A "current amplifier" insofar as this application is concerned is an active circuit having an output current related to and responsive to an input current over a portion of their ranges and need not exhibit current gain. The input and output currents may be quiescent currents or may exhibit variations. Those circuits commonly denominated "current mirrors" are included within the category "current amplifier" as defined hereabove.
Three-terminal current amplifiers are known of the type in which first and second transistors have their emitter electrodes direct current conductively coupled to the amplifier common-terminal; their respective collector electrodes direct current conductively coupled to the input and the output amplifier terminals, respectively, and their joined base electrodes direct coupled to the amplifier input terminal. Such a prior art current amplifier operates in the following manner. The first transistor regulates its collector current by means of its collector-to-base negative feedback connection to be substantially equal to the input current. Because of the parallel connection of the base-emitter circuits of the first and second transistors, the second transistor delivers a collector current as output current proportional to the collector current of the first transistor.
The prior art current amplifiers have no provision to maintain the collector potentials of their first and second transistors substantially equal under all conditions of conduction. Such provision is desirable to remove the effects of dissimilar collector potentials which undesirably affect the proportioning of the output and input currents of the current amplifier. It is desirable that the proportioning of the current amplifier output and input currents be determined solely by the relative geometries of the first and second transistors and, in particular, by their relative effective base-emitter junction areas.