Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
1998-08-27
2001-02-13
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S260000
Reexamination Certificate
active
06188280
ABSTRACT:
FIELD OF THE INVENTION
The present claimed invention relates to the field of differential amplifiers. More particularly, the present claimed invention relates to differential amplifiers that generate linear gain through transconductance compensation.
BACKGROUND ART
Differential amplifiers are widely used in analog and digital circuits to amplify a differential voltage between two input signals. Ideally, a differential amplifier amplifies only the difference between the input signals while rejecting common-mode input changes such as noise. Differential amplifiers have found important applications where signals are contaminated by noise signals. For example, digital signals transmitted over a long cable may pick up miscellaneous noise signals during the signal transmission. The differential amplifiers reject the noise signals while amplifying the digital signals, thereby leading to the recovery of the original signals.
Unfortunately, conventional differential amplifiers present several drawbacks. For example, fully differential amplifier circuits often suffer from limited linear operating ranges as described in U.S. Pat. No. 5,289,136 by DeVeirman et al., the disclosure of which is incorporated herein in its entirety. As a result, the differential amplifier circuits can only receive a narrow range of input voltages to produce a linear output. If the input voltages venture outside of the narrow input voltage range, the differential amplifier circuits produce a non-linear output.
In addition, the gain of conventional differential amplifiers may vary in response to a change in temperature. This is because base-emitter voltages of individual transistors in the differential amplifiers are highly sensitive to temperature variation. Since base-emitter voltages affect the transconductance of a transistor, the gain of the differential amplifier may not be constant or linear when such temperature variation occurs. In such cases, the gain of the conventional differential amplifiers may not be predictable due to the temperature variations.
Furthermore, the conventional differential amplifiers may not provide sufficient gain for today's state of the art low current or low voltage applications. For example, the state of the art high speed analog or digital circuits often employ low currents and/or low voltages to speed up the operation of the circuit while reducing power requirements for continuously decreasing die sizes. At such low currents and/or voltages, the conventional differential amplifiers may not provide a large enough gain to operate properly in high speed applications.
Thus, what is needed is a differential amplifier that provides a large and predictable linear gain over a wide input range. What is also needed is a differential amplifier that provides such linear gain even in low current and low voltage applications.
SUMMARY OF THE INVENTION
Broadly speaking, the present invention fills these needs by providing a differential amplifier having transconductance compensating circuitry to provide a linear gain over a wide input range. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, the present invention provides a differential amplifier including an emitter follower pair, a first differential pair, and a main differential amplifier. The emitter follower pair is operative to receive an input differential voltage signal for shifting the input differential voltage signal to develop a shifted differential voltage signal. The first differential pair is configured to feed a first differential current inversely to the emitter follower pair so that a transconductance of the emitter follower pair changes inversely to compensate for a change in a transconductance of the first differential pair. The main differential amplifier is coupled to receive the shifted differential voltage signal and is configured to amplify the shifted differential voltage signal to generate an output voltage signal.
In another embodiment, the present invention provides a transconductance compensating circuit for a differential amplifier having a main differential pair. The compensating circuit includes an emitter follower pair and a first differential pair. The emitter follower pair is operative to receive an input differential voltage signal for shifting the input differential voltage signal to develop a shifted differential voltage signal. The first differential pair is configured to feed a first differential current inversely to the emitter follower pair such that a transconductance of the emitter follower pair changes inversely to compensate for a change in a transconductance of the first differential pair.
In yet another embodiment, the present invention provides a method for compensating variations in transconductance of a differential amplifier. The method includes (a) receiving, by a pair of emitter follower transistors, a differential input voltage; (b) providing, by a pair of differential pair of transistors, a compensating differential current inversely to the emitter follower transistors; (c) changing a transconductance of the emitter follower transistors inversely to compensate for a change in a transconductance of the differential pair transistors; and (d) shifting, by the emitter follower pair of transistors, the DC voltage level of the differential input voltage by a specified voltage in accordance with the change in the transconductance of the emitter follower pair transistors.
In another embodiment, a transconductance compensating circuit for a differential amplifier is disclosed. The transconductance compensating circuit includes input means and differential current providing means. The input means receives and shifts an input differential voltage signal to develop a shifted differential voltage signal. The differential current providing means provides a first differential current inversely to the input means such that a transconductance of the receiving and shifting means changes inversely to compensate for a change in a transconductance of the differential current providing means.
Advantageously, the differential amplifier of the present invention provides a large linear gain over a wide input range by compensating for transconductance variations of differential amplifier transistors. In addition, the differential amplifier provides such linear gain even in low current and low voltage applications by utilizing level shifting circuitry. Furthermore, the gain of the differential amplifier is highly predictable because the gain depends only on a set of resistors and not on the transconductance of transistors in the differential amplifier. Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.
REFERENCES:
patent: 4780688 (1988-10-01), Groom
patent: 5289136 (1994-02-01), DeVeirman et al.
patent: 5392002 (1995-02-01), Delano
patent: 5461342 (1995-10-01), Crabtree
patent: 5481224 (1996-01-01), Kimura
patent: 5525930 (1996-06-01), Pothast et al.
patent: 5552742 (1996-09-01), Perkins
Hickman Coleman & Hughes LLP
Maxim Integrated Products
Mottola Steven J.
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