Output stage using positive feedback to provide large...

Details Output stage using positive feedback to provide large... Output stage using positive feedback to provide large...

2003-05-12

2004-10-26

Choe, Henry (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including push-pull amplifier

C330S288000

Reexamination Certificate

active

06809590

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention generally relates to output stages of electronic components. More particularly, the invention relates to output stages in which increased current sourcing capability is desired.
2. Related Art
FIG. 1
depicts a conventional output stage that is suitable for use as an output buffer in, for example, Class AB amplifiers. While possessing favorable power supply rejection ratio (PSRR) performance, rail-to-rail output swing, and current sinking capability, the
FIG. 1
circuit has relatively weak current sourcing capability (I
OUT
is undesirably small).
Due to current mirroring phenomena in the
FIG. 1
configuration, the maximum output source current I
OUT
is ultimately determined by how much gate-to-source voltage (V
GS
) can be applied to the MP
2
sink. Assuming that all transistors in
FIG. 1
are ideal and the same size, MN
3
conducts I
B
when V
IN
is equal to V
B
, a bias voltage applied to MP
1
. When V
IN
swings an incremental voltage amount &Dgr;V below V
B
, current through MP
2
increases accordingly. In the illustrated configuration, this MP
2
current increment is mirrored to MP
4
.
Unfortunately, only part of the input change &Dgr;V in V
IN
is reflected as an increment in V
GS
for MP
2
. Because the gate-to-source voltage V
GS
for MN
2
(connected above MP
2
) is fixed by the gate's connection to the gate of MN
1
, part of the &Dgr;V is “absorbed” by V
GS
of MN
2
to account for the current increment. Thus, because the maximum output source current I
OUT
is ultimately determined by how much gate-to-source voltage (V
GS
) can be applied to the MP
2
sink, I
OUT
does not adequately increase with &Dgr;V variations in V
IN
, resulting in undesirably small maximum output source current I
OUT
.
Accordingly, there is a need in the art to provide increased current sourcing capability of circuits such as that illustrated in
FIG. 1
, preferably with minimal additional circuitry and without compromising other operational features of the circuit.
SUMMARY
Accordingly, there is provided an output stage for providing an increased current source output. The output stage has a first circuit element that carries a first current that varies in accordance with an input voltage and a feedback voltage that includes a first voltage across the first circuit element. The output stage also has a positive feedback arrangement that dynamically increases the feedback voltage in response to an increase in at least the first current, to supplement an increase in the first current beyond that caused by a change in the input voltage alone. The output stage also has a second circuit element that provides the increased current source output as a mirror of the first current.


REFERENCES:
patent: 4618815 (1986-10-01), Swanson
patent: 5359296 (1994-10-01), Brooks et al.
patent: 5952884 (1999-09-01), Ide

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