity of the mirror circuit.  Base currents of devices that load a
current mirror can also become a significant portion of the total
current and can affect voltage levels in associated circuits.

      The Automatic Beta Compensation (ABC) circuit in the figure is
a current mirror circuit that automatically compensates for low beta
with the addition of only one circuit element.  As beta is reduced
the base currents of all devices increase and a conventional current
mirror circuit will cancel the effects of Vbe and temperature
variations on itself but cannot compensate for low beta nor
anticipate the need for more current to supply additional base
current loads.

      Automatic Beta Compensation is introduced by the addition of R6
in the base of current mirror device T6.  As beta is reduced, the
base current of T6 increases and produces a larger voltage drop
across R6.  This results in a higher voltage at the collector of T6
since its collector node must supply a T6 base current that is
sufficient to support its collector current.  The additional voltage
drop produced across R6 as a result of lower beta is applied to the
base of current source T5 and thus increases its collector current as
beta is reduced.  The amount of compensation is designable by
altering the value of compensation resistor R6...