Amplifiers – With semiconductor amplifying device – Including differential amplifier
Utility Patent
1999-05-18
2001-01-02
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S086000, C330S260000, C330S009000
Utility Patent
active
06169453
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of semiconductor devices and, more particularly, to a differential amplifier.
BACKGROUND OF THE INVENTION
A differential amplifier is an analog circuit that provides an output signal (Vout) corresponding to a difference between two input voltage signals (Va−Vb). The output signal is multiplied by a gain factor, which is defined in the design stage of the differential amplifier.
A traditional prior art scheme of a differential amplifier is shown in
FIG. 1. A
description of this circuit can be found in any basic electronics textbook, and is generally based on an operational feedback amplifier. The main drawback of the traditional differential amplifier of
FIG. 1
is poor common mode rejection. This is because any spread of the value of the resistances about their nominal design value produces a common mode output signal.
A different approach to a differential amplifier is based on the use of a degeneration resistance connected between the emitters of an input pair of transistors, according to either one of the two circuits shown in FIG.
2
. According to this alternative approach, the common mode rejection can be significantly enhanced although a limit is placed on the maximum swing of the input voltages, which can no longer swing as far as the power supply voltage. It is evident that there is a need for a differential amplifier capable of ensuring an enhanced common mode rejection without restricting the voltage swing, and keeping the inputs compatible with the power supply voltage.
SUMMARY OF THE INVENTION
A differential amplifier according to the present invention provides a high common mode rejection while maintaining substantially unchanged the input dynamic range. The differential amplifier includes a comparator having inputs to which are applied two input signals so that the comparator outputs a signal whose sign is indicative of the sign of the difference between the two input signals.
The output of the operational amplifier is feedback to one of the inputs of the operational amplifier through a current mirror. This feedback signal is switched between the non-inverting input of the operational amplifier and the inverting input of the operational amplifier. The switching of the feedback signal ensures negative feedback, and is dependent upon the sign of the difference detected by the comparator.
An output stage is connected in cascade to the output of the operational amplifier. Also connected therebetween is a stage whose function is to invert the sign of the output signal of the operational amplifier. This inversion is dependent upon the sign information provided by the comparator. The output signal, whether inverted or not by the dedicated stage placed in cascade to the operational amplifier, drives the output stage. The output stage includes a transistor and a resistive load. The value of the resistive load may be selected to define a certain multiplying factor for the difference between the input signals.
The output stage forms the input branch of a current mirror that applies a feedback current either on the inverting or on the non-inverting input node of the operational amplifier. The feedback current is applied through a switch controlled by the output of the comparator.
REFERENCES:
patent: 4034308 (1977-07-01), Wermuth et al.
patent: 4641105 (1987-02-01), Albaugh et al.
patent: 5319265 (1994-06-01), Lim
patent: 5548453 (1996-08-01), Sasaki et al.
patent: 5777515 (1998-07-01), Kimura
patent: 6028482 (2000-02-01), Herrie
patent: 6064257 (2000-05-01), Sauer
patent: 0 534 007 A1 (1991-09-01), None
Milanesi Andrea
Poletto Vanni
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Galanthay Theodore E.
Nguyen Khanh Van
Pascal Robert
STMicroelectronics S.r.l.
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