Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2001-11-26
2002-11-26
Tokar, Michael (Department: 2819)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S252000, C330S258000, C330S261000, C330S311000, C327S063000, C327S560000, C327S563000
Reexamination Certificate
active
06486737
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of rail-to-rail amplifiers, and particularly to methods of reducing gain error in the input stage of a rail-to-rail amplifier.
2. Description of the Related Art
A rail-to-rail amplifier's input stage typically includes complementary differential pairs. One differential pair conducts when the common mode voltage V
cm
applied to the amplifier's inputs is above a particular reference voltage V
ref
, and the other pair conducts when V
cm
is less than V
ref
.
To improve slew rate and linear input range, the input stage transistors of a bipolar rail-to-rail amplifier often include emitter degeneration resistors. When so arranged, the transconductance G
m
of the input stage when only one of the differential pairs is conducting is given by:
G
m
=
1
(
k
T
/
q
)
I
T
/
2
+
R
d
where kT/q is a constant for a bipolar transistor, I
T
is the differential pair's tail current, and R
d
is the resistance of the emitter degeneration resistors.
As the common mode voltage moves above and below V
ref
, the active differential pair transitions from one pair to the other. When V
cm
=V
ref
, both pairs are active, with each pair getting one-half of the total tail current I
T
. Under this condition, the transconductance G
m2
of the input stage is given by:
G
m2
=
2
(
k
T
/
q
)
I
T
/
4
+
R
d
By inspection, it is seen that a bipolar rail-to-rail input amplifier with degeneration has a higher gain in the transition region, i.e., with both pairs active, than it does when one pair is operating alone. In fact, when the amplifier is heavily degenerated, i.e., with
R
d
>>
k
T
/
q
I
T
/
2
,
G
m2
≈2G
m
. This results in a gain error when operating in the transition region, which can cause discontinuities and distortion in the output voltage produced by the input stage.
SUMMARY OF THE INVENTION
A bipolar rail-to-rail input stage is presented which overcomes the problem noted above. Gain error which would otherwise arise is reduced by lowering the transconductance of the input stage transistors when operating in the transition region.
The degeneration resistors connected in the emitter circuits of the PNP transistors are connected together at a first common mode node, and the NPN transistors' emitter resistors are connected together at a second common mode node. The transconductance of the input stage is reduced in the transition region with a current reduction circuit, which is arranged to sense the difference in voltage between the two common mode nodes, and to reduce the amount of current which the input pairs would otherwise conduct when V
cm
≈V
ref
. The current reduction circuit is preferably arranged so that the transconductance of the input stage when V
cm
=V
ref
is made equal to the transconductance of the input stage when V
m
>V
ref
or V
cm
<V
ref
and only one pair is conducting.
The current reduction circuit preferably comprises a current shunt circuit made from four bipolar transistors: a pair of PNP transistors, the emitters of which are connected to respective emitters of the PNP input transistors, and a pair of NPN transistors, the emitters of which are connected to respective emitters of the NPN input transistors, with the bases and collectors of all four shunt transistors connected together at a summing node. A current shunt circuit so arranged senses the difference in voltage between the common mode nodes and shunts current around the input pairs to reduce the current they would otherwise conduct and thereby lower their respective transconductances when V
cm
≈V
ref
. When the shunt transistors are properly sized with respect to the input devices, G
m
is made equal to G
m2
.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.
REFERENCES:
patent: 5392003 (1995-02-01), Nag et al.
patent: 5455535 (1995-10-01), Sauer
patent: 5714906 (1998-02-01), Motamed et al.
patent: 5729177 (1998-03-01), Goutti
patent: 5789949 (1998-08-01), Giordano et al.
patent: 6281753 (2001-08-01), Corsi et al.
Analog Devices Inc.
Koppel, Jacobs Patrick & Heybl
Nguyen Linh Van
Tokar Michael
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