Linear and multi-sin h transconductance circuits

Details Linear and multi-sin h transconductance circuits Linear and multi-sin h transconductance circuits

2000-12-05

2002-12-03

Shingleton, Michael B (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including differential amplifier

C330S295000, C330S310000

Reexamination Certificate

active

06489848

ABSTRACT:

TECHNICAL FIELD
The present invention is related to operational amplifier design. More specifically, the present invention teaches a variety of linear and multi-sin h transconductance circuits each well suited for use as a low-distortion input stage in an operational amplifier.
BACKGROUND ART
In the design of operational amplifiers, it is important to provide a highly linear (i.e., low distortion), low noise amplifier capable of wide bandwidth operation. Bandwidth limitations, noise, and distortion can arise at any stage within the operational amplifier, but for present purposes the focus is upon the input stage. The typical input stage is a transconductor or transconductance circuit operable to convert an input voltage signal into an internal current signal more suitable for amplification by the output stage. Hence, the defining feature of the transconductance circuit is its voltage to current transfer function.
Prior Art
FIG. 1
illustrates the prototypical input stage transconductor
10
, i.e., a differential transistor pair. The transconductor
10
includes a pair of transistors Q
1
and Q
2
whose emitters are coupled to a bias current source I
DC
that provides “tail” current for the transconductor
10
. The differential voltage input pair V
IN+
and V
IN−
drive the bases of the transistors Q
1
and Q
2
, essentially steering the resulting differential current pair I
OUT+
and I
OUT−
to a common ground reference
20
. As will be appreciated, the voltage to current transfer function of the differential pair transconductor
10
is ideally a hyperbolic tangent (tan h) function.
While widely applicable and well suited for certain applications, the transconductor
10
suffers many shortcomings. When used within an amplifier having a capacitive feedback loop, as is often the case, the transconductor
10
is extremely limiting on the slew rate. (An amplifier's slew rate defines the maximum rate of change in voltage across the input and output terminals of the amplifier.) Specifically, the total current available to charge the feedback loop compensation capacitor C
C
is limited by the so-call “tail current” of the differential pair, i.e., the bias current I
DC
.
For the present analysis, it is fair to assume that the slew rate is equal to I
DC
/C
C
. Hence to improve the slew rate, one must decrease C
C
and/or increase I
DC
, both of which are undesirable for a variety of well known reasons. Additionally, the tan h transfer function of the differential pair transconductor
10
means that transconductor
10
is a non-linear, distortive circuit.
One common approach for addressing the slew rate limitations of the differential pair transconductor
10
of
FIG. 1
is to use a class AB transconductance amplifier. Prior Art
FIG. 2
illustrates one typical class AB amplifier
100
formed from a pair of differentially coupled diamond followers whose output emitters are coupled through a common load resistance R
DGEN
. Each diamond follower includes a pair of bias current sources I
DC
, and four transistors (one follower is made of transistors Q
1
-Q
4
, the other follower is made of transistors Q
5
-Q
8
).
The voltage to current transfer function of the class AB amplifier
100
without a common load resistance R
DGEN
(i.e., R
DGEN
=0) is ideally a hyperbolic sine (sin h) function. Prior Art
FIG. 4
illustrates such an ideal transconductance of the class AB amplifier
100
(i.e., dIout/dVout) as a function of input voltage. As seen in
FIG. 4
, the ideal transconductance of the class AB amplifier
100
is non-linear at voltages close to zero, but fairly linear elsewhere. The transfer function of the class AB amplifier will vary for different values of R
DGEN
, but the non-linear characteristics are similar and related to the sin h function represented in FIG.
4
.
In practice, the transconductance gain of the class AB amplifier
100
is set by the available bias current, the common load resistor R
DGEN
, and the nonlinear transconductance characteristics of the individual transistors. However, when R
DGEN
is large it dominates the nonlinear effects of the individual transistors, thereby improving the distortion characteristics of the class AB amplifier
100
. Unfortunately, increasing R
DGEN
increases noise in the class AB amplifier
100
due to thermal noise of the resistor.
As mentioned above with reference to the differential pair transconductor
10
of
FIG. 1
, much of the non-linearity of transconductor
10
is due to the tan h nature of its transfer function. One well-known technique for linearizing differential pair transconductors is the so-called “multi-tan h technique.” As will be appreciated, the key to the multi-tan h technique is the placement of multiple nonlinear tan h transconductors (i.e., differential pairs) along the input-voltage axis to achieve in combination a more linear transfer function.
Prior Art
FIG. 3
illustrates a multi-tan h doublet
200
formed from two differential pairs Q
1
-Q
2
and Q
3
-Q
4
and two bias current sources I
DC
. Positive and negative offsets are introduced by forming each differential transistor pair with a gain imbalance. Specifically, a positive offset is introduced into the differential pair Q
1
-Q
2
by forming transistor Q
1
with a gain A that is greater than unity, and transistor Q
2
with a gain of substantially unity. Likewise, a negative offset is introduced into the differential pair Q
3
-Q
4
by forming transistor Q
4
with a gain A that is greater than unity and transistor Q
3
with a gain of substantially unity. Prior Art
FIG. 5
illustrates the combined transconductance gain.
The multi-tan h transconductors do improve the distortion characteristics of an input stage, however the multi-tan h technique does not address the slew rate and other problems of the differential pair transconductor. Likewise, the class AB amplifier provides an improved slew rate, yet suffers from the nonlinearity about zero due to its sin h transfer function. What are needed are a variety of transconductance circuits that are highly linear with low noise, and having bandwidth characteristics not limited by slew rate.
DISCLOSURE OF THE INVENTION
The present invention teaches a variety of transconductance circuits such as transconductance circuits formed having a plurality of class AB transconductor amplifiers coupled in parallel. The class AB transconductor amplifiers have non-linear voltage to current transfer functions. Each class AB transconductor amplifier is designed with an offset chosen such that the individual nonlinear transfer functions are arranged along the input voltage axis to achieve a more linear transfer function for the combined transconductance circuit.
For example, a first embodiment of the present invention discloses a transconductance circuit characterized by a voltage to current transfer function, the transconductance circuit including a pair of class AB transconductance amplifiers coupled in parallel across differential input and output pairs. The first class AB transconductance amplifier has a positive offset. The second class AB transconductance amplifier has a negative offset. These negative and positive offsets are selected to improve linearity of the voltage to current transfer function of the transconductance circuit. It is contemplated that offsets may be of equal or differing magnitudes.
In certain embodiments, the class AB transconductance amplifiers are formed from a pair of differentially coupled diamond followers and a common load resistance R
DGEN
. Each diamond follower has four transistors and two bias current sources. The transconductance of each class AB amplifier is thus a function of transistor gain, the available bias current and the common load resistance R
DGEN
.
The present invention further teaches an operational amplifier having an input stage and a second stage (e.g., gain stage or output stage) coupled in series. The input stage, characterized by a voltage to current transfer function, includes a plurality of class AB transconductance amplifiers coupled

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