Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2001-05-23
2002-10-01
Shingleton, Michael B (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S258000, C330S259000
Reexamination Certificate
active
06459338
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to differential amplifiers, and more particularly to an amplifier with a class AB differential input stage that employs a single loop output common mode feedback circuit to achieve high performance as compared to amplifiers with class AB differential input stages that employ two loop solutions which have loop interplay.
2. Description of the Prior Art
Differential amplifiers are very useful devices when dealing with differential systems. They have several advantages such as very good common mode rejection ratio since everything that is common will get canceled at the differential output. Some applications of differential amplifiers include single ended to differential conversion, differential ADC driver, differential transmitter and receiver, and output level shifter.
Differential amplifiers are not too different from single ended amplifiers. Several single ended topologies can be converted to differential topologies. The most obvious difference between differential amplifiers and their single ended counterparts is that they have two outputs instead of one. With the addition of this output, another difference arises, which is the “output common mode feedback circuit” (CMFC). Extra circuitry is necessary to direct the outputs where to go DC wise. Since this signal is common to both outputs differentially, it will cancel, leaving at the differential output only the amplified differential input signal.
Implementing such a common mode circuit is challenging.
FIG. 1
illustrates one implementation of a differential amplifier
100
that includes an emitter coupled pair input stage
102
with a folded cascode gain stage
104
. The CMFC includes an error amplifier
106
that compares the common mode voltage at the outputs to the voltage set on the V
ocm
pin
108
(seen defaulted to mid rail in FIG.
1
). The error amplifier
106
adjusts the common mode voltage by changing the voltage in the bases of transistors Q
5
and Q
6
. The feedback path of the error amplifier
106
in
FIG. 1
is from the output of the error amplifier
106
to the bases of transistors Q
5
and Q
6
, to the outputs of the main amplifier through the buffers
110
,
112
, and back into the positive input of the error amplifier
106
.
The class AB input stage in a differential configuration has several advantages over the class A single emitter coupled pair input stage illustrated in FIG.
1
. Some of these advantages include better slew rate and better THD performance, especially at high frequencies. It would be desirable to have a way of implementing the foregoing CMFC on this topology.
FIG. 2
is a schematic diagram illustrating a differential class AB amplifier
200
without the CMFC. It can be seen this topology is quite different from the one shown in FIG.
1
. One of the primary differences between this kind of circuit and the circuit in
FIG. 2
is that in
FIG. 1
it is only necessary to manipulate the current of transistors Q
5
and Q
6
to control the output common mode voltage. In the circuit of
FIG. 2
, it is necessary to manipulate the top mirrors Q
5
, Q
6
and Q
13
, Q
14
as well as the bottom mirrors Q
7
, Q
8
and Q
15
, Q
16
. The conventional prior art solution includes the use of two loops to control the two sets of mirrors, and that way control the output common mode voltage. This conventional approach has been problematic however since it results in undesirable distortion associated with additional parasitics and mismatch between the loops, stability problems due to complexity of the multiple control loops and the interaction between them, added complexity, and circuitry that consumes additional die area.
In view of the foregoing, it would be desirable to have a way of implementing the CMFC on a class AB input stage in a differential configuration in a manner that does not result in additional distortion, or stability problems due to increased complexity of multiple control loops, and that does not undesirably increase the required die area.
SUMMARY OF THE INVENTION
The present invention is directed to an amplifier with a class AB differential input stage that employs a single loop output common mode feedback circuit (CMFC) to achieve high performance by controlling the common mode output voltage. The CMFC includes a small amplifier to compare the voltage at the outputs with a desired voltage. Having only one loop to control this voltage instead of two makes the design more reliable and easier to compensate since there is no need to worry about how multiple loops will interact. The distortion caused by the nonlinear parasitics associated with the CMFC are minimized by connecting the CMFC to a point at the input where the voltage does not change as much as in the output of the input stage, where the full voltage signal is seen.
According to one aspect of the invention, an amplifier with a class AB input stage in a differential configuration is implemented with common mode feedback circuitry having only one loop to control the output common mode voltage;
According to another aspect of the invention, an amplifier with a class AB input stage in a differential configuration is implemented with common mode feedback circuitry (CMFC) having only one loop to control the output common mode voltage such that distortion caused by the CMFC is minimized;
According to yet another aspect of the invention, an amplifier with a class AB input stage in a differential configuration is implemented with common mode feedback circuitry (CMFC) having only one loop to control the output common mode voltage such that interaction between multiple loops caused by a CMFC is eliminated;
According to still another aspect of the invention, an amplifier with a class AB input stage in a differential configuration is implemented with common mode feedback circuitry (CMFC) having only one loop to control the output common mode voltage such that the CMFC is less complex and requires less die area than known two loop CMFC implementations associated with class AB differential amplifiers.
According to still another aspect of the invention, an amplifier with a class AB input stage in a differential configuration is implemented with common mode feedback circuitry (CMFC) having only one loop to control the output common mode voltage such that the resultant class AB differential amplifier power consumption is reduced when compared with conventional class AB differential amplifier architectures that employ two loops to control the common mode output voltage.
REFERENCES:
patent: 6281751 (2001-08-01), Mevlik
Acosta Julio E.
Hayat-Dawoodi Kambiz
Brady W. James
Shingleton Michael B
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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