Amplifiers – Sum and difference amplifiers
Patent
1990-12-17
1992-09-08
Mullins, James B.
Amplifiers
Sum and difference amplifiers
330 84, 330295, H03F 345, H03F 368
Patent
active
051461765
DESCRIPTION:
BRIEF SUMMARY
The present invention relates generally to an analogue amplifier circuit suitable for a wide range of uses.
It is widely known that analogue amplifier circuits using differential operational amplifiers have a disadvantage due to the inherent error voltage which appears across the inputs of the differential operational amplifier. As used hereinafter, the term "operational amplifier" will be understood to refer to a differential operational amplifier.
Operational amplifiers have an inverting input and a non-inverting input. The error voltage arises because in practice the amplifier has a finite gain so that a voltage always appears at the inverting input of the amplifier with respect to the non-inverting input. This error voltage results in distortion, particularly at low frequencies, which makes the operational amplifier unsuitable for circumstances where high fidelity in the amplified signal is required. This is unfortunate since operational amplifiers are ideally suited for use with a balanced line input and a balanced line input is almost invariably required for high precision applications such as in medical apparatus and the like.
Various attempts have been made to compensate for the error voltage appearing across the inputs of an operational amplifier, usually achieved by the use of a compensating voltage applied to the output of the amplifier. The present invention seeks to provide an amplifier circuit incorporating an error-correction stage which does not involve the application of a compensating voltage to the output of a gain stage of the amplifier, but rather which acts to apply compensation at the input stage thereof.
According to the present invention, therefore, there is provided an analogue amplifier circuit having a gain stage including two operational amplifiers, in which the input error of at least one of the operational amplifiers is compensated by an error correction stage comprising at least one operational amplifier the inputs of which are connected across the inputs of the said one amplifier of the gain stage and the output of which is connected to the input of the other amplifer of the gain stage.
Preferably the inverting input of the operational amplifier of the error correction stage is connected to the non-inverting input of the said one operational amplifier of the gain stage.
Such a circuit is particularly suitable for use in a single line input mode which is effected by earthing one of two lines of a balanced input applied across the inputs of the two amplifiers of the gain stage.
Preferably the output of the operational amplifier of the error correction stage is connected to the non-inverting input of the said other operational amplifier of the gain stage.
For use with a balanced line input a symmetrical configuration is preferably employed in which the said error correction stage includes a second operational amplifier the inputs of which are connected across the inputs of the said other operational amplifier of the gain stage and the output of which is connected to the input of the said one amplifier of the gain stage. Such a symmetrical configuration thus acts to provide a compensating input to each input of a gain stage operational amplifier to compensate for its inherent error voltage.
The two outputs of the two operational amplifiers of the gain stage may provide a balanced line output, but if a single line output is required a final output stage may be provided having differential inputs connected to respective outputs of the said two operational amplifiers of the gain stage.
In preferred embodiments of the invention the output of the second operational amplifier of the error correction stage is connected to the non-inverting input of the said one operational amplifier of the gain stage and the inverting input of the second operational amplifier of the error correction stage is connected to the non-inverting input of the said other amplifier of the gain stage. Likewise it is preferred that the output of each operational amplifier of the error correction stage is connecte
REFERENCES:
patent: 3768033 (1973-10-01), Lowe
E. C. Audio Limited
Mullins James B.
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