Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – With compensation
Reexamination Certificate
2002-10-02
2004-10-05
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
With compensation
Reexamination Certificate
active
06801075
ABSTRACT:
FIELD OF INVENTION
The present invention relates to amplifier circuits. More particularly, the present invention relates to a base current compensation circuit for use in an amplifier circuit.
BACKGROUND OF THE INVENTION
The increasing demand for higher performance amplifier circuits has resulted in the continued improvement of the precision and accuracy of the various devices and components within the amplifier circuits, as well the inclusion of additional buffers and compensation circuits.
In the implementation of various amplifier circuits, losses in the integrity of referenced currents flowing through the various devices and components can be realized. As a result, the delivered current can be less than the intended current to be provided from the amplifier circuit. One problem that can cause losses in the integrity results from the absorption of base current that is demanded from transistor devices by other devices within the amplifier circuit.
For example, with reference to
FIG. 1
, a circuit
100
comprising a differential pair of transistors Q
1
and Q
2
, such as may be used within a logarithmic amplifier circuit, are illustrated. A first reference current I
C1
is provided to the collector of transistor Q
1
, while a second reference current I
C2
is provided to the collector of transistor Q
2
. It is desirable for a base current I
B1
demanded by transistor Q
1
to be provided without loss, such that a lost current I
LOST
equals zero. However, prior art amplifier circuits unfortunately are not able to provide the base current without loss. As a result, lost current I
LOST
can also generate error voltages that create an output error for amplifier circuit
100
.
One approach for compensating for lost current I
LOST
includes the use of an isolating buffer that creates an additional current for supplementing the lost current I
LOST
and that is provided at the base of the transistor Q
1
. However, such isolating buffer applications are generally more complex than desired. With reference to
FIG. 2
, another approach includes the implementation of a current mirror comprising diode-connected transistor Q
3
and transistor Q
4
configured to approximately provide the base current I
B2
from the base of transistor Q
2
to the base of transistor Q
1
, wherein transistors Q
1
and Q
2
are matched, and transistors Q
3
and Q
4
are matched. However, this approach requires significant headroom since the input current into the collector of transistor Q
2
requires the voltage at the collector of transistor Q
2
to be at least a base-emitter voltage drop V
BE
above the voltage at the base of transistor Q
1
. For many amplifier circuits, such as logarithmic amplifier circuits, it is desirable for the voltage at the collector of transistor Q
2
be equal to the voltage at the base of transistor Q
1
.
Accordingly, a need exists for addressing the lost base current resulting within amplifier circuits.
SUMMARY OF THE INVENTION
In accordance with various aspects of the present invention, a base current compensation circuit is configured for injecting base current to the base of a transistor device to compensate for the lost current demanded by a transistor base. The base current compensation circuit is configured to inject current into the base of the transistor without the headroom requirements, as well as being less complex than other approaches.
In accordance with an exemplary embodiment, a base current compensation circuit comprises a sampling circuit configured for sampling current from the transistor device, and for providing multiples of the base current demanded by the transistor device. The sampling circuit can comprise a first sampling component for sampling current comprising a collector current proportional to the collector current of the transistor device. The sampled collector current can be configured through a second sampling component into a sampled base current proportional to the base current demanded by the transistor device. The sampled base current can then be provided to the base of the transistor device to compensate for current lost at the base. Base current compensation circuit can also comprise a current mirror circuit coupled between the first sampling component and the second sampling component for facilitating the sampling functions.
REFERENCES:
patent: 4263562 (1981-04-01), Seiler
patent: 4345216 (1982-08-01), Schade, Jr.
patent: 4471321 (1984-09-01), Bowers
patent: 4757274 (1988-07-01), Bowers
patent: 5157322 (1992-10-01), Llewellyn
patent: 5382917 (1995-01-01), Miyake et al.
patent: 5617056 (1997-04-01), Main et al.
patent: 6204719 (2001-03-01), Gilbert
Doorenbos Jerry L.
Gammie David A.
Parfenchuck Jeffery B.
Brady W. James
Cunningham Terry D.
Swayze, Jr. W. Danial
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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