Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Unwanted signal suppression
Reexamination Certificate
2002-09-09
2003-04-29
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Unwanted signal suppression
C327S553000, C330S254000
Reexamination Certificate
active
06556073
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a filter circuit having a transconductor and a capacitor, and more specifically to a transconductor for use in such a filter circuit.
2. Description of the Related Art
A transconductor can provide an output current which is proportional to an input voltage wherein the transconductance is a proportional constant. Therefore, the transconductor is used in a filter circuit, an amplifier, a current transformer, a calibrator, etc.
In a disk system such as a DVD system, in order to ensure compatibility with various media and multiple speed reproduction, a filter circuit used for a signal processing with a disk needs to process signals in a wide speed range from a high speed signal to a low speed signal. The high speed signal is about 100 times higher than the low speed signal. Therefore, the maximum value of a cutoff frequency F
c
should be about 100 times or more greater than the minimum value of the cutoff frequency F
c
.
FIG. 11
shows an example of a transconductor used in such a high speed filter circuit. The high-speed filter circuit of
FIG. 11
is an example of a conventional GM-C filter circuit including a transconductor (GM circuit) and capacitors (C). Specifically, in the GM-C filter circuit shown in
FIG. 11
, a plurality of capacitors
902
which are in parallel to one another are connected through a plurality of switches
903
to a transconductor (GM circuit)
901
. The cutoff frequency (F
c
), which is a filter characteristic of the GM-C filter circuit, is represented by expression (1) of gm (transconductance of the transconductor) and C (capacitance):
F
c
=gm/C (1)
Thus, in order to improve the filter characteristic, it is necessary to extend the variation range of the cutoff frequency F
c
. For that purpose, as seen from expression (1), it is necessary to extend the variation range of transconductance gm or the variation range of capacitance C.
According to the conventional technology, it is very difficult to extend the variation range of the transconductance gm of the transconductor
901
. In particular, it is more difficult to extend the variation range of transconductance in a transconductor formed by a CMOS transistor than in a transconductor formed by a bipolar transistor. The transconductor formed by a CMOS transistor typically has a transconductance which varies over a variation range wherein the maximum limit is only about 10 times greater at most than the minimum limit. Therefore, in the case of making a filter circuit shown in
FIG. 11
using a CMOS transistor with such a narrow variation range of conductance, capacitors
902
to be connected to the transconductor
901
are selected by the switches
903
to change the entire capacitance C, whereby the cutoff frequency is changed. That is, by making the capacitance C variable as well as the transconductance gm of the transconductor
901
, the variation range of the cutoff frequency is extended.
Now, consider a case where the cutoff frequency needs to have a variation range wherein the maximum limit is about 100 times greater than the minimum limit, while the transconductance gm of the transconductor
901
has a variation range wherein the maximum limit is only about 10 times greater than the minimum limit. In this case, the whole capacitance C of the capacitors
902
needs to have a variation range in which the maximum limit is about 10 times greater than the minimum limit by the selection of the switches
903
. The minimum value of the whole capacitance C of the capacitors
902
is limited due to noise, stability of circuit performance, difference in capacitance among the respective capacitors, etc. Accordingly, the capacitance C of the capacitors
902
cannot be significantly reduced. Thus, in the case where a filter circuit includes a plurality of capacitors
902
so that the total capacitance in the filter circuit is changed by various combinations of the capacitors
902
over a variation range wherein the maximum limit is about 10 times greater than the minimum limit, the circuit area increases. Furthermore, the on-resistance of the switch
903
which is directly connected to the capacitor
902
deteriorates the group delay of the filter circuit.
Thus, it is preferable to provide a transconductor which has a variation range of transconductance wherein the maximum limit is about 100 times or more greater than the minimum limit. With a transconductor having such a wide variation range of transconductance, a faster filter circuit, a faster current transformer, etc., can be achieved.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, a transconductor has a transconductance gm and receives an input voltage V
in
and outputs in response to the input voltage V
in
an output current I
out
of gm×V
in
, wherein: the transconductor includes a plurality of sub-transconductors which are connected in parallel to one another: and at least one control signal is input to the plurality of sub-transconductors, and the plurality of sub-transconductors are controlled by the at least one control signal such that at least one of the plurality of sub-transconductors has a negative transconductance, whereby the transconductance gm of the transconductor can be varied.
In one embodiment of the present invention, the at least one of sub-transconductors includes a differential input/output transconductor and a plurality of switching sections, the plurality of switching sections are connected to a first input terminal and a second input terminal of the differential input/output transconductor, and the plurality of switching sections are switched in response to the at least one control signal, thereby switching a sign of a transconductance of the differential input/output transconductor.
In another embodiment of the present invention, the plurality of sub-transconductors include one or more first sub-transconductors having a first polarity transconductance and one or more second sub-transconductors having a second polarity transconductance; and an operation (on/off) state of each of the one or more first and second sub-transconductors is selectively switched by the at least one control signal.
In still another embodiment of the present invention, each of the one or more first and second sub-transconductors is a differential input/output transconductor.
In still another embodiment of the present invention, the differential input/output transconductor has an input terminal with a first polarity, an input terminal with a second polarity, an output terminal with a fist polarity, and an output terminal with a second polarity; a difference between a first input voltage which is input to the input terminal with a first polarity and a second input voltage which is input to the input terminal with a second polarity is equal to the input voltage; and a difference between a first output current which is output from the output terminal with a first polarity and a second output current which is output from the output terminal with a second polarity is equal to the output current.
In still another embodiment of the present invention, the transconductor further includes: a first transistor with a first polarity which has a source connected to a power supply with a first polarity, a gate connected to a bias terminal, and a drain connected to the output terminal with a fist polarity; a second transistor with a first polarity which has a source connected to the power supply with a fist polarity, a gate connected to the bias terminal, and a drain connected to the output terminal with a second polarity; first and second sub-transconductor components each having at least one unit transconductor, the first and second sub-transconductor components being connected to the output terminal with a first polarity; and third and fourth sub-transconductor or components each having at least one unit transconductor, the third and fourth sub-transconductor components being connected to the output terminal with a second polarity
Dosho Shiro
Morie Takashi
Callahan Timothy P.
Luu An T.
Matsushita Electric - Industrial Co., Ltd.
RatnerPrestia
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