Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Unwanted signal suppression
Reexamination Certificate
2001-12-31
2004-06-01
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Unwanted signal suppression
C327S558000, C327S559000, C330S303000
Reexamination Certificate
active
06744306
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a filter circuit such as a first-order low-pass filter, a first-order high-pass filter, or a first-order all-pass filter, and particularly to a filter circuit with a wide dynamic range capable of low-voltage operation.
As a conventional example of a filter circuit, for example a first-order low-pass filter with a wide dynamic range capable of low-voltage operation, a filter circuit disclosed in Japanese Patent Laid-Open No. Hei 9-69752 is known, for example. A circuit configuration of the first-order low-pass filter according to the conventional example is shown in FIG.
13
.
In
FIG. 13
, a base electrode of a transistor Q
1
is connected to a circuit input terminal
101
of one of differential inputs. A collector electrode of the transistor Q
1
is connected to a power supply line
103
of a supply voltage VCC. An emitter electrode of the transistor Q
1
is connected with an emitter electrode of a transistor Q
2
. The transistor Q
2
is of a diode-connected configuration, in which a base electrode and a collector electrode of the transistor Q
2
are connected to each other. A current source
111
is connected between a GND line
104
at a ground level and a common emitter connection point of the transistors Q
1
and Q
2
.
The base electrode and the collector electrode of the transistor Q
2
are connected with a base electrode and a collector electrode of a transistor Q
3
. Thus, the transistor Q
3
is also of the diode-connected configuration, and is connected in parallel with the diode-connected transistor Q
2
with a polarity opposite from the transistor Q
2
. A current source
112
is connected between the power supply line
103
and a common connection point of the bases and the collectors of the transistors Q
2
and Q
3
. An emitter electrode of the transistor Q
3
is connected with an emitter electrode of a transistor Q
4
. A current source
113
is connected between the GND line
104
and a common emitter connection point of the transistors Q
3
and Q
4
.
By thereafter repeating the same connecting relation, a total of n transistors Q
1
to Qn are connected to one another. Then, a current source
114
is connected between the GND line
104
and a common emitter connection point of an (n−1)th diode-connected transistor Qn−1 and an nth diode-connected transistor Qn in a final stage. A current source
115
is connected between the power supply line
103
and a common connection point of a base and a collector of the transistor Qn. The common connection point of the base and the collector of the transistor Qn is also connected to a circuit output terminal
105
of one of differential outputs, and connected to one terminal of a capacitor
107
.
A circuit formed by n transistors (transistors Q
2
n to Qn+1) and current sources in exactly the same connecting relation as the above circuit is connected between a circuit input terminal
102
of the other differential input, a circuit output terminal
106
of the other differential output, and the other terminal of the capacitor
107
. It is to be noted that the first-order low-pass filter according to the present example is an example of a circuit when n is an even number; when n is an odd number, the connecting relation of the nth transistor Qn (Qn+1), the circuit output terminal
105
(
106
), and the one terminal (other terminal) of the capacitor
107
is as shown in FIG.
14
.
A circuit equivalent to the thus formed first-order low-pass filter according to the conventional example is shown in FIG.
15
. As is clear from the equivalent circuit, the first-order low-pass filter has a circuit configuration in which n emitter resistances re of the transistors are connected in series with each other between the circuit input terminal
101
and the circuit output terminal
105
and between the circuit input terminal
102
and the circuit output terminal
106
, and the capacitor
107
is connected between the circuit output terminals
105
and
106
.
Letting vi be an input signal, vo be an output signal, I be a current flowing in each of the transistors, C be capacitance of the capacitor
107
, and s be a complex frequency, a transfer function H (=vo/vi) of the first-order low-pass filter is:
[Equation 1]
H
=
1
2
re
·
n
·
C
s
+
1
2
re
·
n
·
C
(
1
)
The emitter resistance re is expressed as re=Vt/I, where Vt=kT/q, k being the Boltzmann constant, T being the absolute temperature, and q being the amount of electron charge. The cut-off frequency fc is:
fc=
¼&pgr;·
re·n·C
As is clear from FIG.
13
and
FIG. 14
, because of the circuit configuration in which only two current sources and one transistor circuit are arranged between the power supply line
103
and the GND line
104
, the first-order low-pass filter according to the conventional example has advantages of being able to operate at a low supply voltage and extend the input dynamic range by a factor of n by increasing the number n of transistors.
However, in the first-order low-pass filter formed as described above according to the conventional example, the extension of the input dynamic range requires an increase of the number n of transistors, and hence when the cut-off frequency fc and the capacitance C of the capacitor
107
are fixed, the increase of the number n of transistors results in an exponential increase in current consumption in accordance with the number n.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above problem, and it is accordingly an object of the present invention to provide a filter circuit of low-voltage operation that can extend the input dynamic range while reducing current consumption.
In order to achieve the above object, according to the present invention, there is provided a filter circuit comprising: a first differential circuit formed by a combination of one transistor and four diodes connected in parallel with each other and each having one electrode connected to a first electrode of the transistor, a first current corresponding to an input signal flowing through the four diodes; and a second differential circuit formed by a combination of one diode and four transistors connected in parallel with each other and each having a first electrode connected to one electrode of the diode, a second current corresponding to the input signal flowing through the one diode. Further, a current source is connected to a common connection node of the four diodes and the one diode. A capacitor through which a current determined by a current of the current source and the first and second currents flows is connected to predetermined nodes, whereby a low-pass filter, a high-pass filter, or an all-pass filter is formed.
Hereinafter, bipolar transistors will be taken as an example of the transistors forming the first and second differential circuits. In this case, the first electrode of the transistor refers to an emitter electrode for injecting a carrier (electron or hole); a second electrode refers to a collector electrode reached by the carrier; and a control electrode refers to a base electrode supplied with a current for controlling movement of the carrier injected from the emitter electrode. The one electrode of a diode refers to a cathode electrode, and when the diode is formed by a transistor, the electrode refers to an emitter electrode.
By providing the thus formed filter circuit with the first differential circuit in which a ratio of the number of transistors to that of diodes is 1:4 and the second differential circuit in which the ratio of the number of transistors to that of diodes is 4:1, and by connecting the current source to the connection node of the diodes, the differential circuits have two operating points. By adding together the first and second differential circuits having two operating points, it is possible to extend the dynamic range. In addition, the cut-off frequency is made variable by changing the current of the current source.
RE
Callahan Timothy P.
Englund Terry L.
Kananen Ronald P.
Rader & Fishman & Grauer, PLLC
Sony Corporation
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