Amplifiers – Combined with automatic amplifier disabling switch means
Reissue Patent
1999-04-15
2001-07-24
Mottola, Steven J. (Department: 2817)
Amplifiers
Combined with automatic amplifier disabling switch means
C330S086000
Reissue Patent
active
RE037291
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a circuit assembly having an operational amplifier, and more particularly to an operational power amplifier that can be switched to different configurations.
BACKGROUND OF THE INVENTION
Certain applications of operational amplifiers, in particular of operational power amplifiers, are known in which it is useful to modify the circuit assembly of which the operational amplifier forms part such that the latter is transformed into a buffer, i.e., into an amplifier in which the input circuit is only slightly sensitive to the variations in the load impedance and such that, therefore, it behaves in the manner of a separator between the output and input.
A typical application of this type is the muting control of an audio amplifier. An example of a known circuit assembly for performing this function is shown schematically in
FIG. 1
of the appended drawings which shows an operational amplifier consisting of a differential input stage
10
, an intermediate gain stage
11
having a compensation capacitance Cc between the input and output, and a power output stage
12
. The output terminal
13
of the output stage
12
and the inverting terminal of the input stage
10
are connected to one another by a feedback system consisting of a resistive divider Ra, Rb. A commutator
14
enables the input of the intermediate stage
11
to be switched such that it can be connected selectively to the output of the input stage
10
or to the output of a further differential input stage
15
of which the non-inverting input is connected to a fixed reference potential source VA and the inverting input is connected to the output
13
of the output stage
12
. In one position of the switch
14
, as shown in this Figure, the circuit assembly behaves in the manner of an operational amplifier with a gain of greater than 1, while, in the other position of the switch
14
, it behaves in the manner of a unitary gain amplifier of which the output is independent of the input signal Vin of the operational amplifier and dependent on the predetermined potential VA of the second input stage
15
. This solution has the disadvantage of requiring a considerable amount of space since two input stages are required, and also a given degree of complex circuitry since it also has to ensure the stability of the unitary gain amplifier, which entails particular design measures.
Applications are also known in which an operational power amplifier has to be converted into a circuit structure having high impedance between its output terminals, in which the amplifier can be controlled at very low power by a device connected to its output. A structure of this type is useful, for example, in circuits controlling electric motors. A technique for producing a structure of this type consists in deactivating the current generators of the entire amplifier or at least of the output stage. However, in this case the deactivation and subsequent reactivation of the current generators cause considerable variations in the operating conditions, giving rise to intense transient and delay phenomena of the output signal.
A further technique which is sometimes used for attaining high impedance between the output terminals is the provision of a switch in series with the amplifier output; when the switch is opened, the impedance between the output terminals of the amplifier in theory becomes infinite. When this technique is to be used in an integrated circuit, there is a disadvantage in that the switch, through which high currents have to pass, has to have large dimensions and therefore does not lend itself to integration.
SUMMARY OF THE INVENTION
The object of the present invention is to produce a circuit assembly having an operational amplifier which can be converted into the buffer configuration or into the “high impedance output” configuration without using additional bulky components, with the simplest possible circuit and such that it operates in a manner substantially free from interference which is due to transient phenomena.
According to the present invention, a circuit assembly for an operational amplifier has an input stage with first and second input terminals and an output terminal. An output stage has a first input terminal coupled to the output terminal of the input stage, a second input terminal, and an output terminal. A feedback circuit is coupled between the output terminal of the output stage and the second input terminal of the input stage. An interconnection circuit is coupled to the first and second input terminals and the output terminal of the output stage and to a reference voltage source. The interconnection circuit has first, second, and third modes, such that the second input terminal of the output stage is coupled to the reference voltage source when the interconnection circuit is in the first mode. The first input terminal of the output stage is coupled to the reference voltage source when the interconnection circuit is in the second mode, and the first input terminal of the output stage is coupled to the output terminal of the output stage and the second input terminal of the output stage is coupled to the reference voltage source when the interconnection circuit is in the third mode.
According to the present invention, the operational amplifier is configured as a normal amplifier when the interconnection circuit is in the first mode. The operational amplifier can be an impedance load for a control device when the interconnection circuit is in the second mode, and the operational amplifier is in a buffer configuration when the interconnection circuit is in the third mode.
REFERENCES:
patent: 5172073 (1992-12-01), Fujii
patent: 5191296 (1993-03-01), Novelli
patent: 5363062 (1994-11-01), Nebuloni et al.
patent: 29 39 017 A1 (1980-04-01), None
patent: 0 503 843 A1 (1992-09-01), None
patent: 1 587 204 (1970-03-01), None
Botti Edoardo
Mandrini Tiziana
Carlson David V.
Galanthay Theodore E.
Jorgenson Lisa K.
Mottola Steven J.
STMicroelectronics S.r.l.
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