Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
1998-09-30
2001-02-13
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S295000
Reexamination Certificate
active
06188281
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention is related to operational amplifier design. More specifically, the present invention teaches a variety of linearized class AB differential transconductance circuits each well suited for use as a low-distortion input stage in an operational amplifier.
2. 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 a subsequent stage such as the output stage. The transconductance circuit's defining characteristic 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 theoretical voltage to current transfer function of the differential pair transconductor
10
is a hyperbolic tangent (tanh) function.
While widely applicable and well suited for certain applications, the differential pair transconductor
10
suffers many shortcomings. When used within a capacitive feedback loop, as is often the case, the transconductor
10
grossly limits the slew rate. (“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 reasonable 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 tanh 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 (sinh) 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 sinh 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
FIG. 1
, much of the non-linearity of transconductor
10
is due to the tanh nature of its transfer function. One well-known technique for linearizing differential pair transconductors is the so-called “multi-tanh technique.” As will be appreciated, the key to the multi-tanh technique is the placement of multiple nonlinear tanh 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-tanh 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-tanh transconductors do improve the distortion characteristics of an input stage, however the multi-tanh 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 sinh 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 formed having a class AB transconductor amplifier coupled in parallel with at least one concave compensation circuit. When the transconductance circuit has only one concave compensation circuit, the concave compensation circuit is preferably designed without offset so that the concave transconductance gain of the compensation circuit compensates for the convex transconductance gain of the class AB amplifier thereby providing a more linear transconductance circuit. When the transconductance circuit includes multiple concave compensation circuits, each transconductance circuit is preferably designed with an offset chosen such that the combination of the individual concave transfer functions achieve a more linear transconductance circuit.
For example, one transconductance circuit of the present invention has a class AB transconductance amplifier parallel coupled with two concave compensation circuits. The first concave compensation circuit has a positive offset while the second compensation circuit has a negative offset. In preferred embodiments, the offsets' magnitudes are substantially equivalent and the compensation circuits are of the same type. In one particular embodiment, the compensation circuits are formed as tanh doublets. The offsets for these tanh doublets can be brought about by a variety of mechanisms such as an imbalance in the sizes or current saturation of the transistors, an imbalance in the bias current sources, or the placement of resistors in series with the signal path.
The present invention also teaches an operational amplifier having multiple stages including an input stage. The input stage includes a class AB transconductance amplifier and a concave compensation circuit coupled in parallel across differential input and output pairs. As described above, the concave transconductance gain of th
McLeod Scott C.
Smith Douglas L.
Hickman Coleman & Hughes LLP
Maxim Integrated Products Inc.
Mottola Steven J.
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