Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
1999-11-12
2001-07-24
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S259000
Reexamination Certificate
active
06265941
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to electrical signal amplifiers, and more particularly to a differential amplifier having a balanced output and common-mode feedback.
2. Description of Related Art
A balanced or fully differential amplifier typically requires common-mode feedback to ensure that the amplifier operates in the region where its full differential dynamic range can be utilized. Certain implementations of common-mode feedback, however, may add additional voltage offset to the amplifier, move parasitic poles closer to the dominant pole, degrade stability or increasing the power required to obtain desired stability, causes common-mode to differential-mode signal conversion due to random mismatch of components, and/or may result in having latching or dead-zones from which the amplifier cannot recover and operate normally.
In an attempt to overcome these drawbacks, common-mode feedback using input stage tail current servoing and/or common-mode servoing using input stage active loads or folded-cascode active loads have been employed.
While common-mode feedback using input stage tail current servoing can have dead zones or start-up problems, it does nevertheless have certain desirable properties. For example, it does not permit common-mode to differential conversion and tends to exhibit a relatively enhanced bandwidth and offset properties. Common-mode servoing using input stage active loads or folded-cascode active loads can increase differential offset due to component mismatch, can degrade stability, can increase supply current required to maintain stability, and can allow common-mode to differential conversion.
SUMMARY
Accordingly, it is an object of the present invention to provide an improvement in differential amplifier circuitry.
It is another object of the invention to provide an improvement in a balanced differential amplifier utilizing common-mode feedback.
It is a further object of the invention to overcome the inherent problems and deficiencies associated with conventional balanced or fully differential amplifiers employing common-mode feedback.
The foregoing and other objects of the invention are directed to a balanced differential amplifier using common-mode feedback with input stage tail current servoing which solves the latch-up/dead-zone problem by inclusion of a kick-start circuit that ensures that the common-mode servo loop will always start up and force the amplifier into the intended operating region.
The preferred embodiment of the present invention is directed to a differential amplifier fabricated in an integrated circuit and comprising: a balanced differential amplifier circuit including first and second input ports and first and second output ports; a common-mode feedback circuit connected to the first and second output ports and generating a common-mode feedback signal which is coupled back to a predetermined circuit node of the amplifier circuit so as to form a common-mode servo loop; and, a kick-start circuit connected to the common-mode feedback circuit and generating a kick-start feedback signal which is coupled to a predetermined circuit node of the differential amplifier circuit to ensure that the circuit will start up and operate in its intended region.
Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be noted, however, that the detailed description and specific examples, which indicate the preferred embodiments of the invention, are provided by way of illustration only since various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from the following detailed description.
REFERENCES:
patent: 5254956 (1993-10-01), Nishijima
patent: 5838200 (1998-11-01), Opris
patent: 5955922 (1999-09-01), Nicollini et al.
patent: 6052025 (2000-04-01), Chang et al.
patent: 6147552 (2000-11-01), Sauer
Electronic Circuit Design, An Engineering Approach, Savant, Jr. et al, The Benjamin/Cummings Publishing Company, Inc., 1987, Chapter 7, pp. 300, 302, 304, 308, 312, and 316-319.
Agere Systems Guardian Corp.
Choe Henry
Pascal Robert
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