Filter circuit

Details Filter circuit Filter circuit

2003-01-13

2004-11-16

Le, Dinh T. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific identifiable device, circuit, or system

Unwanted signal suppression

C327S103000

Reexamination Certificate

active

06819167

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to a filter circuit and, more particularly, to a filter circuit having a transconductance amplifier.
2. Description of the Related Art
The conventional filter circuit is disclosed in “A circuit design method of an analog filter”, page 146 lines 11-17 and FIG. 4.57, Toshio Horikawa, SOGODENSHISYUPANSYA, and Japanese Patent Application Laid Open Number 11-112286.
Horikawa discloses, as shown in FIGS.
6
(A)-
6
(C), an LC ladder type filter circuit which has three types. One type, as shown in FIG.
6
(A), is that a resistance is supplied between two input ports and between two output ports, respectively (R-R type). Another type, as shown in FIG.
6
(B), is that a resistance is only supplied between two input ports (R-∞ type). The other type, as shown in FIG.
6
(C), is that a resistance is only supplied between two output ports (
0
-R type).
FIG. 7
is a circuit diagram showing a differential input type Low Pass Filter (LPF) circuit which is R-R type. In
FIG. 7
, the LPF circuit has an input resistance portion
710
, a filter portion
720
and an output end resistance portion
730
. The input resistance portion
710
comprises two transconductance amplifiers
711
and
712
.
However, the input dynamic range of the LPF circuit depends on the input dynamic range of the transconductance amplifier
711
. The voltage amplitude of the differential input signal to be input to the LPF circuit, is a value that one transconductance amplifier shares.
FIG. 8
is wave diagram showing frequency characteristics of the differential input type LPF circuit. In
FIG. 8
, the vertical axis shows gain and the horizontal axis shows frequency. The cutoff frequency of the differential input type LPF circuit is shown as “fc”. The left side of fc is a passing band and the right side of fc is a cutoff band. When a mixed signal having frequency f
1
which is in the passing band and frequency f
2
which is in the cutoff band is input to the differential input type LPF circuit, a voltage amplitude peak of the mixed signal is the sum of voltage amplitude of the frequency f
1
and the frequency f
2
. When the input dynamic range of the transconductance amplif
1
er
711
is Vdr and when the voltage amplitude peak of the mixed signal to be input is over Vdr, the operation of the differential input type LPF circuit is not guaranteed.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided a filter circuit of an input resistance portion, wherein said input resistance portion includes a first transconductance means having a positive input port, a negative input port, a positive output port and a negative output port, wherein the positive input port is impressed with a first voltage, and wherein the negative input port is impressed with a second voltage; a second transconductance means having a positive input port, a negative input port, a positive output port and a negative output port, wherein the positive input port is impressed with the second voltage, and wherein the negative input port is impressed with a third voltage; and a third transconductance means having a positive input port, a negative input port, a positive output port and a negative output port, wherein the positive input port is coupled to the positive output ports of the first and second transconductance means and the negative output port of the third transconductance means, and wherein the negative input port is coupled to the negative output ports of the first and second transconductance means and the positive output port of the third transconductance means.


REFERENCES:
patent: 5245565 (1993-09-01), Petersen et al.
patent: 6441682 (2002-08-01), Vinn et al.
patent: 6525621 (2003-02-01), van Zeijl et al.

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