Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2001-07-02
2003-06-03
Cunningham, Terry D. (Department: 2816)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C327S562000
Reexamination Certificate
active
06573794
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to operational amplifiers and in particular to improvements in operational amplifier performance, and is more particularly directed toward a filter interposed between the g
m
stage and the integrator in a conventional two-stage operational amplifier architecture to improve the amplifier's signal slew rate, gain, and harmonic distortion.
BACKGROUND OF THE INVENTION
The conventional operational amplifier (op amp), illustrated in
FIG. 1
in block diagram form, and generally depicted by the numeral
100
, comprises two gain stages. The first functions as a differential transconductance (g
m
) stage
101
and the second as an integrator
103
, separated by a differential to single-ended converter
102
. The conventional op amp
100
is illustrated in more detail in FIG.
2
.
As shown in
FIG. 2
, the g
m
stage
101
comprises a differential pair
201
,
202
with a single current source “tail”
203
(both typically, and as an example, p-type insulated-gate field effect transistors), and two current source loads
204
,
205
(typically, and as an example, provided by n-type transistors). By selecting an output
206
from only one of the differential input stages, differential to single-ended conversion is accomplished or, conventionally, current sources
204
and
205
are implemented as a mirror with single-ended output
206
derived from the high impedance side of the mirror.
This single-ended output
206
is then applied to the integrator stage
103
. In the implementation shown, the integrator
103
includes a p-type output transistor
207
with a current source tail
210
, and Miller capacitor
208
. A nulling resistor
209
has been added for the sake of stability.
In conventionally designed op amps, the compensation capacitor
208
must be fairly large to forestall the possibility of oscillation. A large capacitor utilizes integrated circuit (IC) area that might otherwise be dedicated to other IC components. In addition, a large compensation capacitor negatively impacts amplifier gain at high frequencies and signal slew rate. Consequently, a need arises for an op amp design that provides improved high frequency gain and signal slew rate while reducing overall circuit area.
SUMMARY OF THE INVENTION
These needs and others are satisfied by the operational amplifier design of the present invention. The performance of an op amp is improved by placing a current mode filter between the g
m
stage and the integrator, which has a current gain of much less than one and is substantially without phase shift at the op amp's resonant frequency, permitting stabilization with a relatively small compensation capacitor. This improves the signal slew rate and harmonic distortion.
In accordance with one aspect of the present invention, an improved operational amplifier includes a transconductance amplifier input stage and an integrator output stage. The improvement comprises a current-mode low pass filter interposed between the transconductance amplifier and the integrator.
In one form of the invention, the current mode low pass filter comprises a series resistor coupled between the transconductance amplifier output and the integrator input, a first capacitor connected in parallel with the resistor, and a shunt capacitor coupled between the transconductance amplifier output and ground.
In a preferred form, the series resistor has a resistance value of approximately 2.5 kilohms, the first capacitor connected in parallel with the resistor has a value of approximately 4 picofarads, and the shunt capacitor has a value of approximately 10 picofarads.
In accordance with another form of the invention, the improved operational amplifier further comprises a series damping resistor connected in series with the shunt capacitor. Preferably, the series damping resistor has a value of approximately 100 ohms.
Further objects, features, and advantages of the present invention will become apparent from the following description and drawings.
REFERENCES:
patent: 4205276 (1980-05-01), Wright et al.
patent: 4667164 (1987-05-01), Doluca
patent: 4731553 (1988-03-01), Van Lehn et al.
patent: 5124663 (1992-06-01), McEntarfer et al.
patent: 5252868 (1993-10-01), Miida et al.
patent: 5365126 (1994-11-01), Krenik et al.
patent: 5825250 (1998-10-01), Tomasini et al.
patent: 5963047 (1999-10-01), Kwong et al.
patent: 6087853 (2000-07-01), Huber et al.
patent: 6249876 (2001-06-01), Balakrishnan et al.
Copy of U.S. patent application Ser. No. 09/925,074.
Analog Devices Inc.
Cunningham Terry D.
Tra Quan
Wolf Greenfield & Sacks P.C.
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