U.S. patent application number 09/903948 was filed with the patent office on 2002-01-24 for transconductance-capacitance filter system.
This patent application is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Dosho, Shiro, Fujiyama, Hirokuni, Hayashi, Hiroki, Katada, Tomoyuki, Morie, Takashi.
Application Number | 20020008572 09/903948 |
Document ID | / |
Family ID | 18715320 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020008572 |
Kind Code |
A1 |
Hayashi, Hiroki ; et
al. |
January 24, 2002 |
Transconductance-capacitance filter system
Abstract
A gm-C filter system having low power consumption is provided.
An adjusting circuit 2 is equipped with an oscillator 3 constructed
of a gm amplifier 3a having the same arrangement as that of a gm
amplifier 1a of a gm-C filter circuit 1. The adjusting circuit 2
generates a digital adjusting value "D.sub.gm" based upon an
oscillation signal OSC outputted from this oscillation 3, and this
digital adjusting value "D.sub.gm" is used to adjust a gm value of
the gm amplifier 3a of the oscillator 3. This digital adjusting
value "D.sub.gm" is held in a register 10. The digital adjusting
value "D.sub.gm" held in this register 10 is converted into an
analog adjusting value (bias current) by a D/A converter 8, and
then, this analog adjusting value is supplied to the gm amplifier
1a of the gm-C filter circuit 1 so as to adjust the gm value. The
adjusting circuit 2 is operated in an intermittent manner based
upon, for example, a change contained in ambient temperatures of
the gm-C filter system.
Inventors: |
Hayashi, Hiroki;
(Kawasaki-shi, JP) ; Dosho, Shiro; (Osaka, JP)
; Morie, Takashi; (Osaka, JP) ; Fujiyama,
Hirokuni; (Osaka, JP) ; Katada, Tomoyuki;
(Kawasaki-shi, JP) |
Correspondence
Address: |
PEARNE & GORDON LLP
526 SUPERIOR AVENUE EAST
SUITE 1200
CLEVELAND
OH
44114-1484
US
|
Assignee: |
Matsushita Electric Industrial Co.,
Ltd.
|
Family ID: |
18715320 |
Appl. No.: |
09/903948 |
Filed: |
July 12, 2001 |
Current U.S.
Class: |
327/552 |
Current CPC
Class: |
H03H 11/0422
20130101 |
Class at
Publication: |
327/552 |
International
Class: |
H03B 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 21, 2000 |
JP |
P. 2000-220778 |
Claims
What is claimed is:
1. A transconductance-capacitance filter system comprising: a
transconductance-capacitance filter circuit including a
transconductance amplifier and a capacitor; an adjusting circuit
including an oscillator containing a transconductance amplifier
having the same structure as that of the transconductance amplifier
of said transconductance-capacitance filter circuit, said adjusting
circuit producing a digital adjusting value used to adjust the
transconductance of the transconductance amplifier of said
oscillator based upon an oscillation signal outputted from said
oscillator; a register for holding said digital adjusting value
supplied from said adjusting circuit; and a D/A converter for
converting said digital adjusting value held in said register into
an analog adjusting value which is used to adjust the
transconductance of the transconductance amplifier of said
transconductance-capacitance filter circuit, wherein said adjusting
circuit is operated in an intermittent manner.
2. A transconductance-capacitance filter system as claimed in claim
1, further comprising a temperature sensing circuit for sensing an
ambient temperature of said transconductance-capacitance filter
system, wherein said adjusting circuit is operated in the
intermittent manner based upon a change contained in said ambient
temperatures.
3. A transconductance-capacitance filter system as claimed in claim
1 further comprising a power supply voltage sensing circuit for
sensing a power supply voltage of said transconductance-capacitance
filter system, wherein said adjusting circuit is operated in the
intermittent manner based upon a change contained in said power
supply voltages.
4. A transconductance-capacitance filter system as claimed in claim
1 further comprising a temperature sensing circuit for sensing an
ambient temperature of said transconductance-capacitance filter
system, and a power supply voltage sensing circuit for sensing a
power supply voltage of said transconductance-capacitance filter
system, wherein said adjusting circuit is operated in the
intermittent manner based upon either a change contained in the
ambient temperatures or a variation of said power supply
voltages.
5. A transconductance-capacitance filter system comprising: a
transconductance-capacitance filter circuit including a
transconductance amplifier and a capacitor; an adjusting circuit
including an oscillator containing a transconductance amplifier
having the same structure as that of the transconductance amplifier
of said transconductance-capacitance filter circuit, said adjusting
circuit producing a digital adjusting the transconductance of the
transconductance amplifier of said oscillator based upon an
oscillation signal outputted from said oscillator; a register for
holding said digital adjusting value supplied from said adjusting
circuit; a D/A converter for converting said digital adjusting
value held in said register into an analog adjusting value which is
used to adjust the transconductance of the transconductance
amplifier of said transconductance-capacitance filter circuit; and
a temperature compensating circuit for producing such a drive bias
current capable of compensating for a variation component of the
transconductance values of the transconductance amplifier of said
transconductance-capacitance filter circuit with respect to a
change contained in ambient temperatures of said
transconductance-capacitance filter system based upon
externally-supplied temperature data, and capable of driving said
D/A converter by said drive bias current, wherein said adjusting
circuit is operated only when said transconductance-capacitance
filter system is initiated.
6. A transconductance-capacitance filter system comprising: a
transconductance-capacitance filter circuit including a
transconductance amplifier and a capacitor; an adjusting circuit
including an oscillator containing a transconductance amplifier
having the same structure as that of the transconductance amplifier
of said transconductance-capacitance filter circuit, said adjusting
circuit producing a digital adjusting the transconductance of the
transconductance amplifier of said oscillator based upon an
oscillation signal outputted from said oscillator; a register for
holding said digital adjusting value supplied from said adjusting
circuit; a temperature compensating circuit for producing such a
drive bias current capable of compensating for a variation
component of the transconductance values of the transconductance
amplifier of said transconductance-capacitance filter circuit with
respect to a change contained in ambient temperatures of said
transconductance-capacitance filter system based upon
externally-supplied temperature data, and capable of driving said
D/A converter by said drive bias current; an adder for executing a
digital calculation with respect to said compensating digital
adjusting value supplied from said temperature compensating circuit
and the digital adjusting value held in said register; and a D/A
converter for converting a digital calculation result supplied form
said adder into an analog adjusting value which is used to adjust
the transconductance of the transconductance amplifier of said
transconductance-capacitance filter circuit, wherein said adjusting
circuit is operated only when said transconductance-capacitance
filter system is initiated.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention is related to a
transconductance-capacitance filter system equipped with a
transconductance-capacitance filter circuit and an adjusting
circuit thereof.
[0002] For instance, transconductance-capacitance filter systems
(will be referred to as a "gm-C filer system" hereinafter) are
employed in portable electronic appliances such as portable
telephone sets. FIG. 7 schematically represents an example of
conventional gm-C filter systems. This gm-C filter system is
equipped with a transconductance-capacitance filter circuit (will
be referred to as a "gm-C filter circuit hereinafter) 1, and an
adjusting circuit 16 for adjusting a cut-off frequency of this gm-C
filter circuit 1. The gm-C filter circuit 1 is arranged by
employing a transconductance amplifier (will be referred to as a
"gm amplifier" hereinafter) 1a and a capacitor 1b, and may
constitute, for example, a low-pass filter. On the other hand, the
adjusting circuit 1b is provided with an oscillator 3 having a gm
amplifier 3a and a capacitor 3b, comparators 14 and 15 designed for
waveform shaping operation, and a frequency comparator 13. In this
case, the gm amplifier 3a of the oscillator 3 owns the same
structure as that of the gm amplifier 1a of the gm-C filter circuit
1.
[0003] In the gm-C filter system arranged in the above-explained
manner, an oscillation signal OSC is supplied from the oscillator 3
via the comparator 14 to the frequency comparator 13, and also a
reference clock signal CK is supplied from an externally provided
crystal oscillator (not shown) via the comparator 15 to this
frequency comparator 13, so that the frequency of the oscillation
signal OSC is compared with the frequency of the reference clock
signal CK. In other words, in the frequency comparator 13, a bias
current "i.sub.BIAS" is produced based upon a frequency error of
the oscillation signal OSC with respect to the reference clock
signal CK. This bias current "IBIAS" is supplied to the gm
amplifier 3a employed in the oscillator 3 so as to adjust a value
of a transconductance (will be referred to as a "gm value"
hereinafter) of the gm amplifier 3a. For example, in such a case
that the oscillation frequency of the oscillator 3 is higher than
the frequency of the reference clock signal CK corresponding to the
set value, such a bias current "i.sub.BIAS" capable of reducing the
gm value of the gm amplifier 3a employed in the oscillator 3 is
outputted from the frequency comparator 13, so that the oscillation
frequency of the oscillator 3 is reduced. Conversely, in such a
case that the oscillation frequency of the oscillator 3 is lower
than the frequency of the reference clock signal CK corresponding
to the set value, such a bias current "i.sub.BIAS" capable of
increasing the gm value of the gm amplifier 3a employed in the
oscillator 3 is outputted from the frequency comparator 13, so that
the oscillation frequency of the oscillator 3 is increased. In
other words, the bias current "i.sub.BIAS" is varied in such a
manner that the oscillation frequency of the oscillator 3 is made
coincident with the frequency of the reference clock signal CK, so
that the gm value of the gm amplifier 3a employed in the oscillator
3 is adjusted.
[0004] On the other hand, the bias current "i.sub.BIAS" supplied
from the frequency comparator 13 is also supplied to the gm
amplifier 1a provided in the gm-C filter circuit 1 so as to adjust
the gm value of this gm amplifier. As a result, the cut-off
frequency is adjusted. In this case, since the gm amplifier 3a of
the oscillator 3 owns the same structure as that of the gm-C filter
circuit 1, the oscillation frequency of the oscillator 3 may
correspond to the cut-off frequency of the gm-C filter circuit 1 in
an one-to-one correspondence relationship. As a consequence, in
order to set the cut-off frequency of the gm-C filter circuit 1 to
a desirable frequency value, the oscillation frequency of the
oscillator 3 may be adjusted based upon such a frequency clock
signal CK having a frequency corresponding to this desirable
frequency value.
[0005] However, in the above-explained conventional gm-C filter
system, there is such a serious problem. That is, the adjusting
circuit 16 arranged by the oscillator 3, the comparators 14/15, and
the frequency comparator 13 is continuously operated so as to
adjust the cut-off frequency of the gm-C filter circuit 1. Since
this adjusting circuit 16 is continuously operated, the power
consumption of the entire gm-C filter system would be
increasede.
SUMMARY OF THE INVENTION
[0006] The present invention has been made to solve such a
conventional problem, and therefore, has an object to provide a
gm-C filter system having low power consumption.
[0007] To achieve the above-described object, according to a first
aspect of the present invention, a transconductance-capacitance
filter system comprises: a transconductance-capacitance filter
circuit including a transconductance amplifier and a capacitor; an
adjusting circuit including an oscillator containing a
transconductance amplifier having the same structure as that of the
transconductance amplifier of the transconductance-capacitance
filter circuit, the adjusting circuit producing a digital adjusting
value used to adjust the transconductance of the transconductance
amplifier of the oscillator based upon an oscillation signal
outputted from the oscillator; a register for holding the digital
adjusting value supplied from the adjusting circuit; and a D/A
converter for converting the digital adjusting value held in the
register into an analog adjusting value which is used to adjust the
transconductance of the transconductance amplifier of the
transconductance capacitance filter circuit; wherein the adjusting
circuit is operated in an intermittent manner.
[0008] A transconductance-capacitance filter system, according to a
second aspect of the present invention,
transconductancetransconductancefurther comprises a temperature
sensing circuit for sensing an ambient temperature of the
transconductance-capacitance filter system, and wherein the
adjusting circuit is operated in the intermittent manner based upon
a change contained in the ambient temperatures.
[0009] A transconductance-capacitance filter system, according to a
third aspect of the present invention,
transconductancetransconductancefurther comprises a power supply
voltage sensing circuit for sensing a power supply voltage of the
transconductance-capacitance filter system, and wherein the
adjusting circuit is operated in the intermittent manner based upon
a change contained in the power supply voltages.
[0010] A transconductance-capacitance filter system, according to a
fourth aspect of the present invention,
transconductancetransconductancefurther comprises a temperature
sensing circuit for sensing an ambient temperature of the
transconductancecapacitance filter system, and a power supply
voltage sensing circuit for sensing a power supply voltage of the
transconductance-capacitance filter system, and wherein the
adjusting circuit is operated in the intermittent manner based upon
either a change contained in the ambient temperatures or a
variation of the power supply voltages.
[0011] Also, according to a fifth aspect of the present invention,
a transconductance-capacitance filter system comprises: a
transconductance-capacitance filter circuit including a
transconductance amplifier and a capacitor; an adjusting circuit
including an oscillator containing a transconductance amplifier
having the same structure as that of the transconductance amplifier
of the transconductance-capacitance filter circuit, the adjusting
circuit producing a digital adjusting the transconductance of the
transconductance amplifier of the oscillator based upon an
oscillation signal outputted from the oscillator; a register for
holding the digital adjusting value supplied from the adjusting
circuit; a D/A converter for converting the digital adjusting value
held in the register into an analog adjusting value which is used
to adjust the transconductance of the transconductance amplifier of
the transconductance-capacitance filter circuit; and a temperature
compensating circuit for producing such a drive bias current
capable of compensating for a variation component of the
transconductance values of the transconductance amplifier of the
transconductance-capacitance filter circuit with respect to a
change contained in ambient temperatures of the
transconductance-capacitance filter system based upon
externally-supplied temperature data, and capable of driving the
D/A converter by the drive bias current, wherein the adjusting
circuit is operated only when the transconductancecapacitance
filter system is initiated.
[0012] Further, according to a sixth aspect of the present
invention, a transconductance-capacitance filter system comprises a
transconductance-capacitance filter circuit including a
transconductance amplifier and a capacitor; an adjusting circuit
including an oscillator containing a transconductance amplifier
having the same structure as that of the transconductance amplifier
of the transconductance-capacitance filter circuit, the adjusting
circuit for producing a digital adjusting the transconductance of
the transconductance amplifier of the oscillator based upon an
oscillation signal outputted from the oscillator; a register for
holding the digital adjusting value supplied from the adjusting
circuit; a temperature compensating circuit for producing such a
drive bias current capable of compensating for a variation
component of the transconductance values of the transconductance
amplifier of the transconductance-capacitance filter circuit with
respect to a change contained in ambient temperatures of the
transconductance-capacitance filter system based upon
externally-supplied temperature data, and capable of driving the
D/A converter by the drive bias current; an adder for executing a
digital calculation with respect to the compensating digital
adjusting value supplied from the temperature compensating circuit
and the digital adjusting value held in the register; and a D/A
converter for converting a digital calculation result supplied form
the adder into an analog adjusting value which is used to adjust
the transconductance of the transconductance amplifier of the
transconductance-capacitance filter circuit, wherein the adjusting
circuit is operated only when the transconductance-capacitance
filter system is initiated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a block diagram for showing an arrangement of a
gm-C filter system according to a first embodiment mode of the
present invention;
[0014] FIG. 2 is an explanatory diagram for explaining a method for
setting a digital adjusting value (two-dividing method) outputted
from a digital control circuit, corresponding to an oscillation
frequency of an oscillator 3;
[0015] FIG. 3 is a waveform diagram for describing a method for
adjusting an oscillation amplitude of the oscillator 3;
[0016] FIG. 4 is a time chart for explaining operation of an
adjusting circuit 2;
[0017] FIG. 5 is a block diagram for indicating an arrangement of a
gm-C filter system according to a second embodiment mode of the
present invention;
[0018] FIG. 6 is a block diagram for indicating an arrangement of a
gm-C filter system according to a third embodiment mode of the
present invention;
[0019] FIG. 7 is a block diagram for indicating an example of the
conventional gm-C filter system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] Referring now to drawing, embodiment modes of the present
invention will be described in detail.
[0021] (First Embodiment Mode)
[0022] FIG. 1 is a block diagram for representing an arrangement of
a gm-C filter system according to a first embodiment of the present
invention. In FIG. 1, the gm-C filter system is provided with a
gm-C filter circuit 1, an adjusting circuit 2, registers 9 and 10,
digital-to-analog converters (will be referred to as "D/A
converters" hereinafter) 7 and 8, and a temperature sensing circuit
12. The adjusting circuit 2 adjusts a filter characteristic (both
cut-off frequency and output amplitude) of this gm-C filter circuit
1. The registers 9 and 10 are employed so as to hold digital
adjusting values outputted from the adjusting circuit 2. The D/A
converters 7 and 8 convert the digital adjusting values held in
these registers 9 and 10 into analog adjusting values, and supply
the analog adjusting values to the gm-C filter circuit 1. The
temperature sensing circuit 12 senses an ambient temperature of
this gm-C filter system based upon temperature data which is
supplied from an external device so as to control the operation of
the adjusting circuit 2.
[0023] The gm-C filter circuit 1 is arranged by employing a gm
amplifier 1a and a capacitor 1b, and constitutes, for example, a
low-pass filter. On the other hand, the adjusting circuit 2 is
arranged by employing an oscillator 3, a digital control circuit 4,
a D/A converter 5, and another D/A converter 6. The oscillator 3 is
constructed of a gm amplifier 3a and a capacitor 3b. The digital
control circuit 4 produces a digital adjusting value based upon an
oscillation signal OSC outputted from this oscillator 3 to output
the produced digital adjusting value. The D/A converter 5 converts
an adjusting value "Dor.sub.or" into a corresponding analog
adjusting value, and then supplies this analog adjusting value as a
bias current to the gm amplifier 3a. This adjusting value
"D.sub.or" is to adjust output resistance values of the gm
amplifiers 3a and 1a among the digital adjusting values outputted
from the digital control circuit 4. The D/A converter 6 converts
another adjusting value "D.sub.gm" into a corresponding analog
adjusting value, and supplies this analog adjusting value as
another bias current to the gm amplifier 3a. The adjusting value
"D.sub.gm" is to adjust the gm values of the gm amplifiers 3a and
1a among the above-explained digital adjusting values. In this
case, since the output resistance value of the gm amplifier 3a is
adjusted, the oscillation amplitude of the oscillator 3 is
adjusted. Also, since the gm value of the gm amplifier 3a is
adjusted, the oscillation frequency of the oscillator 3 is
adjusted. It should be noted that the gm amplifier 3a of the
oscillator 3 owns the same construction as that of the gm amplifier
1a of the gm-C filter circuit 1.
[0024] Also, the digital adjusting value "D.sub.or" outputted from
the digital control circuit 4 is also supplied as a analog
adjusting value (bias current) via both the register 9 and the D/A
converter 7 to the gm amplifier 1a. On the other hand, the digital
adjusting value "D.sub.gm" outputted from the digital control
circuit 4 is also supplied via both the register 10 and the D/A
converter 8 as another analog adjusting value (bias current) to the
gm amplifier 1a. In this case, since the output resistance value of
the gm amplifier 1a is adjusted so as to become zero, an error
contained in the filter characteristic of the gm-C filter circuit 1
is reduced. Also, since the gm value of the gm amplifier 1a is
adjusted, the cut-off frequency of the gm-C filter circuit 1 is
adjusted.
[0025] Since the gm amplifier 3a of the oscillator 3 owns the same
construction as that of the gm amplifier 1a of the gm-C filter
circuit 1, the oscillation frequency of the oscillator 3
corresponds to the cut-off frequency of the gm-C filter circuit 1
in an one-to-one correspondence relationship. As a result, in order
to set the cut-off frequency of the gm-C filter circuit 1 to a
desirable value, the oscillation frequency of the oscillator 3
maybe adjusted to be equal to a value corresponding to this
desirable value.
[0026] In the case that the digital adjusting value D.sub.gm
outputted from the digital control circuit 4 corresponding to the
oscillation frequency of the oscillator 3 is set, for instance, a
so-called "two-dividing method" is employed. Referring now to FIG.
2, this two-dividing method will be described. In FIG. 2, an
ordinate shows the digital adjusting value "Dgm" (oscillation
frequency), and an abscissa indicates a time operation. It should
be understood that the digital adjusting value "D.sub.gm" is
constituted by "n" bits (symbol "n"=natural number). In this case,
a description will now be made of such a case that the digital
adjusting value "D.sub.gm" is constructed of 7 bits. First,
assuming now that a 7-bit digital value (for example, maximum
value) is equal to "X" when the gm-C filter system is initiated,
this digital adjusting value "D.sub.gm" is set to {fraction (X/2)}
equal to a {fraction (1/2)} value of this 7-bit digital value
(namely, condition shown as "A" of FIG. 2). Then, while the
oscillation frequency of the oscillator 3 is monitored, in such a
case that this monitored oscillation frequency is compared with a
desirable frequency and then this oscillation frequency is lower
than the desirable frequency, the digital adjusting value
"D.sub.gm" is set to {{fraction (X/2)}-(X-{fraction
(X/2)}).times.1/2}=3{fraction (X/4)}. On the other hand, when this
monitored oscillation frequency is higher than the desirable
frequency, the digital adjusting value D.sub.gm is set to
{{fraction (X/2)}-(X-{fraction (X/2)}).times.1/2}=X/4. In the
example, shown in FIG. 2, since the oscillation frequency is higher
than the desirable frequency under state "A", the digital adjusting
value "D.sub.gm" is set to "{fraction (X/4)}", (namely, state "B"
shown in FIG. 2). Then, the oscillation frequency is again
monitored. Now since the oscillation frequency is lowered than the
desirable frequency, {{fraction (X/4)}+{fraction (X/2)}-{fraction
(X/4)}.times.1/2}=3{fraction (X/8)} is set as a next digital
adjusting value. Subsequently, the digital adjusting value
"D.sub.gm" is controlled in such a manner that the oscillation
frequency is made coincident with a desirable frequency. In such a
case that the digital adjusting value D.sub.gm is arranged by 7
bits, an adjusting value corresponding to the desirable frequency
can be obtained in a seventh time operation.
[0027] Referring now to FIGS. 3A to 3C, a description will be made
of a method for adjusting an oscillating amplitude of the
oscillator 3. As indicated in FIG. 3A to FIG. 3C, the oscillation
amplitude of the oscillator 3 is varied based upon the output
resistance value of the gm amplifier 3a thereof. In other words, in
the case that the output resistance value is negative, an
oscillation signal is diverged as indicated in FIG. 3A. Conversely,
in the case that the output resistance value is positive, an
oscillation signal is attenuated, so that an oscillation state is
not maintained as indicated in FIG. 3C. In accordance with this
embodiment mode, as represented in FIG. 3B, in order to realize
such a state that the oscillation is maintained with keeping a
constance amplitude, the output resistance value of the gm
amplitude 3a is adjusted as follows. In other words, for instance,
while the output resistance value of the gm amplitude 3a is changed
in 5 stages, a judgement is made as to whether the oscillation is
diverged, or attenuated in each stage. Then, the output resistance
value of the gm amplifier 3a is adjusted to be equal to such a
value defined between the output resistance value obtained when the
oscillation is diverged and the output resistance value obtained
when the oscillation is attenuated.
[0028] It should also be noted that when the above-explained
oscillation frequency adjusting method (two-dividing method) is
combined with the oscillation amplitude adjusting method, if the
oscillation frequency is adjusted, then the oscillator 3 is
required to be brought into the oscillation state. As a result, the
output resistance value of the gm amplifier 3a is set in such a
manner that the oscillation may be likely diverged, and the
oscillation frequency is adjusted. Thereafter, the oscillation
amplitude is adjusted.
[0029] Next, operations of the adjusting circuit 2 will now be
explained with reference to a time chart of FIGS. 4A and 4B. FIG.
4A indicates a change contained in ambient temperatures of the
filter system, and FIG. 4B shows a change contained in operation
conditions of the adjusting circuit 2. First, the adjusting circuit
2 commences the adjusting operation of the filter characteristic
when the gm-C filter system is initiated (time instance "t.sub.A").
After the adjusting operation has been ended, both the digital
adjusting values "D.sub.or" and "D.sub.gm" supplied from the
adjusting circuit 2 are held in the registers 9 and 10,
respectively, and the operation of the adjusting circuit 20 is
stopped (power off state, time instance t.sub.A'). While the
operation of the adjusting circuit 2 is stopped (power off), both
the digital adjusting values D.sub.or and D.sub.gm saved in the
registers 9 and 10 are supplied via the D/A converts 7/8 as analog
adjusting values (bias currents) to the gm amplifier 1a of the gm-C
filter circuit 1. Thereafter, the adjusting circuit 2 is controlled
in response to a control "CTL" supplied from the temperature
sensing circuit 12. The temperature sensing circuit 12 senses the
ambient temperature of the gm-C filter system in response to the
externally supplied temperature data. This temperature sensing
circuit 12 supplies the control signal CTR in order that at a time
instant (time instant t.sub.B) when the ambient temperature is
changed by, for example, 10 degrees, the operation of the adjusting
circuit 2 is commenced. In response to this control signal CTR, the
adjusting circuit 2 commences the adjusting operation of the filter
characteristic. After the adjusting operation has been
accomplished, the respective adjusting values D.sub.or and D.sub.gm
saved in the resisters 9 and 10 are updated, the operation of the
adjusting circuit 2 is stopped (power off) at a time instant
t.sub.B'. Subsequently, the adjusting circuit 2 repeatedly performs
such an intermittent adjusting operation (time instants tc and
tc').
[0030] As previously described, in accordance with the gm-C filter
system of this embodiment mode, the adjusting circuit 2 is operated
in the intermittent manner in order to adjust the filter
characteristic. As a result, the power consumption of this gm-C
filter system can be reduced, as compared with that of the
conventional filter system.
[0031] It should be noted that the temperature sensing circuit 12
may be replaced by a power supply voltage sensing circuit. In this
alternative case, for instance, the power supply voltage of this
gm-C filter system is subdivided into just a half of this supply
voltage by employing a resistor, and then, this voltage may be
entered to the power supply voltage sensing circuit. In the power
supply voltage sensing circuit, for example, while an input voltage
is converted into a digital value by an A/D converter, a digital
signal processing operation is carried out in such a manner that
the control signal CTL is outputted to the control circuit 2 by
which the operation of the adjusting circuit 2 is commenced when
the voltage is changed by 0.1 V. In this operation manner, the
adjusting circuit may be intermittently carried out based upon the
variation of the power supply voltage.
[0032] Alternatively, both the above-explained temperature sensing
circuit 12 and the power supply voltage sensing circuit may be
employed. In this alternative case, the control signal outputted
from the temperature sensing circuit is AND-gated with the control
signal outputted from the power supply voltage sensing circuit, and
then, the AND-gated control signal is used to control the adjusting
circuit 2. Since such an AND-gated control signal is used, the
adjusting circuit 2 may be intermittently operated based upon
either a change contained in the ambient temperatures or a
variation contained in the power supply voltages.
[0033] Also, the temperature sensing circuit 12 may be replaced by
a counter circuit. In this alternative case, while the reference
clock signal is input into this counter circuit, at such a time
instant when a count value of this counter circuit is reached to a
predetermined, the control signal CTL is supplied from this counter
circuit with respect to the adjusting circuit 2 in order to
commence the operation of the adjusting circuit 2. At this time,
the counter circuit is reset. As a result, the adjusting circuit 2
may be intermittently operated in response to an elapse of
time.
[0034] Alternatively, the operation of the adjusting circuit 2 may
be commenced within systematically empty time of an electronic
appliance in which the gm-C filter system is employed.
[0035] It should also be understood that the adjustment of the
output amplitude of the gm-C filter circuit 1 is no longer
required, the above-explained D/A converter 5, register 9, and D/A
converter 7 may be omitted.
[0036] (Second Embodiment Mode)
[0037] FIG. 5 is a block diagram for representing an arrangement of
a gm-C filter system according to a second embodiment mode of the
present invention. It should be noted that the same reference
numerals used in the first embodiment mode shown in FIG. 1 will be
employed as those for denoting the same, or similar constructions
of this second embodiment mode, and therefore, descriptions thereof
are omitted. In FIG. 5, a temperature compensating circuit 11
produces a drive bias current "i.sub.TEMP" based upon
externally-supplied temperature data, and this drive bias current
"i.sub.TEMP" is to compensate a variation component of output
resistance values derived from the gm amplifiers 3a and 1a with
respect to a change contained in ambient temperature. This
temperature compensating circuit 11 drives the D/A converters 5 and
7 based upon this drive bias current "i.sub.TEMP1" capable of
compensating for variation components in gm values of both the gm
amplifiers 3a and 1a with respect to a change contained in ambient
temperatures. Then, the temperature compensating circuit 11 drives
the D/A converters 6 and 8 based upon this drive bias current
i.sub.TEMP2. The temperature compensating circuit 11 produces both
the drive bias currents "i.sub.TEMP1" and "i.sub.TEMP2" based upon,
for instance, data related to temperature-to-drive bias currents
stored in a ROM.
[0038] Next, a description will now be made of operations of the
gm-C filter system according to this embodiment mode. First, the
adjusting circuit 2 commences the adjusting operation of the filter
characteristic when the gm-C filter system is initiated. After the
adjusting operation has been ended, both the digital adjusting
values "D.sub.or" and "D.sub.gm" supplied from the adjusting
circuit 2 are held in the registers 9 and 10, respectively, and the
operation of the adjusting circuit 20 is stopped (power off state).
While the operation of the adjusting circuit 2 is stopped (power
off), both the digital adjusting value D.sub.or and D.sub.gm saved
in the registers 9 and 10 are supplied via the D/A converters 7/8
as analog adjusting values (bias currents) to the gm amplifier 1a
of the gm-C filter circuit 1. Thereafter, once the operation of the
adjusting circuit 2 is stopped (power off), even when the ambient
temperature is changed, the adjusting circuit 2 is not operated,
which is different from the above-explained operation of the first
embodiment mode. Instead, both the drive bias currents "i.sub.TEMP"
and "i.sub.TEMP2" are supplied from the temperature compensating
circuit 11 to the D/A converters 7/8, respectively, so that the
filter characteristic is adjusted.
[0039] As previously explained, in accordance with the gm-C filter
system of this embodiment mode, the adjusting circuit 2 is operated
only when the gm-C filter system is initiated in order to adjust
the filter characteristic, so that the power consumption can be
reduced, as compared with that of the conventional filter
system.
[0040] (Third Embodiment Mode)
[0041] FIG. 6 is a block diagram for representing an arrangement of
a gm-C filter system according to a third embodiment mode of the
present invention. It should be noted that the same reference
numeral used in the first embodiment mode shown in FIG. 1 will be
employed as those for denoting the same, or similar constructions
of this third embodiment mode, and therefore, descriptions thereof
are omitted. In FIG. 6, a temperature compensating circuit 11'
produces such a compensating digital adjusting value "D.sub.orc"
capable of compensating for a variation component of output
resistance values of the gm amplifier 1a with respect to a change
contained in ambient temperatures based upon externally supplied
temperature data, and also produces another compensating digital
adjusting value "D.sub.gmc" capable of compensating a variation
component of gm values of the gm amplifier 1a with respect to a
change contained in ambient temperatures. An adder 21 executes a
digital calculation with respect to both the compensating digital
adjusting value "D.sub.orc" supplied from the temperature
compensating circuit 11', and also a digital adjusting value
"D.sub.or" held in the register 9. Then, this adder 21 supplies the
digitally calculated result to the D/A converter 7. Also, the adder
22 performs a digital calculation with respect to both the
compensating digital adjusting value "D.sub.gmc" supplied from the
temperature compensating circuit 11' and also a digital adjustment
value "D.sub.gm" held in the register 10. Then, this adder 21
supplies the digitally calculated result to the D/A converter
8.
[0042] Next, a description will now be made of operations of the
gm-C filter system according to this embodiment mode. First, the
adjusting circuit 2 commences the adjusting operation of the filter
characteristic when the gm-C filter system is initiated. After the
adjusting operation has bee ended, both the digital adjusting
values "D.sub.or" and "D.sub.gm" supplied from the adjusting
circuit 2 are held in the registers 9 and 10, respectively, and the
operation of the adjusting circuit 20 is stopped (power off state).
While the operation of the adjusting circuit 2 is stopped (power
off), both the digital adjusting values D.sub.or and D.sub.gm saved
in the registers 9 and 10 are supplied via the D/A converters 7 and
8 as analog adjusting values (bias currents) to the gm amplifier 1a
of the gm-C filter circuit 1. Thereafter, once the operation of the
adjusting circuit 2 is stopped (power off state), even when the
ambient temperature is changed, the adjusting circuit 2 is not
operated, which is different from the above-explained operation of
the first embodiment mode. Instead, both the compensating digital
adjusting values "D.sub.orc" and "D.sub.gmc" are supplied from the
temperature compensating circuit 11' to the adders 21 and 22,
respectively, so that the filter characteristic is adjusted.
[0043] As previously explained, in accordance with the gm-C filter
system of this embodiment mode, the adjusting circuit 2 is operated
only when the gm-C filter system is initiated in order to adjust
the filter characteristic, so that the power consumption can be
reduced, as compared with that of the conventional filter
system.
[0044] As apparent from the above-explained description, in
accordance with the present invention, since the adjusting circuit
for adjusting the filter characteristic is operated in the
intermittent manner (otherwise, only when filter system is
initiated), the gm-C filter system whose power consumption is low
can be provided. The gm-C filter system of the present invention
may be effectively used when, for instance, this gm-C filter system
is used as such a filter system mounted on an LSI designed for a
portable electronic appliance such as a portable telephone set.
* * * * *